RESUMO
Myosin-binding protein-C (MyBP-C or C-protein) is a ca. 130 kDa protein present in the thick filaments of all vertebrate striated muscle. The protein contains ten domains, each of ca. 90-100 amino acids; seven are members of the IgI family of proteins, three of the fibronectin type III family. The motifs are arranged in the following order (from N- to C-terminus): Ig-Ig-Ig-Ig-Ig-Fn-Fn-Ig-Fn-Ig. The C-terminal Ig motif (domain X or CX) contains its light meromyosin-binding site. A recombinant form of CX, beginning at Met-1027, exhibits saturable binding to myosin with an affinity comparable to the C-terminal 13 kDa chymotryptic fragment of native MyBP-C. To identify the surface in CX involved in its interaction with myosin, nine site-directed mutants (R37E, K43E, N49D, E52R, D56K, R73E, R74E, G80D and R103E) were constructed. Using a new assay for assessing the binding of CX with the light meromyosin (LMM) portion of myosin, we demonstrate that recombinant CX, just as the full-length protein, is able to facilitate LMM polymerization. Moreover, we show that residues Arg-37, Glu-52, Asp-56, Arg-73, and Arg-74 are involved in this interaction with the myosin rod. All of these amino acids interact with negatively charged residues of LMM, since the mutants R37E, R73E and R74E are unable to bind myosin, whereas E52R and D56K bind myosin with higher affinity than wild-type CX. Residues Lys-43 and Arg-103 show a small but significant influence on the binding reaction; residues Asn-49 and Gly-80 seem not to be involved in this interaction. Based on these data, a model is proposed for the interaction between MyBP-C CX and myosin filaments. In this model, CX interacts with four molecules of LMM at four different sites of the binding protein, thus explaining the effects of MyBP-C on the critical concentration of myosin polymerization.
Assuntos
Proteínas de Transporte/química , Miosinas/química , Sequência de Aminoácidos , Animais , Sítios de Ligação , Proteínas de Transporte/genética , Proteínas de Transporte/metabolismo , Dimerização , Humanos , Dados de Sequência Molecular , Mutagênese Sítio-Dirigida , Miosinas/genética , Miosinas/metabolismo , Ligação ProteicaRESUMO
The case of a 72-year-old woman presenting sensory neuropathy and anti-Hu antibodies is reported. She was admitted in November 1995 with a one year history of sensory neuropathy. Her first symptoms were painful numbness and dysesthesias in both feet. She experienced progression of the sensory symptoms affecting upper limbs, and clumsiness of gait. One month before admission she complained of diminished strength in both hands. The neurologic examination showed anicocoric fixed pupils, with no reaction to light; convergence miosis was evident in the right eye (Argyll-Robertson pupil). In the lower limbs she had very mild distal weakness, and tendon reflexes were universally abolished. Pin and touch sensation, position sense and pallesthesia were absent in all four limbs. Romberg test was elicited, and a tabetic gait was patent. Pseudoathetotic movements were observed in hands and feet. An ulcer was present in the fifth finger of the right foot. Routine blood biochemistry and hematology showed a ESR of 105 and an increased IgG in the immune-electrophoretic run. Neurophysiologic evaluation disclosed a mild demyelinating neuropathy. Positive anti-Hu antibodies were found in the serum (Western blot - Athena Diagnostics); CSF was normal but not tested for anit-Hu. An abdominal CT scan disclosed multiple hypodense nodules in liver, right adrenal gland and peritoneum. A chest CT scan showed a hyperdense mass in the lower right pulmonary lobe and enlarged retrocava-pretracheal lymph nodes. A biopsy of the peritoneal nodule was performed, showing a metastatic small cell carcinoma. The patient died eight days after discharge. Although multiple organs were affected, she was independent until death, showing an indolent clinical course.
Assuntos
Carcinoma de Células Pequenas/diagnóstico , Neoplasias Pulmonares/patologia , Síndromes Paraneoplásicas/diagnóstico , Síndromes Paraneoplásicas/imunologia , Transtornos de Sensação/diagnóstico , Transtornos de Sensação/imunologia , Idoso , Anticorpos Antinucleares , Carcinoma de Células Pequenas/patologia , Humanos , Masculino , Tomografia Computadorizada por Raios XRESUMO
Of the several proteins located within sarcomeric A-bands, C-protein, a myosin binding protein (MyBP) is thought to regulate and stabilize thick filaments during assembly. This paper reports the characterization of C-protein isoforms in juvenile and adult axolotls, Ambystoma mexicanum, by means of immunofluorescent microscopy and Western blot analyses. C-protein and myosin are found specifically within the A-bands, whereas tropomyosin and alpha-actin are detected in the I-bands of axolotl myofibrils. The MF1 antibody prepared against the fast skeletal muscle isoform of chicken C-protein specifically recognizes a cardiac isoform (Axcard1) in juvenile and adult axolotls but does not label axolotl skeletal muscle. The ALD66 antibody, which reacts with the C-protein slow isoform in chicken, local- izes only in skeletal muscle of the axolotl. This slow axolotl isoform (Axslow) displays a heterogeneous distribution in fibers of dorsalis trunci skeletal muscle. The C315 antibody against the chicken C-protein cardiac isoform identifies a second axolotl cardiac isoform (Axcard2), which is present also in axolotl skeletal muscle. No C-protein was detected in smooth muscle of the juvenile and adult axolotl with these antibodies.