RESUMEN
The landscape paradigm of protein folding can enable preferred pathways on a funnel-like energy surface. Hierarchical preferences may be manifest as a nonrandom pathway of disulfide pairing. Stepwise stabilization of structural subdomains among on-pathway intermediates is proposed to underlie the disulfide pathway of proinsulin and related molecules. Here, effects of pairwise serine substitution of insulin's exposed interchain disulfide bridge (Cys(A7)-Cys(B7)) are characterized as a model of a late intermediate. Untethering cystine A7-B7 in an engineered monomer causes significantly more marked decreases in the thermodynamic stability and extent of folding than occur on pairwise substitution of internal cystine A6-A11 [Weiss, M. A., Hua, Q. X., Jia, W., Chu, Y. C., Wang, R. Y., and Katsoyannis, P. G. (2000) Biochemistry 39, 15429-15440]. Although substantially disordered and without significant biological activity, the untethered analogue contains a molten subdomain comprising cystine A20-B19 and a native-like cluster of hydrophobic side chains. Remarkably, A and B chains make unequal contributions to this folded moiety; the B chain retains native-like supersecondary structure, whereas the A chain is largely disordered. These observations suggest that the B subdomain provides a template to guide folding of the A chain. Stepwise organization of insulin-like molecules supports a hierarchic view of protein folding.
Asunto(s)
Insulina/análogos & derivados , Insulina/química , Dicroismo Circular , Disulfuros/química , Estabilidad de Medicamentos , Humanos , Técnicas In Vitro , Insulina/síntesis química , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Oxidación-Reducción , Proinsulina/síntesis química , Proinsulina/química , Conformación Proteica , Desnaturalización Proteica , Pliegue de Proteína , Estructura Terciaria de Proteína , Subunidades de Proteína , TermodinámicaRESUMEN
Previous studies have demonstrated that the potency and thermodynamic stability of human insulin are enhanced in concert by substitution of Thr(A8) by arginine or histidine. These surface substitutions stabilize the N-terminal alpha-helix of the A chain, a key element of hormone-receptor recognition. Does enhanced stability necessarily imply enhanced activity? Here, we test by structure-based mutagenesis the relationship between the stability and activity of the hormone. To circumvent confounding effects of insulin self-association, A chain analogs were combined with a variant B chain (Asp(B10), Lys(B28), and Pro(B29) (DKP)) to create a monomeric template. Five analogs were obtained by chain combination; disulfide pairing proceeded in each case with native yield. CD and (1)H NMR spectra of the DKP analogs are essentially identical to those of DKP-insulin, indicating a correspondence of structures. Receptor binding affinities were determined by competitive displacement of (125)I-insulin from human placental membranes. Thermodynamic stabilities were measured by CD titration; unfolding was monitored as a function of guanidine concentration. In this broader collection of analogs receptor binding affinities are uncorrelated with stability. We suggest that receptor binding affinities of A8 analogs reflect local features of the hormone-receptor interface rather than the stability of the free hormone or the intrinsic C-capping propensity of the A8 side chain.
Asunto(s)
Insulina/análogos & derivados , Secuencia de Aminoácidos , Estabilidad de Medicamentos , Guanidina , Calor , Insulina/química , Modelos Moleculares , Datos de Secuencia Molecular , Unión Proteica , Conformación Proteica , Desnaturalización Proteica , Receptor de Insulina/metabolismo , Homología de Secuencia de Aminoácido , Relación Estructura-Actividad , TermodinámicaRESUMEN
A hierarchical pathway of protein folding can enable segmental unfolding by design. A monomeric insulin analogue containing pairwise substitution of internal A6-A11 cystine with serine [[Ser(A6),Ser(A11),Asp(B10),Lys(B28),Pro(B29)]insulin (DKP[A6-A11](Ser))] was previously investigated as a model of an oxidative protein-folding intermediate [Hua, Q. X., et al. (1996) J. Mol. Biol. 264, 390-403]. Its structure exhibits local unfolding of an adjoining amphipathic alpha-helix (residues A1-A8), leading to a 2000-fold reduction in activity. Such severe loss of function, unusual among mutant insulins, is proposed to reflect the cost of induced fit: receptor-directed restoration of the alpha-helix and its engagement in the hormone's hydrophobic core. To test this hypothesis, we have synthesized and characterized the corresponding alanine analogue [[Ala(A6),Ala(A11),Asp(B10),Lys(B28), Pro(B29)]insulin (DKP[A6-A11](Ala))]. Untethering the A6-A11 disulfide bridge by either amino acid causes similar perturbations in structure and dynamics as probed by circular dichroism and (1)H NMR spectroscopy. The analogues also exhibit similar decrements in thermodynamic stability relative to that of the parent monomer as probed by equilibrium denaturation studies (Delta Delta G(u) = 3.0 +/- 0.5 kcal/mol). Despite such similarities, the alanine analogue is 50 times more active than the serine analogue. Enhanced receptor binding (Delta Delta G = 2.2 kcal/mol) is in accord with alanine's greater helical propensity and more favorable hydrophobic-transfer free energy. The success of an induced-fit model highlights the applicability of general folding principles to a complex binding process. Comparison of DKP[A6-A11](Ser) and DKP[A6-A11](Ala) supports the hypothesis that the native A1-A8 alpha-helix functions as a preformed recognition element tethered by insulin's intrachain disulfide bridge. Segmental unfolding by design provides a novel approach to dissecting structure-activity relationships.
Asunto(s)
Insulina/química , Pliegue de Proteína , Secuencia de Aminoácidos , Sustitución de Aminoácidos , Animales , Diseño de Fármacos , Insulina/metabolismo , Datos de Secuencia Molecular , Ratas , Receptor de Insulina/química , Receptor de Insulina/metabolismo , Alineación de Secuencia , Relación Estructura-Actividad , SulfurosRESUMEN
Maturity-onset diabetes of the young, a monogenic form of Type II diabetes mellitus, is most commonly caused by mutations in hepatic nuclear factor 1alpha (HNF-1alpha). Here, the dimerization motif of HNF-1alpha is shown to form an intermolecular four-helix bundle. One face contains an antiparallel coiled coil whereas the other contains splayed alpha-helices. The "mini-zipper" is complementary in structure and symmetry to the top surface of a transcriptional coactivator (dimerization cofactor of homeodomains). The bundle is destabilized by a subset of mutations associated with maturity-onset diabetes of the young. Impaired dimerization of a beta-cell transcription factor thus provides a molecular mechanism of metabolic deregulation in diabetes mellitus.
Asunto(s)
Proteínas de Unión al ADN , Diabetes Mellitus Tipo 2/genética , Islotes Pancreáticos/metabolismo , Mutación , Proteínas Nucleares , Estructura Secundaria de Proteína , Factores de Transcripción/química , Secuencia de Aminoácidos , Dimerización , Factor Nuclear 1 del Hepatocito , Factor Nuclear 1-alfa del Hepatocito , Factor Nuclear 1-beta del Hepatocito , Humanos , Concentración de Iones de Hidrógeno , Modelos Moleculares , Datos de Secuencia Molecular , Factores de Transcripción/genética , Factores de Transcripción/metabolismoRESUMEN
A model of transcriptional activator-coactivator recognition is provided by the mammalian CREB activation domain and the KIX domain of coactivator CBP. The CREB kinase-inducible activation domain (pKID, 60 residues) is disordered in solution and undergoes an alpha-helical folding transition on binding to CBP [Radhakrishan, I., Perez-Alvarado, G. C., Parker, D., Dyson, H. J., Montminy, M. R., and Wright, P. E. (1997) Cell 91, 741-752]. Binding requires phosphorylation of a conserved serine (RPpSYR) in pKID associated in vivo with the biological activation of CREB signaling pathways. The CBP-bound structure of CREB contains two alpha-helices (designated alphaA and alphaB) flanking the phosphoserine; the bound structure is stabilized by specific interactions with CBP. Here, the nascent structure of an unbound pKID domain is characterized by multidimensional NMR spectroscopy. The solubility of the phosphopeptide (46 residues) was enhanced by truncation of N- and C-terminal residues not involved in pKID-CBP interactions. Although disordered under physiologic conditions, the pKID fragment and its unphosphorylated parent peptide exhibit partial folding at low temperatures. One recognition helix (alphaA) is well-defined at 4 degreesC, whereas the other (alphaB) is disordered but inducible in 40% trifluoroethanol (TFE). Such nascent structure is independent of serine phosphorylation and correlates with the relative extent of engagement of the two alpha-helices in the pKID-KIX complex; whereas alphaA occupies a peripheral binding site with few intermolecular contacts, the TFE-inducible alphaB motif is deeply engaged in a hydrophobic groove. Our results support the use of TFE as an empirical probe of hidden structural propensities and define a correspondence between induced fit and the nascent structure of peptide fragments.
Asunto(s)
Proteína de Unión a Elemento de Respuesta al AMP Cíclico/química , Proteínas Quinasas Dependientes de AMP Cíclico/metabolismo , Proteínas Nucleares/química , Pliegue de Proteína , Transactivadores/química , Factores de Transcripción/química , Activación Transcripcional , Secuencia de Aminoácidos , Proteína de Unión a CREB , Dicroismo Circular , Proteína de Unión a Elemento de Respuesta al AMP Cíclico/metabolismo , Humanos , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Datos de Secuencia Molecular , Proteínas Nucleares/metabolismo , Unión Proteica , Estructura Secundaria de Proteína , Estructura Terciaria de Proteína , Relación Estructura-Actividad , Transactivadores/metabolismo , Factores de Transcripción/metabolismoRESUMEN
Protein minimization highlights essential determinants of structure and function. Minimal models of proinsulin and insulin-like growth factor I contain homologous A and B domains as single-chain analogues. Such models (designated mini-proinsulin and mini-IGF-I) have attracted wide interest due to their native foldability but complete absence of biological activity. The crystal structure of mini-proinsulin, determined as a T3R3 hexamer, is similar to that of the native insulin hexamer. Here, we describe the solution structure of a monomeric mini-proinsulin under physiologic conditions and compare this structure to that of the corresponding two-chain analogue. The two proteins each contain substitutions in the B-chain (HisB10-->Asp and ProB28-->Asp) designed to destabilize self-association by electrostatic repulsion; the proteins differ by the presence or absence of a peptide bond between LysB29 and GlyA1. The structures are essentially identical, resembling in each case the T-state crystallographic protomer. Differences are observed near the site of cross-linking: the adjoining A1-A8 alpha-helix (variable among crystal structures) is less well-ordered in mini-proinsulin than in the two-chain variant. The single-chain analogue is not completely inactive: its affinity for the insulin receptor is 1500-fold lower than that of the two-chain analogue. Moreover, at saturating concentrations mini-proinsulin retains the ability to stimulate lipogenesis in adipocytes (native biological potency). These results suggest that a change in the conformation of insulin, as tethered by the B29-A1 peptide bond, optimizes affinity but is not integral to the mechanism of transmembrane signaling. Surprisingly, the tertiary structure of mini-proinsulin differs from that of mini-IGF-I (main-chain rms deviation 4.5 A) despite strict conservation of non-polar residues in their respective hydrophobic cores (side-chain rms deviation 4.9 A). Three-dimensional profile scores suggest that the two structures each provide acceptable templates for threading of insulin-like sequences. Mini-proinsulin and mini-IGF-I thus provide examples of homologous protein sequences encoding non-homologous structures.
Asunto(s)
Factor I del Crecimiento Similar a la Insulina/química , Proinsulina/química , Secuencia de Aminoácidos , Animales , Humanos , Insulina/metabolismo , Factor I del Crecimiento Similar a la Insulina/metabolismo , Espectroscopía de Resonancia Magnética , Datos de Secuencia Molecular , Proinsulina/metabolismo , Unión Proteica , Conformación Proteica , Pliegue de Proteína , Ratas , Soluciones , Electricidad EstáticaRESUMEN
Functional surfaces of a protein are often mapped by combination of X-ray crystallography and mutagenesis. Such studies of insulin have yielded paradoxical results, suggesting that the native state is inactive and reorganizes on receptor binding. Of particular interest is the N-terminal alpha-helix of the A-chain. Does this segment function as an alpha-helix or reorganize as recently proposed in a prohormone-convertase complex? To correlate structure and function, we describe a mapping strategy based on protein design. The solution structure of an engineered monomer ([AspB10, LysB28, ProB29]-human insulin) is determined at neutral pH as a template for synthesis of a novel A-chain analogue. Designed by analogy to a protein-folding intermediate, the analogue lacks the A6-A11 disulphide bridge; the cysteine residues are replaced by serine. Its solution structure is remarkable for segmental unfolding of the N-terminal A-chain alpha-helix (A1 to A8) in an otherwise native subdomain. The structure demonstrates that the overall orientation of the A and B chains is consistent with reorganization of the A-chain's N-terminal segment. Nevertheless, the analogue's low biological activity suggests that this segment, a site of clinical mutation causing diabetes mellitus, functions as a preformed recognition alpha-helix.
Asunto(s)
Insulina/química , Secuencia de Aminoácidos , Humanos , Insulina/análogos & derivados , Espectroscopía de Resonancia Magnética , Datos de Secuencia Molecular , Estructura Molecular , Pliegue de Proteína , Soluciones , Relación Estructura-ActividadRESUMEN
The structure of a metastable folding intermediate of human insulin-like growth factor I (IGF-I) and an engineered model are investigated by circular dichroism and two-dimensional 1H NMR spectroscopy. The intermediate, which contains two of three native disulfide bonds, was trapped by acid quenching and isolated by reverse-phase HPLC. The reduced cysteine residues were mapped to residues 47 and 52 (corresponding to A6-A11 in insulin). In the native state this disulfide bridge anchors an adjoining amphipathic alpha-helix (helix 2; residues 42 to 49) against the hydrophobic core. Comparison of CD and 1H-NMR spectra demonstrates that the acid-quenched intermediate is partially folded and contains elements of native secondary and tertiary structure. Spectra are similar to those of an equilibrium model in which the reduced cysteine residues are replaced by alanine. Complete 1H-NMR sequential assignment of the alanine model has been obtained and demonstrates that removal of the disulfide bond is associated with local unfolding of the adjoining alpha-helix. Native secondary structure (including helices 1 and 3) is otherwise retained and defines a folded subdomain. Long-range nuclear Overhauser effects (NOE) within this subdomain are similar to those of native IGF-I; no non-native NOE is observed. Our results support the hypothesis that folding of the insulin motif is directed by a subset of native structural elements and that these elements form at an early step in the pathway. Formation of helix 2, despite its prominence in the native state, is likely to represent a late step. Hydrophobic collapse of this segment appears to precede helix formation.
Asunto(s)
Factor I del Crecimiento Similar a la Insulina/química , Pliegue de Proteína , Alanina/química , Secuencia de Aminoácidos , Dicroismo Circular , Humanos , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Datos de Secuencia Molecular , Oxidación-Reducción , Conformación Proteica , Precursores de Proteínas/química , Proteínas Recombinantes/químicaRESUMEN
The structure and dynamics of the R6 human insulin hexamer are investigated by two- and three-dimensional homonuclear 1H-NMR spectroscopy. The R6 hexamer, stabilized by Zn2+ and phenol, provides a model of an allosteric protein assembly and is proposed to mimic aspects of receptor recognition. Despite the large size of the assembly (36 kDa), its extreme thermal stability permits high-resolution spectra to be observed at 55 degrees C. Each spin system is represented uniquely, implying either 6-fold symmetry or fast exchange among allowed protomeric conformations. Dramatic changes in chemical shifts and long-range nuclear Overhauser enhancements (NOEs) are observed relative to the spectra of insulin monomers. Complete sequential assignment is obtained and demonstrates native secondary structure with distinctive R-state N-terminal extension of the B-chain alpha-helix (residues B1 to B19). The distance-geometry structure of an R-state promoter is similar to those of R6 crystal structures. Specific long-range intra- and intersubunit NOEs, assigned by stepwise analysis of engineered insulin monomer and dimers, demonstrate that tertiary and quaternary contacts are also similar. Although the hexamer is well-ordered in solution, binding of phenol to an internal cavity occurs within milliseconds, implying the existence of "gatekeeper" residues whose flexibility provides a portal of entry and release. Changes in 1H-NMR chemical shifts on hexamer assembly are readily rationalized by analysis of aromatic ring-currents and provide sensitive probes for sites of protein-protein interaction and phenol binding. Our results provide a foundation for the interaction and phenol binding. Our results provide a foundation for the studies of insulin analogues (such as "designed" insulins of therapeutic interest) under conditions of clinical formulation and for the investigation of the effects of protein assembly on the dynamics of individual protomers.
Asunto(s)
Insulina/química , Conformación Proteica , Dicroismo Circular , Cristalografía por Rayos X , Calor , Humanos , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Pliegue de ProteínaRESUMEN
We have determined the structure of a metastable disulphide isomer of human insulin. Although not observed for proinsulin folding or insulin-chain recombination, the isomer retains ordered secondary structure and a compact hydrophobic core. Comparison with native insulin reveals a global rearrangement in the orientation of A- and B-chains. One face of the protein's surface is nevertheless in common between native and non-native structures. This face contains receptor-binding determinants, rationalizing the partial biological activity of the isomer. Structures of native and non-native disulphide isomers also define alternative three-dimensional templates. Threading of insulin-like sequences provide an experimental realization of the inverse protein-folding problem.
Asunto(s)
Insulina/química , Modelos Moleculares , Conformación Proteica , Secuencia de Aminoácidos , Dicroismo Circular , Humanos , Insulina/metabolismo , Isomerismo , Cinética , Espectroscopía de Resonancia Magnética , Datos de Secuencia Molecular , Proinsulina/química , Pliegue de Proteína , Estructura Secundaria de Proteína , Receptor de Insulina/metabolismo , Moldes Genéticos , TermodinámicaRESUMEN
Can one protein sequence encode two structures? Oxidative folding of human insulin-like growth factor 1 (IGF-1), a globular protein of 70 residues, is shown to yield two products of similar thermodynamic stability. This observation is of particular interest in light of the recent demonstration that two of the three disulfide bonds in native IGF-1 rearrange in the presence of dithiothreitol [Hober, S., et al. (1992) Biochemistry 31, 1749-1756]. Kinetics of the IGF-1 folding pathway were monitored by high-performance liquid chromatography (rp-HPLC). Disulfide-pairing schemes of intermediates and products were established by peptide mapping. Two disulfide isomers were obtained as products: one with native insulin-like pairing [6-48; 18-61; 47-52] (designated native IGF-1; 60% yield) and the other with alternative pairing [6-47; 18-61; 48-52] (designated IGF-swap; 40% yield). The predominant early intermediate contains the single disulfide 18-61, which is shared in common by the two products. Relative yields of native IGF-1 and IGF-swap are independent of protein concentration under dilute conditions. In the absence of an added thiol reagent, each isomer is stable indefinitely at neutral pH; in the presence of an added thiol reagent, the two isomers interconvert with an Arrhenius activation barrier of 12 kcal/mol. Interconversion does not require complete reduction and yields the same ratio of products as initial folding, demonstrating thermodynamic control. Spectroscopic studies using circular dichroism (CD), infrared spectroscopy (FTIR), two-dimensional 1H-NMR (2D-NMR), and photochemical dynamic nuclear polarization (photo-CIDNP) suggest that IGF-1 and IGF-swap adopt similar secondary structures but distinct tertiary folds. Implications of these observations for understanding the topology of protein-folding pathways are discussed.
Asunto(s)
Factor I del Crecimiento Similar a la Insulina/química , Secuencia de Aminoácidos , Cromatografía Líquida de Alta Presión , Dicroismo Circular , Disulfuros/química , Estabilidad de Medicamentos , Humanos , Concentración de Iones de Hidrógeno , Cinética , Espectroscopía de Resonancia Magnética , Datos de Secuencia Molecular , Oxidación-Reducción , Mapeo Peptídico , Desnaturalización Proteica , Pliegue de Proteína , Proteínas Recombinantes/química , Espectrofotometría Infrarroja , TermodinámicaRESUMEN
Insulin and insulin-related proteins contain three motif-specific disulfide bonds. Here we examine the role of these disulfide bonds in the folding and function of one family member, human insulin-like growth factor 1 (IGF-1). Analogues containing pariwise Cys-->Ser or Cys-->Ala substitutions were expressed in Escherichia coli, purified, and analyzed with respect to receptor-binding, solution structure, and thermodynamic stability. An analogue lacking all three disulfide bonds (designated des-Cys-IGF-1) is inactive and unfolded. Introduction of the [18-61] disulfide bond, previously shown to occur in an early intermediate in oxidative refolding [Miller, J. A., Owers-Narhi, L., Hua, Q. X., Rosenfeld, R., Arakawa, T., Rohde, M., Prestrelski, S., Lauren, S., S. Stoney, K. S., Tsai, L., & Weiss, M. A. (1993) Biochemistry (preceding paper in this issue)], results in a compact partially folded state with low but significant biological activity. Additional but incomplete structural organization and biological activity are observed following introduction of either the [6-48] or the [47-52] disulfide bonds. Native function, structure, and stability require the presence of all three disulfide bonds. These analogues provide genetic models of IGF-1 protein-folding intermediates. Their characterization suggests that bifurcation of the IGF-1 folding pathway reflects alternative late steps in the folding of a molten-globule intermediate.
Asunto(s)
Disulfuros/química , Factor I del Crecimiento Similar a la Insulina/química , Secuencia de Aminoácidos , Dicroismo Circular , Estabilidad de Medicamentos , Escherichia coli/genética , Humanos , Factor I del Crecimiento Similar a la Insulina/genética , Factor I del Crecimiento Similar a la Insulina/fisiología , Espectroscopía de Resonancia Magnética , Datos de Secuencia Molecular , Mutagénesis Sitio-Dirigida , Pliegue de Proteína , Receptor IGF Tipo 1/metabolismo , Soluciones , Espectrometría de Fluorescencia , Relación Estructura-Actividad , Termodinámica , Tirosina/químicaRESUMEN
Dynamic differences between an engineered insulin monomer and dimer are investigated under physiological conditions by an adaptation of the method of acid-quenched amide proton exchange. Although exchange lifetimes of amide protons involved in intrachain hydrogen bonds are similar in the two analogs, dimerization specifically stabilizes two interchain hydrogen bonds (LeuB6-NH...O = C-CysA6 and CysA11-NH...O = C-GluB4) that are distant from the dimer interface. Such non-local stabilization demonstrates that fluctuations in the tertiary structure of the monomer are damped by dimerization. As the B6-A6 and A11-B4 hydrogen bonds are specific to the crystallographic T-state, their stabilization also indicates that the R-state (an allosteric feature of hexamer assembly) is not significantly populated in an isolated dimer. Our results are discussed in reference to recent hypotheses that crystal structures of insulin depict inactive conformers and that detachment of interchain contacts accompany receptor binding.
Asunto(s)
Insulina/análogos & derivados , Insulina/química , Estructura Secundaria de Proteína , Amidas , Secuencia de Aminoácidos , Estabilidad de Medicamentos , Enlace de Hidrógeno , Sustancias Macromoleculares , Espectroscopía de Resonancia Magnética/métodos , Modelos Estructurales , Datos de Secuencia Molecular , Ingeniería de Proteínas , Difracción de Rayos X/métodosRESUMEN
The structure of insulin exhibits local and nonlocal differences among crystal forms and so provides an important model for analysis of protein dynamics. A novel combination of order and disorder has recently been inferred from 2D-NMR studies of the monomeric analogue des-pentapeptide(B26-B30) insulin (DPI) under acidic conditions [the molten-globule hypothesis; Hua, Q.X., Kochoyan, M., & Weiss, M.A. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 2379-2383]. Distance-geometry structures are similar in general to crystal structures but differ by rigid-body displacements of alpha-helices; the hydrophobic core is not well ordered due to insufficient long-range restraints. To test whether such informational uncertainty may represent physical disorder, we have performed complementary studies of the thermal unfolding of DPI and its interaction with 1-anilino-8-naphthalenesulfonate (ANS). Experimental design is based on a predicted analogy between DPI and A-state models of protein-folding intermediates (the "molten globule"). Unfolding is monitored by five distinct biophysical probes: photochemical dynamic nuclear polarization (photo-CIDNP), differential scanning calorimetry (DSC), circular dichroism (CD), 1H-NMR chemical shifts, and slowly exchanging amide 1H-NMR resonances in D2O solution. The results provide evidence that DPI adopts a compact partially folded state. Because the 2D-NMR spectrum of an engineered insulin monomer under physiological conditions is similar to that of DPI under acidic conditions [Weiss, M.A., Hua, Q.X., Frank, B.H., Lynch, C., & Shoelson, S.E. (1991) Biochemistry 30, 7373-7389], we propose that the functional form of insulin is a molten globule.
Asunto(s)
Insulina/análogos & derivados , Insulina/química , Conformación Proteica , Secuencia de Aminoácidos , Naftalenosulfonatos de Anilina , Rastreo Diferencial de Calorimetría/métodos , Dicroismo Circular , Colorantes Fluorescentes , Cinética , Sustancias Macromoleculares , Espectroscopía de Resonancia Magnética/métodos , Modelos Moleculares , Pliegue de Proteína , Espectrometría de Fluorescencia/métodosRESUMEN
The solution structure of a diabetes-associated mutant human insulin (insulin Los Angeles; PheB24-->Ser) was determined by 13C-edited NMR spectroscopy and distance-geometry/simulated annealing calculations. Among vertebrate insulins PheB24 is invariant, and in crystal structures the aromatic ring appears to anchor the putative receptor-binding surface through long-range packing interactions in the hydrophobic core. B24 substitutions are of particular interest in relation to the mechanism of receptor binding. In one analogue ([GlyB24]insulin), partial unfolding of the B chain has been observed with paradoxical retention of near-native bioactivity. The present study of [SerB24]insulin extends this observation: relative to [GlyB24]insulin, near-native structure is restored despite significant loss of function. To our knowledge, our results provide the first structural study of a diabetes-associated mutant insulin and support the hypothesis that insulin undergoes a change in conformation on receptor binding.
Asunto(s)
Insulina/análogos & derivados , Insulina/química , Secuencia de Aminoácidos , Diabetes Mellitus Tipo 2/etiología , Diabetes Mellitus Tipo 2/genética , Humanos , Insulina/genética , Insulina/metabolismo , Insulina/fisiología , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Datos de Secuencia Molecular , Mutación , Fenilalanina , Conformación Proteica , Estructura Secundaria de Proteína , Receptor de Insulina/metabolismo , Serina , Relación Estructura-ActividadRESUMEN
Insulin's mechanism of receptor binding is not well understood despite extensive study by mutagenesis and X-ray crystallography. Of particular interest are "anomalous" analogues whose bioactivities are not readily rationalized by crystal structures. Here the structure and dynamics of one such analogue (GlyB24-insulin) are investigated by circular dichroism (CD) and isotope-aided 2D-NMR spectroscopy. The mutant insulin retains near-native receptor-binding affinity despite a nonconservative substitution (PheB24-->Gly) in the receptor-binding surface. Relative to native insulin, GlyB24-insulin exhibits reduced dimerization; the monomer (the active species) exhibits partial loss of ordered structure, as indicated by CD studies and motional narrowing of selected 1H-NMR resonance. 2D-NMR studies demonstrate that the B-chain beta-turn (residues B20-23) and beta-strand (residues B24-B28) are destabilized; essentially native alpha-helical secondary structure (residues A3-A8, A13-A18, and B9-B19) is otherwise maintained. 13C-Isotope-edited NOESY studies demonstrate that long-range contacts observed between the B-chain beta-strand and the alpha-helical core in native insulin are absent in the mutant. Implications for the mechanism of insulin's interaction with its receptor are discussed.
Asunto(s)
Glicina/química , Insulina/química , Espectroscopía de Resonancia Magnética , Mutagénesis , Fenilalanina/química , Receptor de Insulina/metabolismo , Secuencia de Aminoácidos , Dicroismo Circular , Humanos , Insulina/metabolismo , Datos de Secuencia Molecular , Conformación Proteica , Estructura Secundaria de Proteína , Relación Estructura-ActividadRESUMEN
We have investigated the role of tyrosine residues in the insulin receptor cytoplasmic juxtamembrane region (Tyr953 and Tyr960) during endocytosis. Analysis of the secondary structure of the juxtamembrane region by the Chou-Fasman algorithms predicts that both the sequences GPLY953 and NPEY960 form tyrosine-containing beta-turns. Similarly, analysis of model peptides by 1-D and 2-D NMR show that these sequences form beta-turns in solution, whereas replacement of the tyrosine residues with alanine destabilizes the beta-turn. CHO cell lines were prepared expressing mutant receptors in which each tyrosine was mutated to phenylalanine or alanine, and an additional mutant contained alanine at both positions. These mutations had no effect on insulin binding or receptor autophosphorylation. Replacements with phenylalanine had no effect on the rate of [125I]insulin endocytosis, whereas single substitutions with alanine reduced [125I]insulin endocytosis by 40-50%. Replacement of both tyrosines with alanine reduced internalization by 70%. These data suggest that the insulin receptor contains two tyrosine/beta-turns which contribute independently and additively to insulin-stimulated endocytosis.
Asunto(s)
Endocitosis , Receptor de Insulina/química , Tirosina/química , Secuencia de Aminoácidos , Animales , Células CHO , Cricetinae , Insulina/metabolismo , Datos de Secuencia Molecular , Mutagénesis Sitio-Dirigida , Mutación , Fosforilación , Conformación Proteica , Receptor de Insulina/metabolismoRESUMEN
The structure of the glycoprotein hormones (LH, CG, FSH, and TSH) and their mechanism of receptor recognition are problems of long-standing interest and speculation. Here we describe the two-dimensional [1H]nuclear magnetic resonance ([1H]NMR) analysis of a linear peptide model for the intercysteine sequence (38-57) from the beta-subunit of human (h) LH. This sequence contains functional determinants for receptor binding and postreceptor activation and is predicted by computer-based modeling to fold as a compact minidomain containing a central amphipathic helix. To test this prediction, an Arg-extended disulfide-free (38-57) analog of enhanced solubility was prepared for complementary circular-dichroic and two-dimensional NMR studies. The linear peptide retains ovarian membrane receptor-binding activity. Although the peptide is not highly structured in aqueous solution, circular-dichroic analysis shows partial alpha-helix formation in a lipophilic medium (50% trifluoroethanol). Complete sequential assignment is obtained in 50% trifluoroethanol based on homonuclear and [15N]edited heteronuclear NMR methods. alpha-Helix-related (i,i + 3) connectivities are observed by nuclear-Overhauser effect spectroscopy that define an amphipathic alpha-helical segment (residues 41-48). Additional long range nuclear-Overhauser effects are observed in the C-terminal region that are consistent with beta-turns involving one or more proline residues; these may serve to reverse the direction of the peptide chain. A nuclear-Overhauser effect contact is identified between residues 38 and 55 at opposite ends of the linear sequence, suggesting that a loop configuration is significantly populated in this solvent system. These results, taken together, characterize elements of ordered structure in the 38-57 peptide, which appear to be distinguishing features of hLH (and the homologous region of hCG). We propose that the structure of this peptide provides a model for the structure of the corresponding region of native hLH in the hormone-receptor complex.
Asunto(s)
Hormona Luteinizante/química , Secuencias Reguladoras de Ácidos Nucleicos , Secuencia de Aminoácidos , Sitios de Unión , Humanos , Hidrógeno , Hormona Luteinizante/genética , Espectroscopía de Resonancia Magnética , Datos de Secuencia Molecular , Isótopos de Nitrógeno , Fragmentos de Péptidos/síntesis química , Fragmentos de Péptidos/química , Conformación ProteicaRESUMEN
Structures of insulin in different crystal forms exhibit significant local and nonlocal differences, including correlated displacement of elements of secondary structure. Here we describe the solution structure and dynamics of a monomeric insulin analogue, des-pentapeptide-(B26-B30)-insulin (DPI), as determined by two-dimensional NMR spectroscopy and distance geometry/restrained molecular dynamics (DG/RMD). Although the solution structure of DPI exhibits a general similarity to its crystal structure, individual DG/RMD structures in the NMR ensemble differ by rigid-body displacements of alpha-helices that span the range of different crystal forms. These results suggest that DPI exists as a partially folded state formed by coalescence of distinct alpha-helix-associated microdomains. The physical reality of this model is investigated by comparison of the observed two-dimensional nuclear Overhauser enhancement (NOE) spectroscopy (NOESY) spectrum with that predicted from crystal and DG/RMD structures. The observed NOESY spectrum contains fewer tertiary contacts than predicted by any single simulation, but it matches their shared features; such "ensemble correspondence" is likely to reflect the effect of protein dynamics on observed NOE intensities. We propose (i) that the folded state of DPI is analogous to that of a compact protein-folding intermediate rather than a conventional native state and (ii) that the molten state is the biologically active species. This proposal (the molten-globule hypothesis) leads to testable thermodynamic predictions and has general implications for protein design.
Asunto(s)
Insulina/análogos & derivados , Calorimetría , Disulfuros , Insulina/química , Sustancias Macromoleculares , Espectroscopía de Resonancia Magnética/métodos , Modelos Moleculares , Conformación Proteica , Soluciones , Termodinámica , Difracción de Rayos X/métodosRESUMEN
Crystal structures of insulin have been determined in various distinct forms, the relevance of which to receptor recognition has long been the subject of speculation. Recently the crystal structure of an inactive insulin analogue has been determined and, surprisingly, found to have a conformation identical to native insulin. On this basis Dodson and colleagues have suggested that the known insulin crystal structures reflect an inactive conformation, and that a change in conformation is required for activity--specifically, the carboxy terminal residues of the B-chain are proposed to separate from the amino terminal residues of the A-chain. Here we report the solution structure of an active insulin mutant, determined by two-dimensional NMR, which supports this hypothesis. In the mutant, the carboxy terminal beta-turn and beta-strand of the B-chain are destabilized and do not pack across the rest of the molecule. We suggest that analogous detachment of the carboxy terminal region of the B-chain occurs in native insulin on binding to its receptor. Our finding that partial unfolding of the B-chain exposes an alternative protein surface rationalizes the receptor-binding properties of a series of anomalous insulin analogues, including a mutant insulin associated with diabetes mellitus in man.