Molecular features of the UNC-45 chaperone critical for binding and folding muscle myosin

Optogenetic induction of mechanical muscle stress identifies myosin regulatory ubiquitin ligase NHL-1 in C. elegans

Mechanical stress during muscle contraction is a constant threat to proteome integrity. However, there is a lack of experimental systems to identify critical proteostasis regulators under mechanical stress conditions. Here, we present the transgenic Caenorhabditis elegans model OptIMMuS (Optogenetic Induction of Mechanical Muscle Stress) to study changes in the proteostasis network associated with mechanical forces. Repeated blue light exposure of a muscle-expressed Chlamydomonas rheinhardii channelrhodopsin-2 variant results in sustained muscle contraction and mechanical stress. Using OptIMMuS, combined with proximity labeling and mass spectrometry, we identify regulators that cooperate with the myosin-directed chaperone UNC-45 in muscle proteostasis. One of these is the TRIM E3 ligase NHL-1, which interacts with UNC-45 and muscle myosin in genetic epistasis and co-immunoprecipitation experiments. We provide evidence that the ubiquitylation activity of NHL-1 regulates myosin levels and functionality under mechanical stress. In the future, OptIMMuS will help to identify muscle-specific proteostasis regulators of therapeutic relevance.

UNC-45 assisted myosin folding depends on a conserved FX3HY motif implicated in Freeman Sheldon Syndrome

Myosin motors are critical for diverse motility functions, ranging from cytokinesis and endocytosis to muscle contraction. The UNC-45 chaperone controls myosin function mediating the folding, assembly, and degradation of the muscle protein. Here, we analyze the molecular mechanism of UNC-45 as a hub in myosin quality control. We show that UNC-45 forms discrete complexes with folded and unfolded myosin, forwarding them to downstream chaperones and E3 ligases. Structural analysis of a minimal chaperone:substrate complex reveals that UNC-45 binds to a conserved FX3HY motif in the myosin motor domain. Disrupting the observed interface by mutagenesis prevents myosin maturation leading to protein aggregation in vivo. We also show that a mutation in the FX3HY motif linked to the Freeman Sheldon Syndrome impairs UNC-45 assisted folding, reducing the level of functional myosin. These findings demonstrate that a faulty myosin quality control is a critical yet unexplored cause of human myopathies.

UNC-45A: A potential therapeutic target for malignant tumors

Uncoordinated mutant number-45 myosin chaperone A (UNC-45A), a protein highly conserved throughout evolution, is ubiquitously expressed in somatic cells. It is correlated with tumorigenesis, proliferation, metastasis, and invasion of multiple malignant tumors. The current understanding of the role of UNC-45A in tumor progression is mainly related to the regulation of non-muscle myosin II (NM-II). However, many studies have suggested that the mechanisms by which UNC-45A is involved in tumor progression are far greater than those of NM-II regulation. UNC-45A can also promote tumor cell proliferation by regulating checkpoint kinase 1 (ChK1) phosphorylation or the transcriptional activity of nuclear receptors, and induces chemoresistance to paclitaxel in tumor cells by destabilizing microtubule activity. In this review, we discuss the recent advances illuminating the role of UNC-45A in tumor progression. We also put forward therapeutic strategies targeting UNC-45A, in the hope of paving the way the development of UNC-45A-targeted therapies for patients with malignant tumors.

Regulation of Epithelial and Endothelial Barriers by Molecular Chaperones

The integrity and permeability of epithelial and endothelial barriers depend on the formation of tight junctions, adherens junctions, and a junction-associated cytoskeleton. The establishment of this junction-cytoskeletal module relies on the correct folding and oligomerization of its protein components. Molecular chaperones are known regulators of protein folding and complex formation in different cellular compartments. Mammalian cells possess an elaborate chaperone network consisting of several hundred chaperones and co-chaperones. Only a small part of this network has been linked, however, to the regulation of intercellular adhesions, and the systematic analysis of chaperone functions at epithelial and endothelial barriers is lacking. This review describes the functions and mechanisms of the chaperone-assisted regulation of intercellular junctions. The major focus of this review is on heat shock protein chaperones, their co-chaperones, and chaperonins since these molecules are the focus of the majority of the articles published on the chaperone-mediated control of tissue barriers. This review discusses the roles of chaperones in the regulation of the steady-state integrity of epithelial and vascular barriers as well as the disruption of these barriers by pathogenic factors and extracellular stressors. Since cytoskeletal coupling is essential for junctional integrity and remodeling, chaperone-assisted assembly of the actomyosin cytoskeleton is also discussed.

What can molecular assembly learn from catalysed assembly in living organisms?

Molecular assembly is the process of organizing individual molecules into larger structures and complex systems. The self-assembly approach is predominantly utilized in creating artificial molecular assemblies, and was believed to be the primary mode of molecular assembly in living organisms as well. However, it has been shown that the assembly of many biological complexes is "catalysed" by other molecules, rather than relying solely on self-assembly. In this review, we summarize these catalysed-assembly (catassembly) phenomena in living organisms and systematically analyse their mechanisms. We then expand on these phenomena and discuss related concepts, including catalysed-disassembly and catalysed-reassembly. Catassembly proves to be an efficient and highly selective strategy for synergistically controlling and manipulating various noncovalent interactions, especially in hierarchical molecular assemblies. Overreliance on self-assembly may, to some extent, hinder the advancement of artificial molecular assembly with powerful features. Furthermore, inspired by the biological catassembly phenomena, we propose guidelines for designing artificial catassembly systems and developing characterization and theoretical methods, and review pioneering works along this new direction. Overall, this approach may broaden and deepen our understanding of molecular assembly, enabling the construction and control of intelligent assembly systems with advanced functionality.

SPR spectroscopic analysis of myosin binding to wild type and mutant UNC45B

UNC45B is a multidomain molecular chaperone that is essential for the proper folding and function of myosin. It has previously been demonstrated that the UCS domain is responsible for the chaperoning function of UNC45B and that removing its client-binding loop leads to a significant change in its solution conformation and a reduced chaperoning function. Here, we report the direct quantification of affinities of myosin binding to wild type and mutant UNC45B using surface plasmon resonance (SPR) spectroscopy. We found that deletion of the client-binding loop in UNC45B resulted in a dramatic decrease in myosin affinity.

Uncovering the Relationship Between Genes and Phenotypes Beyond the Gut in Microvillus Inclusion Disease

Microvillus inclusion disease (MVID) is a rare condition that is present from birth and affects the digestive system. People with MVID experience severe diarrhea that is difficult to control, cannot absorb dietary nutrients, and struggle to grow and thrive. In addition, diverse clinical manifestations, some of which are life-threatening, have been reported in cases of MVID. MVID can be caused by variants in the MYO5B, STX3, STXBP2, or UNC45A gene. These genes produce proteins that have been functionally linked to each other in intestinal epithelial cells. MVID associated with STXBP2 variants presents in a subset of patients diagnosed with familial hemophagocytic lymphohistiocytosis type 5. MVID associated with UNC45A variants presents in most patients diagnosed with osteo-oto-hepato-enteric syndrome. Furthermore, variants in MYO5B or STX3 can also cause other diseases that are characterized by phenotypes that can co-occur in subsets of patients diagnosed with MVID. Recent studies involving clinical data and experiments with cells and animals revealed connections between specific phenotypes occurring outside of the digestive system and the type of gene variants that cause MVID. Here, we have reviewed these patterns and correlations, which are expected to be valuable for healthcare professionals in managing the disease and providing personalized care for patients and their families.

UNC-82/NUAK kinase is required by myosin A, but not myosin B, to assemble and function in the thick filament arms of C. elegans striated muscle

The mechanisms that ensure proper assembly, activity, and turnover of myosin II filaments are fundamental to a diverse range of cellular processes. In Caenorhabditis elegans striated muscle, thick filaments contain two myosins that are functionally distinct and spatially segregated. Using transgenic double mutants, we demonstrate that the ability of increased myosin A expression to restore muscle structure and movement in myosin B mutants requires UNC-82/NUAK kinase activity. Myosin B function appears unaffected in the kinase-impaired unc-82(e1220) mutant: the recessive antimorphic effects on early assembly of paramyosin and myosin A in this mutant are counteracted by increased myosin B expression and exacerbated by loss of myosin B. Using chimeric myosins and motility assays, we mapped the region of myosin A that requires UNC-82 activity to a 531-amino-acid region of the coiled-coil rod. This region includes the 264-amino-acid Region 1, which is sufficient in chimeric myosins to rescue the essential filament-initiation function of myosin A, as well as two sites that interact with myosin head domains in the Interacting Heads Motif. A specific physical interaction between myosin A and UNC-82::GFP is supported by GFP labeling of ectopic myosin A filaments but not thin filaments. We hypothesize that UNC-82 regulates assembly competence of myosin A during parallel assembly in the filament arms.

Metabolomics and Proteomics Responses of Largemouth Bass (Micropterus salmoides) Muscle under Organic Selenium Temporary Rearing

Organic selenium has been widely studied as a nutritional supplement for animal feed. However, there are few studies on the effect of organic selenium on flesh quality. In this study, the effects of organic selenium (yeast selenium (YS), Se 0.002 mg/L) on the metabolism and protein expression in Micropterus salmoides muscle under temporary fasting condition (6 weeks) were investigated. The muscle structure was observed through a microscope, and regulatory pathways were analyzed using proteomics and metabolomics methods. Electron microscopy showed that YS made the muscle fibers of M. salmoides more closely aligned. Differential analysis identified 523 lipid molecules and 268 proteins. The numbers of upregulated and downregulated proteins were 178 and 90, respectively, including metabolism (46.15%), cytoskeleton (11.24%) and immune oxidative stress (9.47%), etc. Integrated analyses revealed that YS enhanced muscle glycolysis, the tricarboxylic acid cycle and oxidative phosphorylation metabolism. In the YS group, the content of eicosapentaenoic acid was increased, and that of docosahexaenoic acid was decreased. YS slowed down protein degradation by downregulating ubiquitin and ubiquitin ligase expression. These results suggest that organic selenium can improve M. salmoides muscle quality through the aforementioned pathways, which provides potential insights into the improvement of the quality of aquatic products, especially fish.

Virus-free transfection, transient expression, and purification of human cardiac myosin in mammalian muscle cells for biochemical and biophysical assays

Myosin expression and purification is important for mechanistic insights into normal function and mutation induced changes. The latter is particularly important for striated muscle myosin II where mutations cause several debilitating diseases. However, the heavy chain of this myosin is challenging to express and the standard protocol, using C2C12 cells, relies on viral infection. This is time and work intensive and associated with infrastructural demands and biological hazards, limiting widespread use and hampering fast generation of a wide range of mutations. We here develop a virus-free method to overcome these challenges. We use this system to transfect C2C12 cells with the motor domain of the human cardiac myosin heavy chain. After optimizing cell transfection, cultivation and harvesting conditions, we functionally characterized the expressed protein, co-purified with murine essential and regulatory light chains. The gliding velocity (1.5-1.7 µm/s; 25 °C) in the in vitro motility assay as well as maximum actin activated catalytic activity (kcat; 8-9 s-1) and actin concentration for half maximal activity (KATPase; 70-80 µM) were similar to those found previously using virus based infection. The results should allow new types of studies, e.g., screening of a wide range of mutations to be selected for further characterization.

A ubiquitin fusion reporter to monitor muscle proteostasis in C. elegans

Muscle is a highly dynamic tissue in which a variety of folding and degradation processes are active to maintain protein homeostasis (proteostasis) and functionality. The muscle-specific chaperone UNC-45 folds the motor protein myosin and assembles it into myofilaments. Malfunction of this chaperone leads to misfolding of myosin, disorganization of myofilaments, and degradation of misfolded myosin molecules by the proteasome. Here, we present a new muscle-specific ubiquitin fusion degradation (UFD) model substrate in C. elegans that helps clarify how UNC-45 dysfunction affects muscle proteostasis.

Beyond Chaperoning: UCS Proteins Emerge as Regulators of Myosin-Mediated Cellular Processes

The UCS (UNC-45/CRO1/She4p) family of proteins has emerged as chaperones specific for the folding, assembly, and function of myosin. UCS proteins participate in various myosin-dependent cellular processes including myofibril organization and muscle functions, cell differentiation, striated muscle development, cytokinesis, and endocytosis. Mutations in the genes that code for UCS proteins cause serious defects in myosin-dependent cellular processes. UCS proteins that contain an N-terminal tetratricopeptide repeat (TPR) domain are called UNC-45. Vertebrates usually possess two variants of UNC-45, the ubiquitous general-cell UNC-45 (UNC-45A) and the striated muscle UNC-45 (UNC-45B), which is exclusively expressed in skeletal and cardiac muscles. Except for the TPR domain in UNC-45, UCS proteins comprise of several irregular armadillo (ARM) repeats that are organized into a central domain, a neck region, and the canonical C-terminal UCS domain that functions as the chaperoning module. With or without TPR, UCS proteins form linear oligomers that serve as scaffolds that mediate myosin folding, organization into myofibrils, repair, and motility. This chapter reviews emerging functions of these proteins with a focus on UNC-45 as a dedicated chaperone for folding, assembly, and function of myosin at protein and potentially gene levels. Recent experimental evidences strongly support UNC-45 as an absolute regulator of myosin, with each domain of the chaperone playing different but complementary roles during the folding, assembly, and function of myosin, as well as recruiting Hsp90 as a co-chaperone to optimize key steps. It is becoming increasingly clear that UNC-45 also regulates the transcription of several genes involved in myosin-dependent cellular processes.

Rapid transcriptomic and physiological changes in the freshwater pennate diatom Mayamaea pseudoterrestris in response to copper exposure

Diatoms function as major primary producers, accumulating large amounts of biomass in most aquatic environments. Given their rapid responses to changes in environmental conditions, diatoms are used for the biological monitoring of water quality and for performing ecotoxicological tests in aquatic ecosystems. However, the molecular basis for their toxicity to chemical compounds remains largely unknown. Here, we sequenced the genome of a freshwater diatom, Mayamaea pseudoterrestris NIES-4280, which has been proposed as an alternative strain of Navicula pelliculosa UTEX 664 for performing the Organisation for Economic Co-operation and Development ecotoxicological test. This study shows that M. pseudoterrestris has a small genome and carries the lowest number of genes among freshwater diatoms. The gene content of M. pseudoterrestris is similar to that of the model marine diatom, Phaeodactylum tricornutum. Genes related to cell motility, polysaccharide metabolism, oxidative stress alleviation, intracellular calcium signalling, and reactive compound detoxification showed rapid changes in their expression patterns in response to copper exposure. Active gliding motility was observed in response to copper addition, and copper exposure decreased intracellular calcium concentration. These findings enhance our understanding of the environmental adaptation of diatoms, and elucidate the molecular basis of toxicity of chemical compounds in algae.

Sarcomere maturation: function acquisition, molecular mechanism, and interplay with other organelles

During postnatal cardiac development, cardiomyocytes mature and turn into adult ones. Hence, all cellular properties, including morphology, structure, physiology and metabolism, are changed. One of the most important aspects is the contractile apparatus, of which the minimum unit is known as a sarcomere. Sarcomere maturation is evident by enhanced sarcomere alignment, ultrastructural organization and myofibrillar isoform switching. Any maturation process failure may result in cardiomyopathy. Sarcomere function is intricately related to other organelles, and the growing evidence suggests reciprocal regulation of sarcomere and mitochondria on their maturation. Herein, we summarize the molecular mechanism that regulates sarcomere maturation and the interplay between sarcomere and other organelles in cardiomyocyte maturation. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

UCS Chaperone Folding of the Myosin Head: A Function That Evolved before Animals and Fungi Diverged from a Common Ancestor More than a Billion Years Ago

The folding of the myosin head often requires a UCS (Unc45, Cro1, She4) domain-containing chaperone. Worms, flies, and fungi have just a single UCS protein. Vertebrates have two; one (Unc45A) which functions primarily in non-muscle cells and another (Unc45B) that is essential for establishing and maintaining the contractile apparatus of cardiac and skeletal muscles. The domain structure of these proteins suggests that the UCS function evolved before animals and fungi diverged from a common ancestor more than a billion years ago. UCS proteins of metazoans and apicomplexan parasites possess a tetratricopeptide repeat (TPR), a domain for direct binding of the Hsp70/Hsp90 chaperones. This, however, is absent in the UCS proteins of fungi and largely nonessential for the UCS protein function in Caenorhabditis elegans and zebrafish. The latter part of this review focusses on the TPR-deficient UCS proteins of fungi. While these are reasonably well studied in yeasts, there is little precise information as to how they might engage in interactions with the Hsp70/Hsp90 chaperones or might assist in myosin operations during the hyphal growth of filamentous fungi.

A myosin chaperone, UNC-45A, is a novel regulator of intestinal epithelial barrier integrity and repair

The actomyosin cytoskeleton serves as a key regulator of the integrity and remodeling of epithelial barriers by controlling assembly and functions of intercellular junctions and cell-matrix adhesions. Although biochemical mechanisms that regulate the activity of non-muscle myosin II (NM-II) in epithelial cells have been extensively investigated, little is known about assembly of the contractile myosin structures at the epithelial adhesion sites. UNC-45A is a cytoskeletal chaperone that is essential for proper folding of NM-II heavy chains and myofilament assembly. We found abundant expression of UNC-45A in human intestinal epithelial cell (IEC) lines and in the epithelial layer of the normal human colon. Interestingly, protein level of UNC-45A was decreased in colonic epithelium of patients with ulcerative colitis. CRISPR/Cas9-mediated knock-out of UNC-45A in HT-29cf8 and SK-CO15 IEC disrupted epithelial barrier integrity, impaired assembly of epithelial adherence and tight junctions and attenuated cell migration. Consistently, decreased UNC-45 expression increased permeability of the Drosophila gut in vivo. The mechanisms underlying barrier disruptive and anti-migratory effects of UNC-45A depletion involved disorganization of the actomyosin bundles at epithelial junctions and the migrating cell edge. Loss of UNC-45A also decreased contractile forces at apical junctions and matrix adhesions. Expression of deletion mutants revealed roles for the myosin binding domain of UNC-45A in controlling IEC junctions and motility. Our findings uncover a novel mechanism that regulates integrity and restitution of the intestinal epithelial barrier, which may be impaired during mucosal inflammation.

Mutations in conserved residues of the myosin chaperone UNC-45 result in both reduced stability and chaperoning activity

Proper muscle development and function depend on myosin being properly folded and integrated into the thick filament structure. For this to occur the myosin chaperone UNC-45, or UNC-45B, must be present and able to chaperone myosin. Here we use a combination of in vivo C. elegans experiments and in vitro biophysical experiments to analyze the effects of six missense mutations in conserved regions of UNC-45/UNC-45B. We found that the phenotype of paralysis and disorganized thick filaments in 5/6 of the mutant nematode strains can likely be attributed to both reduced steady state UNC-45 protein levels and reduced chaperone activity. Interestingly, the biophysical assays performed on purified proteins show that all of the mutations result in reduced myosin chaperone activity but not overall protein stability. This suggests that these mutations only cause protein instability in the in vivo setting and that these conserved regions may be involved in UNC-45 protein stability/regulation via posttranslational modifications, protein-protein interactions, or some other unknown mechanism.

Artificial Intelligence Deconstructs Drug Targeting In Vivo by Leveraging a Transformer Platform

Lead optimization in structure-based drug design ultimately requires that the therapeutic agent be evaluated in the cellular context. However, the in vivo control of the target structure remains unyielding to computational modeling. This situation may change as transformer technologies enable a deconstruction of in vivo cooperativity steering drug-induced protein folding.

Artificial Intelligence Set to Reverse Engineer Drug Targeting in the Cell

Therapeutic drugs are required to target proteins in the cell, not in vitro. Yet, drug-induced protein folding in vivo is off limits to computational modeling efforts. This situation may change as artificial intelligence empowers molecular dynamics and enables the deconstruction of in vivo cooperativity for structural adaptation.

The ATPase mechanism of myosin 15, the molecular motor mutated in DFNB3 human deafness

Cochlear hair cells each possess an exquisite bundle of actin-based stereocilia that detect sound. Unconventional myosin 15 (MYO15) traffics and delivers critical molecules required for stereocilia development and thus is essential for building the mechanosensory hair bundle. Mutations in the human MYO15A gene interfere with stereocilia trafficking and cause hereditary hearing loss, DFNB3, but the impact of these mutations is not known, as MYO15 itself is poorly characterized. To learn more, we performed a kinetic study of the ATPase motor domain to characterize its mechanochemical cycle. Using the baculovirus-Sf9 system, we purified a recombinant minimal motor domain (S1) by coexpressing the mouse MYO15 ATPase, essential and regulatory light chains that bind its IQ domains, and UNC45 and HSP90A chaperones required for correct folding of the ATPase. MYO15 purified with either UNC45A or UNC45B coexpression had similar ATPase activities (kcat = ∼ 6 s-1 at 20 °C). Using stopped-flow and quenched-flow transient kinetic analyses, we measured the major rate constants describing the ATPase cycle, including ATP, ADP, and actin binding; hydrolysis; and phosphate release. Actin-attached ADP release was the slowest measured transition (∼12 s-1 at 20 °C), although this did not rate-limit the ATPase cycle. The kinetic analysis shows the MYO15 motor domain has a moderate duty ratio (∼0.5) and weak thermodynamic coupling between ADP and actin binding. These findings are consistent with MYO15 being kinetically adapted for processive motility when oligomerized. Our kinetic characterization enables future studies into how deafness-causing mutations affect MYO15 and disrupt stereocilia trafficking necessary for hearing.

Alpha and beta myosin isoforms and human atrial and ventricular contraction

Human atrial and ventricular contractions have distinct mechanical characteristics including speed of contraction, volume of blood delivered and the range of pressure generated. Notably, the ventricle expresses predominantly β-cardiac myosin while the atrium expresses mostly the α-isoform. In recent years exploration of the properties of pure α- & β-myosin isoforms have been possible in solution, in isolated myocytes and myofibrils. This allows us to consider the extent to which the atrial vs ventricular mechanical characteristics are defined by the myosin isoform expressed, and how the isoform properties are matched to their physiological roles. To do this we Outline the essential feature of atrial and ventricular contraction; Explore the molecular structural and functional characteristics of the two myosin isoforms; Describe the contractile behaviour of myocytes and myofibrils expressing a single myosin isoform; Finally we outline the outstanding problems in defining the differences between the atria and ventricles. This allowed us consider what features of contraction can and cannot be ascribed to the myosin isoforms present in the atria and ventricles.

Pathogenic Variants in the Myosin Chaperone UNC-45B Cause Progressive Myopathy with Eccentric Cores

The myosin-directed chaperone UNC-45B is essential for sarcomeric organization and muscle function from Caenorhabditis elegans to humans. The pathological impact of UNC-45B in muscle disease remained elusive. We report ten individuals with bi-allelic variants in UNC45B who exhibit childhood-onset progressive muscle weakness. We identified a common UNC45B variant that acts as a complex hypomorph splice variant. Purified UNC-45B mutants showed changes in folding and solubility. In situ localization studies further demonstrated reduced expression of mutant UNC-45B in muscle combined with abnormal localization away from the A-band towards the Z-disk of the sarcomere. The physiological relevance of these observations was investigated in C. elegans by transgenic expression of conserved UNC-45 missense variants, which showed impaired myosin binding for one and defective muscle function for three. Together, our results demonstrate that UNC-45B impairment manifests as a chaperonopathy with progressive muscle pathology, which discovers the previously unknown conserved role of UNC-45B in myofibrillar organization.

Mutational Analysis of the Structure and Function of the Chaperoning Domain of UNC-45B

UNC-45B is a multidomain molecular chaperone that is essential for the proper folding and assembly of myosin into muscle thick filaments in vivo. It has previously been demonstrated that the UCS domain is responsible for the chaperone-like properties of the UNC-45B. To better understand the chaperoning function of the UCS domain of the UNC-45B chaperone, we engineered mutations designed to 1) disrupt chaperone-client interactions by removing and altering the structure of a putative client-interacting loop and 2) disrupt chaperone-client interactions by changing highly conserved residues in a putative client-binding groove. We tested the effect of these mutations by using a, to our knowledge, novel combination of complementary biophysical assays (circular dichroism, chaperone activity, and small-angle x-ray scattering) and in vivo tools (Caenorhabditis elegans sarcomere structure). Removing the putative client-binding loop altered the secondary structure of the UCS domain (by decreasing the α-helix content), leading to a significant change in its solution conformation and a reduced chaperoning function. Additionally, we found that mutating several conserved residues in the putative client-binding groove did not alter the UCS domain secondary structure or structural stability but reduced its chaperoning activity. In vivo, these groove mutations were found to significantly alter the structure and organization of C. elegans sarcomeres. Furthermore, we tested the effect of R805W, a mutation distant from the putative client-binding region, which in humans, has been known to cause congenital and infantile cataracts. Our in vivo data show that, to our surprise, the R805W mutation appeared to have the most drastic detrimental effect on the structure and organization of the worm sarcomeres, indicating a crucial role of R805 in UCS-client interactions. Hence, our experimental approach combining biophysical and biological tools facilitates the study of myosin-chaperone interactions in mechanistic detail.

X-ray Crystallographic and Molecular Dynamic Analyses of Drosophila melanogaster Embryonic Muscle Myosin Define Domains Responsible for Isoform-Specific Properties

Drosophila melanogaster is a powerful system for characterizing alternative myosin isoforms and modeling muscle diseases, but high-resolution structures of fruit fly contractile proteins have not been determined. Here we report the first x-ray crystal structure of an insect myosin: the D melanogaster skeletal muscle myosin II embryonic isoform (EMB). Using our system for recombinant expression of myosin heavy chain (MHC) proteins in whole transgenic flies, we prepared and crystallized stable proteolytic S1-like fragments containing the entire EMB motor domain bound to an essential light chain. We solved the x-ray crystal structure by molecular replacement and refined the resulting model against diffraction data to 2.2 Å resolution. The protein is captured in two slightly different renditions of the rigor-like conformation with a citrate of crystallization at the nucleotide binding site and exhibits structural features common to myosins of diverse classes from all kingdoms of life. All atom molecular dynamics simulations on EMB in its nucleotide-free state and a derivative homology model containing 61 amino acid substitutions unique to the indirect flight muscle isoform (IFI) suggest that differences in the identity of residues within the relay and the converter that are encoded for by MHC alternative exons 9 and 11, respectively, directly contribute to increased mobility of these regions in IFI relative to EMB. This suggests the possibility that alternative folding or conformational stability within these regions contribute to the observed functional differences in Drosophila EMB and IFI myosins.