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Cellular signalling cues lead to the initiation of apoptotic pathways and often result in the activation of caspases which in turn cause the generation of proteolytically generated protein fragments with new or altered functions. Mounting number of studies reveal that the activity of these proteolytically activated protein fragments can be counteracted via their selective degradation by the N-degron degradation pathways. Here, we investigate the proteolytically generated fragment of the PKC theta kinase, where we demonstrate the first report on the stability of this pro-apoptotic protein fragment. We have determined that the pro-apoptotic cleaved fragment of PKC-theta is unstable in cells because its N-terminal lysine targets it for proteasomal degradation via the N-degron degradation pathway and this degradation is inhibited by mutating the destabilizing N-termini, knockdown of the UBR1 and UBR2 E3 ligases. Tellingly, we demonstrate that the metabolic stabilization of the cleaved fragment of PKC-theta or inhibition of the N-degron degradation augments the apoptosis-inducing effect of staurosporine in Jurkat cells. Notably, we have unveiled that the cleaved fragment of PKC theta, per se, can induce apoptotic cell death in Jurkat T-cell leukemia. Our results expand the functional scope of mammalian N-degron degradation pathways, and support the notion that targeting N-degron degradation machinery may have promising therapeutic implications in cancer cells.

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The neuropeptide VGF was recently proposed as a neurodegeneration biomarker. The Parkinson's disease-related protein leucine-rich repeat kinase 2 (LRRK2) regulates endolysosomal dynamics, a process that involves SNARE-mediated membrane fusion and could regulate secretion. Here we investigate potential biochemical and functional links between LRRK2 and v-SNAREs. We find that LRRK2 directly interacts with the v-SNAREs VAMP4 and VAMP7. Secretomics reveals VGF secretory defects in VAMP4 and VAMP7 knockout (KO) neuronal cells. In contrast, VAMP2 KO "regulated secretion-null" and ATG5 KO "autophagy-null" cells release more VGF. VGF is partially associated with extracellular vesicles and LAMP1+ endolysosomes. LRRK2 expression increases VGF perinuclear localization and impairs its secretion. Retention using selective hooks (RUSH) assays show that a pool of VGF traffics through VAMP4+ and VAMP7+ compartments, and LRRK2 expression delays its transport to the cell periphery. Overexpression of LRRK2 or VAMP7-longin domain impairs VGF peripheral localization in primary cultured neurons. Altogether, our results suggest that LRRK2 might regulate VGF secretion via interaction with VAMP4 and VAMP7.

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Transmissible spongiform encephalopathies (TSEs) or prion diseases consist of a broad range of fatal neurological disorders affecting humans and animals. Contrary to Watson and Crick's 'central dogma', prion diseases are caused by a protein, devoid of DNA involvement. Herein, we briefly review various cellular and biological aspects of prions and prion pathogenesis focusing mainly on historical milestones, biosynthesis, degradation, structure-function of cellular and scrapie forms of prions .

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Interactions between glycan-binding proteins (GBPs) and glycosphingolipids (GSLs) are involved in numerous physiological and pathophysiological processes. Many model membrane systems are available for studying GBP-GSL interactions, but a systematic investigation has not been carried out on how the nature of the model membrane affects binding. In this work, we use electrospray ionization mass spectrometry (ESI-MS), both direct and competitive assays, to measure the binding of cholera toxin B subunit homopentamer (CTB5) to GM1 ganglioside in liposomes, bilayer islands [styrene maleic acid lipid particles (SMALPs), nanodiscs (NDs), and picodiscs (PDs)], and micelles. We find that direct ESI-MS analysis of CTB5 binding to GM1 is unreliable due to non-uniform response factors, incomplete extraction of bound GM1 in the gas phase, and nonspecific CTB5-GM1 interactions. Conversely, indirect proxy ligand ESI-MS measurements show that the intrinsic (per binding site) association constants of CTB5 for PDs, NDs, and SMALPs are similar and comparable to the affinity of soluble GM1 pentasaccharide (GM1os). The observed affinity decreases with increasing GM1 content due to molecular crowding stemming from GM1 clustering. Unlike the smaller model membranes, the observed affinity of CTB5 toward GM1 liposomes is ∼10-fold weaker than GM1os and relatively insensitive to the GM1 content. GM1 glycomicelles exhibit the lowest affinity, ∼35-fold weaker than GM1os. Together, the results highlight experimental design considerations for quantitative GBP-GSL binding studies involving multisubunit GBPs and factors to consider when comparing results obtained with different membrane systems. Notably, they suggest that bilayer islands with a low percentage of GSL, wherein clustering is minimized, are ideal for assessing intrinsic strength of GBP-GSL interactions in a membrane environment, while binding to liposomes, which is sub-optimal due to extensive clustering, may be more representative of authentic cellular environments.

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An inherent challenge that mitochondria face is the continuous exposure to diverse stresses which increase their likelihood of dysregulation. In response, human cells have evolved sophisticated quality control mechanisms to identify and eliminate abnormal dysfunctional mitochondria. One pivotal mitochondrial quality control pathway is PINK1/Parkin-dependent mitophagy which mediates the selective removal of the dysfunctional mitochondria from the cell by autophagy. PTEN-induced putative kinase 1 (PINK1) is a mitochondrial Ser/Thr kinase that was originally identified as a gene responsible for autosomal recessive early-onset Parkinson's disease (PD). Notably, upon failure of mitochondrial import, Parkin, another autosomal-recessive PD gene, is recruited to mitochondria and mediates the autophagic clearance of deregulated mitochondria. Importantly, recruitment of Parkin to damaged mitochondria hinges on the accumulation of PINK1 on the outer mitochondrial membrane (OMM). Normally, PINK1 is imported from the cytosol through the translocase of the outer membrane (TOM) complex, a large multimeric channel responsible for the import of most mitochondrial proteins. After import, PINK1 is rapidly degraded. Thus, at steady-state, PINK1 levels are kept low. However, upon mitochondrial import failure, PINK1 accumulates and forms a high-molecular weight > 700 kDa complex with TOM on the OMM. Thus, PINK1 functions as sensor, tagging dysfunctional mitochondria for Parkin-mediated mitophagy. Although much has been learned about the function of PINK1 in mitophagy, the biochemical and structural basis of negative regulation of PINK1 operation and functions is far from clear. Recent work unveiled new players as PTEN-l as negative regulator of PINK1 function. Herein, we review key aspects of mitophagy and PINK1/Parkin-mediated mitophagy with highlighting the role of negative regulation of PINK1 function and presenting some of the key future directions in PD cell biology.

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Unlike cytosolic proteins, membrane proteins (MPs) are embedded within the plasma membrane and the lipid bilayer of intracellular organelles. MPs serve in various cellular processes and account for over 65% of the current drug targets. The development of membrane mimetic systems such as bicelles, short synthetic polymers or amphipols, and membrane scaffold proteins (MSP)-based nanodiscs has facilitated the accommodation of synthetic lipids to stabilize MPs, yet the preparation of these membrane mimetics remains detergent-dependent. Bio-inspired synthetic polymers present an invaluable tool for excision and liberation of superstructures of MPs and their surrounding annular lipid bilayer in the nanometric discoidal assemblies. In this article, we discuss the significance of self-assembling process in design of biomimetic systems, review development of multiple series of amphipathic polymers and the significance of these polymeric "belts" in biomedical research in particular in unraveling the structures, dynamics and functions of several high-value membrane protein targets.

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Methylstilbene-alt-maleic acid copolymers spontaneously convert biological membranes into bilayer discs with ∼20 nm diameters. This readily functionalizable class of copolymers has the compositional homogeneity, hydrophobicity, dynamics, and charge that may help to achieve optimal structural resolution, membrane dissolution, stability, and broad utility.

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Prions are lipidated proteins that interact with endogenous lipids and metal ions. They also assemble into multimers and propagate into the infectious scrapie form known as PrPSc The high-resolution structure of the infectious PrPSc state remains unknown, and its analysis largely relies on detergent-based preparations devoid of endogenous ligands. Here we designed polymers that allow isolation of endogenous membrane:protein assemblies in native nanodiscs without exposure to conventional detergents that destabilize protein structures and induce fibrillization. A set of styrene-maleic acid (SMA) polymers including a methylamine derivative facilitated gentle release of the infectious complexes for resolution of multimers, and a thiol-containing version promoted crystallization. Polymer extraction from brain homogenates from Syrian hamsters infected with Hyper prions and WT mice infected with Rocky Mountain Laboratories prions yielded infectious prion nanoparticles including oligomers and microfilaments bound to lipid vesicles. Lipid analysis revealed the brain phospholipids that associate with prion protofilaments, as well as those that are specifically enriched in prion assemblies captured by the methylamine-modified copolymer. A comparison of the infectivity of PrPSc attached to SMA lipid particles in mice and hamsters indicated that these amphipathic polymers offer a valuable tool for high-yield production of intact, detergent-free prions that retain in vivo activity. This native prion isolation method provides an avenue for producing relevant prion:lipid targets and potentially other proteins that form multimeric assemblies and fibrils on membranes.

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The development of amphipathic polymers, including various formulations of styrene-maleic acid (SMA) copolymers, has allowed the purification of increasing sizes and complexities of biological membrane protein assemblies in native nanodiscs. However, the factors determining the sizes and shapes of the resulting bio-nano particles remain unclear. Here, we show how grafting on short alkyl amine sidechains onto the polar residues leads to a broad set of nanoparticle sizes with improved solution behavior. The solubilization of lipid vesicles occurs over a wide range of pH levels and calcium concentrations, providing utility across the physiologically relevant range of solution conditions. Furthermore, the active SMA derivatives can contain strictly alternating monomers, which have inherently lower sequence polydispersity. Pronounced differences in the shapes of native nanoparticles were formed from Escherichia coli bacterial outer membrane containing PagP protein using methyl, ethyl and propylamine derivatives of styrene-maleic anhydride. In particular, the methylamine-substituted polymer forms smaller, monodispersed nanodiscs, while the longer alkyl derivatives form worm-like nanostructures. Thus the introduction of hydrophobicity onto the polar sidechains of amphipathic polymers has profound effects on morphology of native nanodisc, with shorter methyl moieties offering more uniformity and utility for structural biology studies.

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Mitochondrial dysregulation is a pivotal hallmark of aging-related disorders. Although there is a considerable understanding of the molecular counteracting responses toward damaged mitochondria, the molecular underpinnings connecting the abnormal aggregation of mitochondrial precursor protein fragments and abrogation of mitochondrial import machinery are far from clear. Recently, proteasomal-dependent degradation was unveiled as a pivotal fine-tuner of TOM machinery-dependent mitochondrial import. Herein, the role of proteasomal-mediated degradation in regulating fidelity of TOM-dependent import is briefly discussed and analyzed. The insights obtained from the characterization of this process may be applied to targeting mitochondrial import dysfunction in some neurodegenerative disorders.

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A pivotal feature that underlies the development of neurodegeneration is the accumulation of protein aggregates. In response, eukaryotic cells have evolved sophisticated quality control mechanisms to identify, repair and/or eliminate the misfolded abnormal proteins. Chaperones identify any otherwise abnormal conformations in proteins and often help them to regain their correct conformation. However, if repair is not an option, the abnormal protein is selectively degraded to prevent its oligomerization into toxic multimeric complexes. Autophagiclysosomal system and the ubiquitin-proteasome system mediate the targeted degradation of the aberrant protein fragments. Despite the increasing understanding of the molecular counteracting responses toward the accumulation of dysfunctional misfolded proteins, the molecular links between the upstream physiological inputs and the clearance of abnormal misfolded proteins is relatively poorly understood. Recent work has demonstrated that certain physiological states such as vigorous exercise and fasting may enhance the ability of mammalian cells to clear misfolded, toxic and aberrant protein fragments. These findings unveil a novel mechanism that activates the cells' protein-disposal machinery, facilitating the adaptation process of cellular proteome to fluctuations in cellular demands and alterations of environmental cues. Herein, we briefly discuss the molecular interconnection between certain physiological cues and proteasomal degradation pathway in the context of these interesting findings and highlight some of the future prospects.

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The plant plasma membrane Na+/H+ antiporter SOS1 (Salt Overlay Sensitive 1) of Arabidopsis thaliana is the major transporter extruding Na+ out of cells in exchange for an intracellular H+. The sodium extrusion process maintains a low intracellular Na+ concentration and thereby facilitates salt tolerance. A. thaliana SOS1 consists of 1146 amino acids, with the first 450 in a N-terminal membrane transport domain and the balance forming a cytosolic regulatory domain. For studies on characterization of the protein, two different constructs of SOS1 comprising of the residues 28 to 460 and 28 to 990 were cloned and overexpressed in methylotropic yeast strain of Pichia pastoris with a C-terminal histidine tag using the expression vector pPICZA. Styrene malic acid copolymers (SMA) were used as a cost-effective alternative to detergent for solubilization and isolation of this membrane protein. Immobilized Ni2+-ion affinity chromatography was used to purify the expressed protein resulting in a yield of ~0.6-2 mg of SOS1 per liter of Pichia pastoris culture. The SMA purified protein containing amino acids 28 to 990 was directly reconstituted into liposomes for determination of Na+ transport activity and was functionally active. However, similar reconstitution with amino acids 28-460 did not yield a functional protein. Other results have shown that the truncated SOS1 protein at amino acid 481 is active, which infers the presence of an element between residues 461-481 which is necessary for SOS1 activity. This region contains several conserved segments that may be important in SOS1 structure and function.

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The N-end rule denotes the relationship between the identity of the amino-terminal residue of a protein and its in vivo half-life. Since its discovery in 1986, the N-end rule has generally been described by a defined set of rules for determining whether an amino-terminal residue is stabilizing or not. However, recent studies are revealing that this N-end rule (or N-degron concept) is less straightforward than previously appreciated. For instance, it is unveiled that N-terminal acetylation of N-terminal residues may create a degradation signal (Ac-degron) that promotes the degradation of target proteins. A recent high-throughput dissection of degrons in yeast proteins amino termini intriguingly suggested that the hydrophobicity of amino-terminal residues-but not the N-terminal acetylation status-may be the indispensable feature of amino-terminal degrons. Herein, these recent advances in N-terminal acetylation and the complexity of N-terminal degradation signals in the context of the N-degron pathway are analyzed.

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Protein degradation is a crucial regulatory process in maintaining cellular proteostasis. The selective degradation of intracellular proteins controls diverse cellular and biochemical processes in all kingdoms of life. Targeted protein degradation is implicated in controlling the levels of regulatory proteins as well as eliminating misfolded and any otherwise abnormal proteins. Deregulation of protein degradation is concomitant with the progression of various neurodegenerative disorders such as Parkinson's and Alzheimer's diseases. Thus, methods of measuring metabolic half-lives of proteins greatly influence our understanding of the diverse functions of proteins in mammalian cells including neuronal cells. Historically, protein degradation rates have been studied via exploiting methods that estimate overall protein degradation or focus on few individual proteins. Notably, with the recent technical advances and developments in proteomic and imaging techniques, it is now possible to measure degradation rates of a large repertoire of defined proteins and analyze the degradation profile in a detailed spatio-temporal manner, with the aim of determining proteome-wide protein stabilities upon different physiological conditions. Herein, we discuss some of the classical and novel methods for determining protein degradation rates highlighting the crucial role of some state of art approaches in deciphering the global impact of dynamic nature of targeted degradation of cellular proteins. This article is part of the Special Issue "Proteomics".

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Transmembrane proteins function within a continuous layer of biologically relevant lipid molecules that stabilizes their structures and modulates their activities. Structures and interactions of biological membrane-protein complexes or "memteins" can now be elucidated using native nanodiscs made by poly(styrene co-maleic anhydride) derivatives. These linear polymers contain a series of hydrophobic and polar subunits that gently fragment membranes into water-soluble discs with diameters of 5-50 nm known as styrene maleic acid lipid particles (SMALPs). High-resolution structures of memteins that include endogenous lipid ligands and posttranslational modifications can be resolved without resorting to synthetic detergents or artificial lipids. The resulting ex situ structures better recapitulate the in vivo situation and can be visualized by methods including cryo-electron microscopy (cryoEM), electron paramagnetic resonance (EPR), mass spectrometry (MS), nuclear magnetic resonance (NMR) spectroscopy, small angle x-ray scattering (SAXS), and x-ray diffraction (XRD). Recent progress including 3D structures of biological bilayers illustrates how polymers and native nanodiscs expose previously inaccessible membrane assemblies at atomic resolution and suggest ways in which the SMALP system could be exploited for drug discovery.

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Despite being among the first discovered mammalian innate immune sensor, NLRP1B (NLR pyrin domain-containing1B) activation and its molecular basis have remained elusive. Two recent studies have unveiled N-terminal degradation as a common mechanism for pathogen-mediated NLRP1B inflammasome activation in mammals.

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Unlike prokaryotes, N-terminal formylation has been confined to a handful of mitochondrial proteins in eukaryotes. A recent study unveils a new role for eukaryotic cytoplasmic N-terminal formylation linking diverse cellular stresses to N-terminal-dependent protein degradation. These findings suggest broad cellular implications in higher eukaryotes for N-terminal methionine formylation.

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The concept of a memtein as the minimal unit of membrane function is proposed here, and refers to the complex of a membrane protein together with a continuous layer of biological lipid molecules. The elucidation of the atomic resolution structures and specific interactions within memteins remains technically challenging. Nonetheless, we argue that these entities are critical endpoints for the postgenomic era, being essential units of cellular function that mediate signal transduction and trafficking. Their biological mechanisms and molecular compositions can be resolved using native nanodiscs formed by poly(styrene-co-maleic acid) (SMA) copolymers. These amphipathic polymers rapidly and spontaneously fragment membranes into water-soluble discs holding a section of bilayer. This allows structures of complexes found in vivo to be prepared without resorting to synthetic detergents or artificial lipids. The ex situ structures of memteins can be resolved by methods including cryo-electron microscopy (cEM), X-ray crystallography (XRC), NMR spectroscopy and mass spectrometry (MS). Progress in the field demonstrates that memteins are better representations of how biology actually works in membranes than naked proteins devoid of lipid, spurring on further advances in polymer chemistry to resolve their details.

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A pivotal hallmark of some cancer cells is the evasion of apoptotic cell death. Importantly, the initiation of apoptosis often results in the activation of caspases, which, in turn, culminates in the generation of proteolytically-activated protein fragments with potentially new or altered roles. Recent investigations have revealed that the activity of a significant number of the protease-generated, activated, pro-apoptotic protein fragments can be curbed via their selective degradation by the N-end rule degradation pathways. Of note, previous work revealed that several proteolytically-generated, pro-apoptotic fragments are unstable in cells, as their destabilizing N-termini target them for proteasomal degradation via the N-end rule degradation pathways. Remarkably, previous studies also showed that the proteolytically-generated anti-apoptotic Lyn kinase protein fragment is targeted for degradation by the UBR1/UBR2 E3 ubiquitin ligases of the N-end rule pathway in chronic myeloid leukemia cells. Crucially, the degradation of cleaved fragment of Lyn by the N-end rule counters imatinib resistance in these cells, implicating a possible linkage between the N-end rule degradation pathway and imatinib resistance. Herein, we highlight recent studies on the role of the N-end rule proteolytic pathways in regulating apoptosis in mammalian cells, and also discuss some possible future directions with respect to apoptotic proteolysis signaling.

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