Myosin V from head to tail

Myosin XI-mediated BIK1 recruitment to nanodomains facilitates FLS2-BIK1 complex formation during innate immunity in Arabidopsis

Plants rely on immune receptor complexes at the cell surface to perceive microbial molecules and transduce these signals into the cell to regulate immunity. Various immune receptors and associated proteins are often dynamically distributed in specific nanodomains on the plasma membrane (PM). However, the exact molecular mechanism and functional relevance of this nanodomain targeting in plant immunity regulation remain largely unknown. By utilizing high spatiotemporal resolution imaging and single-particle tracking analysis, we show that myosin XIK interacts with remorin to recruit and stabilize PM-associated kinase BOTRYTIS-INDUCED KINASE 1 (BIK1) within immune receptor FLAGELLIN SENSING 2 (FLS2)-containing nanodomains. This recruitment facilitates FLS2/BIK1 complex formation, leading to the full activation of BIK1-dependent defense responses upon ligand perception. Collectively, our findings provide compelling evidence that myosin XI functions as a molecular scaffold to enable a spatially confined complex assembly within nanodomains. This ensures the presence of a sufficient quantity of preformed immune receptor complex for efficient signaling transduction from the cell surface.

Spire2 and Rab11a synergistically activate myosin-5b motor function

Cellular vesicle long-distance transport along the cytoplasmic actin network has recently been uncovered in several cell systems. In metaphase mouse oocytes, the motor protein myosin-5b (Myo5b) and the actin nucleation factor Spire are recruited to the Rab11a-positive vesicle membrane, forming a ternary complex of Myo5b/Spire/Rab11a that drives the vesicle long-distance transport to the oocyte cortex. However, the mechanism underlying the intermolecular regulation of the Myo5b/Spire/Rab11a complex remains unknown. In this study, we expressed and purified Myo5b, Spire2, and Rab11a proteins, and performed ATPase activity measurements, pulldown and single-molecule motility assays. Our results demonstrate that both Spire2 and Rab11a are required to activate Myo5b motor activity under physiological ionic conditions. The GTBM fragment of Spire2 stimulates the ATPase activity of Myo5b, while Rab11a enhances this activation. This activation occurs by disrupting the head-tail interaction of Myo5b. Furthermore, at the single-molecule level, we observed that the GTBM fragment of Spire2 and Rab11a coordinate to stimulate the Myo5b motility activity. Based on our results, we propose that upon association with the vesicle membrane, Myo5b, Spire2 and Rab11a form a ternary complex, and the inhibited Myo5b is synergistically activated by Spire2 and Rab11a, thereby triggering the long-distance transport of vesicles.

Omecamtiv Mecarbil in the treatment of heart failure: the past, the present, and the future

Heart failure, a prevailing global health issue, imposes a substantial burden on both healthcare systems and patients worldwide. With an escalating prevalence of heart failure, prolonged survival rates, and an aging demographic, an increasing number of individuals are progressing to more advanced phases of this incapacitating ailment. Against this backdrop, the quest for pharmacological agents capable of addressing the diverse subtypes of heart failure becomes a paramount pursuit. From this viewpoint, the present article focuses on Omecamtiv Mecarbil (OM), an emerging chemical compound said to exert inotropic effects without altering calcium homeostasis. For the first time, as a review, the present article uniquely started from the very basic pathophysiology of heart failure, its classification, and the strategies underpinning drug design, to on-going debates of OM's underlying mechanism of action and the latest large-scale clinical trials. Furthermore, we not only saw the advantages of OM, but also exhaustively summarized the concerns in sense of its effects. These of no doubt make the present article the most systemic and informative one among the existing literature. Overall, by offering new mechanistic insights and therapeutic possibilities, OM has carved a significant niche in the treatment of heart failure, making it a compelling subject of study.

Differential Myosin 5a splice variants in innervation of pelvic organs

Introduction: Myosin proteins interact with filamentous actin and translate the chemical energy generated by ATP hydrolysis into a wide variety of mechanical functions in all cell types. The classic function of conventional myosins is mediation of muscle contraction, but myosins also participate in processes as diverse as exocytosis/endocytosis, membrane remodeling, and cytokinesis. Myosin 5a (Myo5a) is an unconventional motor protein well-suited to the processive transport of diverse molecular cargo within cells and interactions with multiprotein membrane complexes that facilitate exocytosis. Myo5a includes a region consisting of six small alternative exons which can undergo differential splicing. Neurons and skin melanocytes express characteristic splice variants of Myo5a, which are specialized for transport processes unique to those cell types. But less is known about the expression of Myo5a splice variants in other tissues, their cargos and interactive partners, and their regulation. Methods: In visceral organs, neurotransmission-induced contraction or relaxation of smooth muscle is mediated by Myo5a. Axons within urogenital organs and distal colon of rodents arise from cell bodies located in the major pelvic ganglion (MPG). However, in contrast to urogenital organs, the distal colon also contains soma of the enteric nervous system. Therefore, the rodent pelvic organs provide an opportunity to compare the expression of Myo5a splice variants, not only in different tissues innervated by the pelvic nerves, but also in different subcellular compartments of those nerves. This study examines the expression and distribution of Myo5a splice variants in the MPG, compared to the bladder, corpus cavernosum of the penis (CCP) and distal colon using immunohistochemistry and mRNA analyses. Results/discussion: We report detection of characteristic Myo5a variants in these tissues, with bladder and CCP displaying a similar variant pattern but one which differed from that of distal colon. In all three organs, Myo5a variants were distinct compared to the MPG, implying segregation of one variant within nerve soma and its exclusion from axons. The expression of distinct Myo5a variant arrays is likely to be adaptive, and to underlie specific functions fulfilled by Myo5a in those particular locations.

Phosphorus Chemistry at the Roots of Bioenergetics: Ligand Permutation as the Molecular Basis of the Mechanism of ATP Synthesis/Hydrolysis by FOF1-ATP Synthase

The integration of phosphorus chemistry with the mechanism of ATP synthesis/hydrolysis requires dynamical information during ATP turnover and catalysis. Oxygen exchange reactions occurring at β-catalytic sites of the FOF1-ATP synthase/F1-ATPase imprint a unique record of molecular events during the catalytic cycle of ATP synthesis/hydrolysis. They have been shown to provide valuable time-resolved information on enzyme catalysis during ATP synthesis and ATP hydrolysis. The present work conducts new experiments on oxygen exchange catalyzed by submitochondrial particles designed to (i) measure the relative rates of Pi-ATP, Pi-HOH, and ATP-HOH isotope exchanges; (ii) probe the effect of ADP removal on the extent of inhibition of the exchanges, and (iii) test their uncoupler sensitivity/resistance. The objectives have been realized based on new experiments on submitochondrial particles, which show that both the Pi-HOH and ATP-HOH exchanges occur at a considerably higher rate relative to the Pi-ATP exchange, an observation that cannot be explained by previous mechanisms. A unifying explanation of the kinetic data that rationalizes these observations is given. The experimental results in (ii) show that ADP removal does not inhibit the intermediate Pi-HOH exchange when ATP and submitochondrial particles are incubated, and that the nucleotide requirement of the intermediate Pi-HOH exchange is adequately met by ATP, but not by ADP. These results contradicts the central postulate in Boyer's binding change mechanism of reversible catalysis at a F1 catalytic site with Keq~1 that predicts an absolute requirement of ADP for the occurrence of the Pi-HOH exchange. The prominent intermediate Pi-HOH exchange occurring under hydrolytic conditions is shown to be best explained by Nath's torsional mechanism of energy transduction and ATP synthesis/hydrolysis, which postulates an essentially irreversible cleavage of ATP by mitochondria/particles, independent from a reversible formation of ATP from ADP and Pi. The explanation within the torsional mechanism is also shown to rationalize the relative insensitivity of the intermediate Pi-HOH exchange to uncouplers observed in the experiments in (iii) compared to the Pi-ATP and ATP-HOH exchanges. This is shown to lead to new concepts and perspectives based on ligand displacement/substitution and ligand permutation for the elucidation of the oxygen exchange reactions within the framework of fundamental phosphorus chemistry. Fast mechanisms that realize the rotation/twist, tilt, permutation and switch of ligands, as well as inversion at the γ-phosphorus synchronously and simultaneously and in a concerted manner, have been proposed, and their stereochemical consequences have been analyzed. These considerations take us beyond the binding change mechanism of ATP synthesis/hydrolysis in bioenergetics.

Elucidating Events within the Black Box of Enzyme Catalysis in Energy Metabolism: Insights into the Molecular Mechanism of ATP Hydrolysis by F1-ATPase

Oxygen exchange reactions occurring at β-catalytic sites of the FOF1-ATP synthase/F1-ATPase imprint a unique record of molecular events during the catalytic cycle of ATP synthesis/hydrolysis. This work presents a new theory of oxygen exchange and tests it on oxygen exchange data recorded on ATP hydrolysis by mitochondrial F1-ATPase (MF1). The apparent rate constant of oxygen exchange governing the intermediate Pi-HOH exchange accompanying ATP hydrolysis is determined by kinetic analysis over a ~50,000-fold range of substrate ATP concentration (0.1-5000 μM) and a corresponding ~200-fold range of reaction velocity (3.5-650 [moles of Pi/{moles of F1-ATPase}-1 s-1]). Isotopomer distributions of [18O]Pi species containing 0, 1, 2, and 3 labeled oxygen atoms predicted by the theory have been quantified and shown to be in perfect agreement with the experimental distributions over the entire range of medium ATP concentrations without employing adjustable parameters. A novel molecular mechanism of steady-state multisite ATP hydrolysis by the F1-ATPase has been proposed. Our results show that steady-state ATP hydrolysis by F1-ATPase occurs with all three sites occupied by Mg-nucleotide. The various implications arising from models of energy coupling in ATP synthesis/hydrolysis by the ATP synthase/F1-ATPase have been discussed. Current models of ATP hydrolysis by F1-ATPase, including those postulated from single-molecule data, are shown to be effectively bisite models that contradict the data. The trisite catalysis formulated by Nath's torsional mechanism of energy transduction and ATP synthesis/hydrolysis since its first appearance 25 years ago is shown to be in better accord with the experimental record. The total biochemical information on ATP hydrolysis is integrated into a consistent model by the torsional mechanism of ATP synthesis/hydrolysis and shown to elucidate the elementary chemical and mechanical events within the black box of enzyme catalysis in energy metabolism by F1-ATPase.

Expression of hypoxia-inducible genes is suppressed in altered gravity due to impaired nuclear HIF1α accumulation

Extravehicular activities, the backbone of manned space exploration programs, set astronauts into mild hypoxia. Unfortunately, microgravity aggravates threatening symptoms of hypoxia such as vision impairment and brain edema. Hypoxia-inducible factors (HIFs) sense cellular hypoxia and, subsequently, change the cells' expression profile instantaneously by rapidly translocating-most likely cytoskeleton-dependently-into the nucleus and subsequently forming transcription complexes with other proteins. We tested the hypothesis that this fundamental process could be altered by sudden changes in gravitational forces in parabolic flights using a newly developed pocket-size cell culture lab that deoxygenizes cells within 15 min. Sudden gravity changes (SGCs 1g-1.8g-0g-1.8g-1g) during hypoxic exposure suppressed expression of the HIF1α-dependent genes investigated as compared with hypoxia at constant 1g. Normoxic cells subjected to SGCs showed reduced nuclear but not cytoplasmatic HIF1α signal and appeared to have disturbed cytoskeleton architecture. Inhibition of the actin-dependent intracellular transport using a combination of myosin V and VI inhibitors during hypoxia mimicked the suppression of the HIF1α-dependent genes observed during hypoxic exposure during SGCs. Thus, SGCs seem to disrupt the cellular response to hypoxia by impairing the actin-dependent translocation of HIF1α into the nucleus.

Autoinhibition and activation mechanisms revealed by the triangular-shaped structure of myosin Va

As the prototype of unconventional myosin motor family, myosin Va (MyoVa) transport cellular cargos along actin filaments in diverse cellular processes. The off-duty MyoVa adopts a closed and autoinhibited state, which can be relieved by cargo binding. The molecular mechanisms governing the autoinhibition and activation of MyoVa remain unclear. Here, we report the cryo-electron microscopy structure of the two full-length, closed MyoVa heavy chains in complex with 12 calmodulin light chains at 4.78-Å resolution. The MyoVa adopts a triangular structure with multiple intra- and interpolypeptide chain interactions in establishing the closed state with cargo binding and adenosine triphosphatase activity inhibited. Structural, biochemical, and cellular analyses uncover an asymmetric autoinhibition mechanism, in which the cargo-binding sites in the two MyoVa heavy chains are differently protected. Thus, specific and efficient MyoVa activation requires coincident binding of multiple cargo adaptors, revealing an intricate and elegant activity regulation of the motor in response to cargos.

Myosins and membrane trafficking in intestinal brush border assembly

Myosins are a class of motors that participate in a wide variety of cellular functions including organelle transport, cell adhesion, endocytosis and exocytosis, movement of RNA, and cell motility. Among the emerging roles for myosins is regulation of the assembly, morphology, and function of actin protrusions such as microvilli. The intestine harbors an elaborate apical membrane composed of highly organized microvilli. Microvilli assembly and function are intricately tied to several myosins including Myosin 1a, non-muscle Myosin 2c, Myosin 5b, Myosin 6, and Myosin 7b. Here, we review the research progress made in our understanding of myosin mediated apical assembly.

Case Report: MYO5B Homozygous Variant c.2090+3A>T Causes Intron Retention Related to Chronic Cholestasis and Diarrhea

Background: Biallelically mutated MYO5B is associated with microvillus inclusion disease (MVID, MIM: 251850), cholestasis, or both. This study aims at validating the splicing alteration and clinical features of an intron variant for diagnosis.

Case Presentation: A homozygous variant of MYO5B, NM_001080467.2:c.2090+3A > T (NP_001073936.1:p.?) in intron 17, was identified in a patient suffering from chronic cholestasis and diarrhea. Functional validation showed that this variant caused 185 bp of intron retention in its mRNA and was predicted to present a premature translation termination site for myoVb (p.Arg697fs*47) in the head motor domain. In addition, bowel biopsy revealed decreased microvilli and local lesions of microvillus inclusion in the duodena of the patient. The patient was presented with neonatal cholestasis leading to cirrhosis, intractable diarrhea, cholelithiasis, hepatic cyst, corneal opacity, and failure to thrive.

Conclusion: Our study demonstrated an intronic homozygous variant of MYO5B that affected an intron, subsequently altering splicing and leading to combined cholestasis and MVID. Our results further supported the underlying genotype–phenotype correlations and extended clinical practices toward its diagnosis and management.

Simulated actin reorganization mediated by motor proteins

Cortical actin networks are highly dynamic and play critical roles in shaping the mechanical properties of cells. The actin cytoskeleton undergoes significant reorganization in many different contexts, including during directed cell migration and over the course of the cell cycle, when cortical actin can transition between different configurations such as open patched meshworks, homogeneous distributions, and aligned bundles. Several types of myosin motor proteins, characterized by different kinetic parameters, have been involved in this reorganization of actin filaments. Given the limitations in studying the interactions of actin with myosin in vivo, we propose stochastic agent-based models and develop a set of data analysis measures to assess how myosin motor proteins mediate various actin organizations. In particular, we identify individual motor parameters, such as motor binding rate and step size, that generate actin networks with different levels of contractility and different patterns of myosin motor localization, which have previously been observed experimentally. In simulations where two motor populations with distinct kinetic parameters interact with the same actin network, we find that motors may act in a complementary way, by tuning the actin network organization, or in an antagonistic way, where one motor emerges as dominant. This modeling and data analysis framework also uncovers parameter regimes where spatial segregation between motor populations is achieved. By allowing for changes in kinetic rates during the actin-myosin dynamic simulations, our work suggests that certain actin-myosin organizations may require additional regulation beyond mediation by motor proteins in order to reconfigure the cytoskeleton network on experimentally-observed timescales.

Cell shape is dictated by a scaffolding network called the cytoskeleton. Actin filaments, a main component of the cytoskeleton, are found predominantly at the periphery of the cell, where they organize into different patterns in response to various stimuli, such as progression through the cell cycle. The actin filament reorganizations are mediated by motor proteins from the myosin superfamily. Using a realistic stochastic model that simulates actin filament and motor protein dynamics and interactions, we systematically vary motor protein kinetics and investigate their effect on actin filament organization. Using novel measures of spatial organization, we quantify conditions under which motor proteins, either alone or in combination, can produce the different actin filament organizations observed in vitro and in vivo. These results yield new insights into the role of motor proteins, as well as into how multiple types of motors can work collectively to produce specific actomyosin network patterns.

Multimodal regulation of myosin VI ensemble transport by cargo adaptor protein GIPC

A range of cargo adaptor proteins are known to recruit cytoskeletal motors to distinct subcellular compartments. However, the structural impact of cargo recruitment on motor function is poorly understood. Here, we dissect the multimodal regulation of myosin VI activity through the cargo adaptor GAIP-interacting protein, C terminus (GIPC), whose overexpression with this motor in cancer enhances cell migration. Using a range of biophysical techniques, including motility assays, FRET-based conformational sensors, optical trapping, and DNA origami-based cargo scaffolds to probe the individual and ensemble properties of GIPC-myosin VI motility, we report that the GIPC myosin-interacting region (MIR) releases an autoinhibitory interaction within myosin VI. We show that the resulting conformational changes in the myosin lever arm, including the proximal tail domain, increase the flexibility of the adaptor-motor linkage, and that increased flexibility correlates with faster actomyosin association and dissociation rates. Taken together, the GIPC MIR-myosin VI interaction stimulates a twofold to threefold increase in ensemble cargo speed. Furthermore, the GIPC MIR-myosin VI ensembles yield similar cargo run lengths as forced processive myosin VI dimers. We conclude that the emergent behavior from these individual aspects of myosin regulation is the fast, processive, and smooth cargo transport on cellular actin networks. Our study delineates the multimodal regulation of myosin VI by the cargo adaptor GIPC, while highlighting linkage flexibility as a novel biophysical mechanism for modulating cellular cargo motility.

Understanding the key functions of Myosins in viral infection

Myosins, a class of actin-based motor proteins existing in almost any organism, are originally considered only involved in driving muscle contraction, reshaping actin cytoskeleton, and anchoring or transporting cargoes, including protein complexes, organelles, vesicles. However, accumulating evidence reveals that myosins also play vital roles in viral infection, depending on viral species and infection stages. This review systemically summarizes the described various myosins, the performed functions, and the involved mechanisms or molecular pathways during viral infection. Meanwhile, the existing issues are also discussed. Additionally, the important technologies or agents, including siRNA, gene editing, and myosin inhibitors, would facilitate dissecting the actions and mechanisms for described and undescribed myosins, which could be adopted to prevent or control viral infection are also characterized.

Quantitative cell biology of tip growth in moss

KEY MESSAGE

Here we review, from a quantitative point of view, the cell biology of protonemal tip growth in the model moss Physcomitrium patens. We focus on the role of the cytoskeleton, vesicle trafficking, and cell wall mechanics, including reviewing some of the existing mathematical models of tip growth. We provide a primer for existing cell biological tools that can be applied to the future study of tip growth in moss. Polarized cell growth is a ubiquitous process throughout the plant kingdom in which the cell elongates in a self-similar manner. This process is important for nutrient uptake by root hairs, fertilization by pollen, and gametophyte development by the protonemata of bryophytes and ferns. In this review, we will focus on the tip growth of moss cells, emphasizing the role of cytoskeletal organization, cytoplasmic zonation, vesicle trafficking, cell wall composition, and dynamics. We compare some of the existing knowledge on tip growth in protonemata against what is known in pollen tubes and root hairs, which are better-studied tip growing cells. To fully understand how plant cells grow requires that we deepen our knowledge in a variety of forms of plant cell growth. We focus this review on the model plant Physcomitrium patens, which uses tip growth as the dominant form of growth at its protonemal stage. Because mosses and vascular plants shared a common ancestor more than 450 million years ago, we anticipate that both similarities and differences between tip growing plant cells will provide mechanistic information of tip growth as well as of plant cell growth in general. Towards this mechanistic understanding, we will also review some of the existing mathematical models of plant tip growth and their applicability to investigate protonemal morphogenesis. We attempt to integrate the conclusions and data across cell biology and physical modeling to our current state of knowledge of polarized cell growth in P. patens and highlight future directions in the field.

Designed Antitumor Peptide for Targeted siRNA Delivery into Cancer Spheroids

Antimicrobial/anticancer peptides (AMPs/ACPs) have shown promising results as new therapeutic agents in cancer thearpy. Among them, the designed amphiphilic α-helical peptide G(IIKK)₃I-NH₂ (G3) displayed great affinity and specificity in targeting cancer cells. Here, we report new insights on how G3 penetrates cancer cells. G3 showed high specificity to HCT-116 colon cancer cells compared to the HDFs (human neonatal primary dermal fibroblasts) control. With high concentrations of peptide, a clear cancer cell membrane disruption was observed through SEM. Gene knockdown of the endocytic pathways demonstrated that an energy-dependent endocytic pathway is required for the uptake of the peptide. In addition, G3 can protect and selectively deliver siRNAs into cancer cells and successfully modulated their gene expression. Gene delivery was also tested in 3D cancer spheroids and showed deep penetration delivery into the cancer spheroids. Finally, the in vivo toxicity of G3 was evaluated on zebrafish embryos, showing an increasing toxicity effect with concentration. However, the toxicity of the peptide was attenuated when complexed with siRNA. In addition, negligible toxicity was observed at the concentration range for efficient gene delivery. The current results demonstrate that G3 is promising as an excellent agent for cancer therapy.

Fusion partners of NTRK3 affect subcellular localization of the fusion kinase and cytomorphology of melanocytes

A subset of Spitz tumors harbor fusions of NTRK3 with ETV6, MYO5A, and MYH9. We evaluated a series of 22 melanocytic tumors in which an NTRK3 fusion was identified as part of the diagnostic workup. Tumors in which NTRK3 was fused to ETV6 occurred in younger patients were predominantly composed of epithelioid melanocytes and were classified by their histopathologic features as Spitz tumors. In contrast, those in which NTRK3 was fused to MYO5A were predominantly composed of spindled melanocytes arrayed in fascicles with neuroid features such as pseudo-Verocay bodies. To further investigate the effects of the fusion kinases ETV6-NTRK3 and MYO5A-NTRK3 in melanocytes, we expressed them in immortalized melanocytes and determined their subcellular localization by immunofluorescence. ETV6-NTRK3 was localized to the nucleus and diffusely within the cytoplasm and caused melanocytes to adopt an epithelioid cytomorphology. In contrast, MYO5A-NTRK3, appeared excluded from the nucleus of melanocytes, was localized to dendrites, and resulted in a highly dendritic cytomorphology. Our findings indicate that ETV6-NTRK3 and MYO5A-NTRK3 have distinct subcellular localizations and effects on cellular morphology.

Roles and regulation of myosin V interaction with cargo

A major question in cell biology is, how are organelles and large macromolecular complexes transported within a cell? Myosin V molecular motors play critical roles in the distribution of organelles, vesicles, and mRNA. Mis-localization of organelles that depend on myosin V motors underlie diseases in the skin, gut, and brain. Thus, the delivery of organelles to their proper destination is important for animal physiology and cellular function. Cargoes attach to myosin V motors via cargo specific adaptor proteins, which transiently bridge motors to their cargoes. Regulation of these adaptor proteins play key roles in the regulation of cargo transport. Emerging studies reveal that cargo adaptors play additional essential roles in the activation of myosin V, and the regulation of actin filaments. Here, we review how motor-adaptor interactions are controlled to regulate the proper loading and unloading of cargoes, as well as roles of adaptor proteins in the regulation of myosin V activity and the dynamics of actin filaments.

Calmodulin binding transcription activators: An interplay between calcium signalling and plant stress tolerance

In plants, next to the secondary messengers lies an array of signal relaying molecules among which Calmodulins convey the unequivocal alarms of calcium influxes to Calmodulin-Binding Transcription Activators (CAMTA). Upon reception, CAMTA transcription factors decode the calcium signatures by transcribing the genes corresponding to the specific stimulus, thus have direct/indirect engagement in the complex signalling crosstalk. CAMTA transcription factors make an important contribution to the genome of all eukaryotes, including plants, from Brassica napus (18) to Carica papaya (2), the number of CAMTA genes varies across the plant species, however they exhibit a similar evolutionarily conserved domain organization including a DNA-Binding Domain (CG-1), a Transcription Factor Immunoglobulin Binding Domain (TIG), a Calmodulin-Binding Domain (CaMBD/IQ) and several Ankyrin repeats. The regulatory region of CAMTA genes possess multiple stress-responsive cis motifs including ABRE, SARE, G-box, W-box, AuXRE, DRE and others. CAMTA TFs in Arabidopsis have been studied extensively, however in other plants (with a few exceptions), the evidence merely bases upon expression analyses. CAMTAs are reported to orchestrate biotic as well as abiotic stresses including those occurring due to water and temperature fluctuations as well as heavy metals, light and salinity. Through CG-1 domain, CAMTA TFs bind the CG-box in the promoter of their target genes and modulate their expression under adverse conditions. Here we present a glimpse of how calcium signatures are coded and decoded and translated into necessary responses. In addition, we have emphasized on exploitation of the multiple-stress responsive nature of CAMTAs in engineering plants with desired traits.

Myosin V-mediated transport of Snc1 and Vps10 toward the trans-Golgi network

Retrieval of cargo proteins from the endosome towards the trans-Golgi network (TGN) is a crucial intracellular process for cellular homeostasis. Its dysfunction is associated with pathogenesis of Alzheimer and Parkinson's diseases. Myosin family proteins are cellular motors walking along actin filaments by utilizing the chemical energy from ATP hydrolysis, known to involve in pleiotropic cellular trafficking pathways. However, the question of whether myosins play a role in the trafficking of Snc1 and Vps10 has not been addressed yet. The present study assesses the potential roles of all five yeast myosins in the recycling of two membrane cargo, Snc1 and Vps10. It appears that all myosins except Myo2 are not required for the Snc1 traffic, while it was found that Myo1 and 2 play important roles for Vps10 retrieval from the endosome and the vacuole. Multiple myo2 mutants harboring a point mutation in the actin binding or the cargo binding tail domain were characterized to demonstrate abnormal Vps10-GFP and GFP-Snc1 distribution phenotypes, suggesting a severe defect in their sorting and trafficking at the endosome. Furthermore, Vps10-GFP patches in all tested myo2 mutants were found to be near stationary with quantitative live cell imaging. Finally, we found that actin cables in the myo2 mutant cells were considerably disrupted, which may aggravate the trafficking of Vps10 from the endosome. Together, our results provide novel insights into the function of Myo-family proteins in the recycling traffic of Vps10 and Snc1 destined for the TGN.

F-actin disassembly factor MICAL1 binding to Myosin Va mediates cargo unloading during cytokinesis

Motor-mediated intracellular trafficking requires motors to position cargoes at proper locations. Myosin Va (MyoVa), an actin-based motor, is a classic model for studying cargo transport. However, the molecular basis underlying cargo unloading in MyoVa-mediated transport has remained enigmatic. We have identified MICAL1, an F-actin disassembly regulator, as a binding partner of MyoVa and shown that MICAL1-MyoVa interaction is critical for localization of MyoVa at the midbody. By binding to MICAL1, MyoVa-mediated transport is terminated, resulting in vesicle unloading at the midbody for efficient cytokinesis. The MyoVa/MICAL1 complex structure reveals that MICAL1 and F-actin assembly factors, Spires, share an overlapped binding surface on MyoVa, suggesting a regulatory role of F-actin dynamics in cargo unloading. Down-regulating F-actin disassembly by a MICAL1 mutant significantly reduces MyoVa and vesicles accumulating at the midbody. Collectively, our findings demonstrate that MyoVa binds to MICAL1 at the midbody destination and triggers F-actin disassembly to unload the vesicle cargo.

Cargo-Mediated Activation of Cytoplasmic Dynein in vivo

Cytoplasmic dynein-1 is a minus-end-directed microtubule motor that transports a variety of cargoes including early endosomes, late endosomes and other organelles. In many cell types, dynein accumulates at the microtubule plus end, where it interacts with its cargo to be moved toward the minus end. Dynein binds to its various cargoes via the dynactin complex and specific cargo adapters. Dynactin and some of the coiled-coil-domain-containing cargo adapters not only link dynein to cargo but also activate dynein motility, which implies that dynein is activated by its cellular cargo. Structural studies indicate that a dynein dimer switches between the autoinhibited phi state and an open state; and the binding of dynactin and a cargo adapter to the dynein tails causes the dynein motor domains to have a parallel configuration, allowing dynein to walk processively along a microtubule. Recently, the dynein regulator LIS1 has been shown to be required for dynein activation in vivo, and its mechanism of action involves preventing dynein from switching back to the autoinhibited state. In this review, we will discuss our current understanding of dynein activation and point out the gaps of knowledge on the spatial regulation of dynein in live cells. In addition, we will emphasize the importance of studying a complete set of dynein regulators for a better understanding of dynein regulation in vivo.

Melanosome transport and regulation in development and disease

Melanosomes are specialized membrane-bound organelles that synthesize and organize melanin, ultimately providing color to the skin, hair, and eyes. Disorders in melanogenesis and melanosome transport are linked to pigmentary diseases, such as Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, and Griscelli syndrome. Clinical cases of these pigmentary diseases shed light on the molecular mechanisms that control melanosome-related pathways. However, only an improved understanding of melanogenesis and melanosome transport will further the development of diagnostic and therapeutic approaches. Herein, we review the current literature surrounding melanosomes with particular emphasis on melanosome membrane transport and cytoskeleton-mediated melanosome transport. We also provide perspectives on melanosome regulatory mechanisms which include hormonal action, inflammation, autophagy, and organelle interactions.

Characterization of ancestral myosin XI from Marchantia polymorpha by heterologous expression in Arabidopsis thaliana

Previous studies have revealed duplications and diversification of myosin XI genes between angiosperms and bryophytes; however, the functional differentiation and conservation of myosin XI between them remain unclear. Here, we identified a single myosin XI gene from the liverwort Marchantia polymorpha (Mp). The molecular properties of Mp myosin XI are similar to those of Arabidopsis myosin XIs responsible for cytoplasmic streaming, suggesting that the motor function of myosin XI is able to generate cytoplasmic streaming. In cultured Arabidopsis cells, transiently expressed green fluorescent protein (GFP)-fused Mp myosin XI was observed as some intracellular structures moving along the F-actin. These intracellular structures were co-localized with motile endoplasmic reticulum (ER) strands, suggesting that Mp myosin XI binds to the ER and generates intracellular transport in Arabidopsis cells. The tail domain of Mp myosin XI was co-localized with that of Arabidopsis myosin XI-2 and XI-K, suggesting that all these myosin XIs bind to common cargoes. Furthermore, expression of GFP-fused Mp myosin XI rescued the defects of growth, cytoplasmic streaming and actin organization in Arabidopsis multiple myosin XI knockout mutants. The heterologous expression experiments demonstrated the cellular and physiological competence of Mp myosin XI in Arabidopsis. However, the average velocity of organelle transport in Marchantia rhizoids was 0.04 ± 0.01 μm s-1 , which is approximately one-hundredth of that in Arabidopsis cells. Taken together, our results suggest that the molecular properties of myosin XI are conserved, but myosin XI-driven intracellular transport in vivo would be differentiated from bryophytes to angiosperms.

A novel labeling strategy reveals that myosin Va and myosin Vb bind the same dendritically polarized vesicle population

Neurons are specialized cells with a polarized geometry and several distinct subdomains that require specific complements of proteins. Delivery of transmembrane proteins requires vesicle transport, which is mediated by molecular motor proteins. The myosin V family of motor proteins mediates transport to the barbed end of actin filaments, and little is known about the vesicles bound by myosin V in neurons. We developed a novel strategy to visualize myosin V-labeled vesicles in cultured hippocampal neurons and systematically characterized the vesicle populations labeled by myosin Va and Vb. We find that both myosins bind vesicles that are polarized to the somatodendritic domain where they undergo bidirectional long-range transport. A series of two-color imaging experiments showed that myosin V specifically colocalized with two different vesicle populations: vesicles labeled with the transferrin receptor and vesicles labeled by low-density lipoprotein receptor. Finally, coexpression with Kinesin-3 family members found that myosin V binds vesicles concurrently with KIF13A or KIF13B, supporting the hypothesis that coregulation of kinesins and myosin V on vesicles is likely to play an important role in neuronal vesicle transport. We anticipate that this new assay will be applicable in a broad range of cell types to determine the function of myosin V motor proteins.

Control of Cytoskeletal Dynamics by β-Arrestin1/Myosin Vb Signaling Regulates Endosomal Sorting and Scavenging Activity of the Atypical Chemokine Receptor ACKR2

The atypical chemokine receptor ACKR2, formerly named D6, is a scavenger chemokine receptor with a non-redundant role in the control of inflammation and immunity. The scavenging activity of ACKR2 depends on its trafficking properties, which require actin cytoskeleton rearrangements downstream of a β-arrestin1-Rac1-PAK1-LIMK1-cofilin-dependent signaling pathway. We here demonstrate that in basal conditions, ACKR2 trafficking properties require intact actin and microtubules networks. The dynamic turnover of actin filaments is required to sustain ACKR2 constitutive endocytosis, while both actin and microtubule networks are involved in processes regulating ACKR2 constitutive sorting to rapid, Rab4-dependent and slow, Rab11-dependent recycling pathways, respectively. After chemokine engagement, ACKR2 requires myosin Vb activity to promote its trafficking from Rab11-positive recycling endosomes to the plasma membrane, which sustains its scavenging activity. Other than cofilin phosphorylation, induction of the β-arrestin1-dependent signaling pathway by ACKR2 agonists also leads to the rearrangement of microtubules, which is required to support the myosin Vb-dependent ACKR2 upregulation and its scavenging properties. Disruption of the actin-based cytoskeleton by the apoptosis-inducing agent staurosporine results in impaired ACKR2 internalization and chemokine degradation that is consistent with the emerging scavenging-independent activity of the receptor in apoptotic neutrophils instrumental for promoting efficient efferocytosis during the resolution of inflammation. In conclusion, we provide evidence that ACKR2 activates a β-arrestin1-dependent signaling pathway, triggering both the actin and the microtubule cytoskeletal networks, which control its trafficking and scavenger properties.

Crystalline chitin hydrolase is a burnt-bridge Brownian motor

Motor proteins are essential units of life and are well-designed nanomachines working under thermal fluctuations. These proteins control moving direction by consuming chemical energy or by dissipating electrochemical potentials. Chitinase A from bacterium Serratia marcescens (SmChiA) processively moves along crystalline chitin by hydrolysis of a single polymer chain to soluble chitobiose. Recently, we directly observed the stepping motions of SmChiA labeled with a gold nanoparticle by dark-field scattering imaging to investigate the moving mechanism. Time constants analysis revealed that SmChiA moves back and forth along the chain freely, because forward and backward states have a similar free energy level. The similar probabilities of forward-step events (83.5%=69.3%+14.2%) from distributions of step sizes and chain-hydrolysis (86.3%=(1/2.9)/(1/2.9+1/18.3)×100) calculated from the ratios of time constants of hydrolysis and the backward step indicated that SmChiA moves forward as a result of shortening of the chain by a chitobiose unit, which stabilizes the backward state. Furthermore, X-ray crystal structures of sliding intermediate and molecular dynamics simulations showed that SmChiA slides forward and backward under thermal fluctuation without large conformational changes of the protein. Our results demonstrate that SmChiA is a burnt-bridge Brownian ratchet motor.

Asymmetric inheritance of mitochondria in yeast

Mitochondria are essential organelles of virtually all eukaryotic organisms. As they cannot be made de novo, they have to be inherited during cell division. In this review, we provide an overview on mitochondrial inheritance in Saccharomyces cerevisiae, a powerful model organism to study asymmetric cell division. Several processes have to be coordinated during mitochondrial inheritance: mitochondrial transport along the actin cytoskeleton into the emerging bud is powered by a myosin motor protein; cell cortex anchors retain a critical fraction of mitochondria in the mother cell and bud to ensure proper partitioning; and the quantity of mitochondria inherited by the bud is controlled during cell cycle progression. Asymmetric division of yeast cells produces rejuvenated daughter cells and aging mother cells that die after a finite number of cell divisions. We highlight the critical role of mitochondria in this process and discuss how asymmetric mitochondrial partitioning and cellular aging are connected.

The Myosin Family of Mechanoenzymes: From Mechanisms to Therapeutic Approaches

Myosins are among the most fascinating enzymes in biology. As extremely allosteric chemomechanical molecular machines, myosins are involved in myriad pivotal cellular functions and are frequently sites of mutations leading to disease phenotypes. Human β-cardiac myosin has proved to be an excellent target for small-molecule therapeutics for heart muscle diseases, and, as we describe here, other myosin family members are likely to be potentially unique targets for treating other diseases as well. The first part of this review focuses on how myosins convert the chemical energy of ATP hydrolysis into mechanical movement, followed by a description of existing therapeutic approaches to target human β-cardiac myosin. The next section focuses on the possibility of targeting nonmuscle members of the human myosin family for several diseases. We end the review by describing the roles of myosin in parasites and the therapeutic potential of targeting them to block parasitic invasion of their hosts.

MARCKS and Lung Disease

MARCKS (myristoylated alanine-rich C kinase substrate) is a prominent PKC substrate expressed in all eukaryotic cells. It is known to bind to and cross-link actin filaments, to serve as a bridge between Ca²⁺/calmodulin and PKC signaling, and to sequester the signaling molecule phosphatidylinositol 4,5-bisphosphate in the plasma membrane. Since the mid-1980s, this evolutionarily conserved and ubiquitously expressed protein has been associated with regulating cellular events that require dynamic actin reorganization, including cellular adhesion, migration, and exocytosis. More recently, translational studies have implicated MARCKS in the pathophysiology of a number of airway diseases, including chronic obstructive pulmonary disease, asthma, lung cancer, and acute lung injury/acute respiratory distress syndrome. This article summarizes the structure and cellular function of MARCKS (also including MARCKS family proteins and MARCKSL1 [MARCKS-like protein 1]). Evidence for MARCKS's role in several lung diseases is discussed, as are the technological innovations that took MARCKS-targeting strategies from theoretical to therapeutic. Descriptions and updates derived from ongoing clinical trials that are investigating inhalation of a MARCKS-targeting peptide as therapy for patients with chronic bronchitis, lung cancer, and ARDS are provided.

Structural basis for power stroke vs. Brownian ratchet mechanisms of motor proteins

Two mechanisms have been proposed for the function of motor proteins: The power stroke and the Brownian ratchet. The former refers to generation of a large downhill free energy gradient over which the motor protein moves nearly irreversibly in making a step, whereas the latter refers to biasing or rectifying the diffusive motion of the motor. Both mechanisms require input of free energy, which generally involves the processing of an ATP (adenosine 5'-triphosphate) molecule. Recent advances in experiments that reveal the details of the stepping motion of motor proteins, together with computer simulations of atomistic structures, have provided greater insights into the mechanisms. Here, we compare the various models of the power stroke and the Brownian ratchet that have been proposed. The 2 mechanisms are not mutually exclusive, and various motor proteins employ them to different extents to perform their biological function. As examples, we discuss linear motor proteins Kinesin-1 and myosin-V, and the rotary motor F₁-ATPase, all of which involve a power stroke as the essential element of their stepping mechanism.

The cargo adaptor proteins RILPL2 and melanophilin co-regulate myosin-5a motor activity

Vertebrate myosin-5a is an ATP-utilizing processive motor associated with the actin network and responsible for the transport and localization of several vesicle cargoes. To transport cargo efficiently and prevent futile ATP hydrolysis, myosin-5a motor function must be tightly regulated. The globular tail domain (GTD) of myosin-5a not only functions as the inhibitory domain but also serves as the binding site for a number of cargo adaptor proteins, including melanophilin (Mlph) and Rab-interacting lysosomal protein-like 2 (RILPL2). In this study, using various biochemical approaches, including ATPase, single-molecule motility, GST pulldown assays, and analytical ultracentrifugation, we demonstrate that the binding of both Mlph and RILPL2 to the GTD of myosin-5a is required for the activation of myosin-5a motor function under physiological ionic conditions. We also found that this activation is regulated by the small GTPase Rab36, a binding partner of RILPL2. In summary, our results indicate that RILPL2 is required for Mlph-mediated activation of Myo5a motor activity under physiological conditions and that Rab36 promotes this activation. We propose that Rab36 stimulates RILPL2 to interact with the myosin-5a GTD; this interaction then induces exposure of the Mlph-binding site in the GTD, enabling Mlph to interact with the GTD and activate myosin-5a motor activity.

Suo Quan Wan Protects Mouse From Early Diabetic Bladder Dysfunction by Mediating Motor Protein Myosin Va and Transporter Protein SLC17A9

Objective: To investigate the effects of Suo Quan Wan (SQW), a traditional Chinese herbal formula, on the overactive bladder (OAB) of type 2 diabetes mellitus (T2DM) mouse models, particularly on its function of mediating the gene and protein expression levels of myosin Va and SLC17A9.

Materials and Methods: After 4 weeks high-fat diet (HFD) feeding, C57BL/6J mice were injected with streptozotocin (100 mg/kg) for four times. After 3 weeks, the diabetic mice were treated with SQW for another 3 weeks. Voided stain on paper assay, fasting blood glucose (FBG) test, and oral glucose tolerance test (OGTT) were conducted. Urodynamic test, tension test [α,β-methylene ATP, electrical-field stimulation (EFS), KCl, and carbachol] and histomorphometry were also performed. Western blot analysis and qPCR assays were used to quantify the expression levels of myosin Va and SLC17A9.

Results: The diabetic mice exhibited decreased weight but increased water intake, urine production, FBG, and OGTT. No significant changes were observed after 3 weeks SQW treatment. Urodynamic test indicated that the non-voiding contraction (NVC) frequency, maximum bladder capacity (MBC), residual volume (RV), and bladder compliance (BC) were remarkably increased in the diabetic mice, whereas the voided efficiency (VE) was decreased as a feature of overactivity. Compared with the model mice, SQW treatment significantly improved urodynamic urination with decreased NVC, MBC, RV, and BC, and increased VE. Histomorphometry results showed that the bladder wall of the diabetic mice thickened, and SQW effectively attenuated the pathological alterations. The contract responses of bladder strips to all stimulators were higher in the DSM strips of diabetic mice, whereas SQW treatment markedly decreased the contraction response for all stimuli. Moreover, the protein and gene expression levels of myosin Va and SLC17A9 were up-regulated in the bladders of diabetic mice, but SQW treatment restored such alterations.

Conclusion: T2DM mice exhibited the early phase of diabetic bladder dysfunction (DBD) characterized by OAB and bladder dysfunction. SQW can improve the bladder storage and micturition of DBD mice by mediating the protein and gene expression levels of myosin Va and SLC17A9 in the bladder, instead of improving the blood glucose level.

Myosin Va and spermine synthase: partners in exosome transport

A recent paper in Bioscience Reports (BSR20182189) describes the discovery of an interaction between the motor protein myosin Va and the metabolic enzyme spermine synthase. Myosin Va is a molecular motor which plays a key role in vesicle transport. Mutations in the gene which encodes this protein are associated with Griscelli syndrome type 1 and the ‘dilute’ phenotype in animals. Spermine synthase catalyzes the conversion of spermidine to spermine. This largely cytoplasmic enzyme can also be localized to the soluble fraction in exosomes. Mutations in the spermine synthase gene are associated with Snyder Robinson mental retardation syndrome. The interaction between the two proteins was detected using the yeast two hybrid method and verified by microscale thermophoresis of recombinant proteins. Knockdown of the MYO5A gene reduced the expression of mRNA coding for spermine synthase. The amount of this transcript was also reduced in cells derived from a patient with Griscelli syndrome type 1. This suggests that, in addition to a direct physical interaction between the two proteins, myosin Va also modulates the transcription of the spermine synthase gene. The mechanism for this modulation is currently unknown. These findings have implications for Griscelli syndrome type 1 and Snyder Robinson mental retardation syndrome. They also suggest that interactions between myosin Va and soluble exosome proteins such as spermine synthase may be important in the mechanism of exosome transport.

Structural mechanism for versatile cargo recognition by the yeast class V myosin Myo2

Class V myosins are actin-dependent motors, which recognize numerous cellular cargos mainly via the C-terminal globular tail domain (GTD). Myo2, a yeast class V myosin, can transport a broad range of organelles. However, little is known about the capacity of Myo2-GTD to recognize such a diverse array of cargos specifically at the molecular level. Here, we solved crystal structures of Myo2-GTD (at 1.9-3.1 Å resolutions) in complex with three cargo adaptor proteins: Smy1 (for polarization of secretory vesicles), Inp2 (for peroxisome transport), and Mmr1 (for mitochondria transport). The structures of Smy1- and Inp2-bound Myo2-GTD, along with site-directed mutagenesis experiments, revealed a binding site in subdomain-I having a hydrophobic groove with high flexibility enabling Myo2-GTD to accommodate different protein sequences. The Myo2-GTD-Mmr1 complex structure confirmed and complemented a previously identified mitochondrion/vacuole-specific binding region. Moreover, differences between the conformations and locations of cargo-binding sites identified here for Myo2 and those reported for mammalian MyoVA (MyoVA) suggest that class V myosins potentially have co-evolved with their specific cargos. Our structural and biochemical analysis not only uncovers a molecular mechanism that explains the diverse cargo recognition by Myo2-GTD, but also provides structural information useful for future functional studies of class V myosins in cargo transport.

Interferometric Scattering Microscopy

Interferometric scattering microscopy (iSCAT) is an extremely sensitive imaging method based on the efficient detection of light scattered by nanoscopic objects. The ability to, at least in principle, maintain high imaging contrast independent of the exposure time or the scattering cross section of the object allows for unique applications in single-particle tracking, label-free imaging of nanoscopic (dis)assembly, and quantitative single-molecule characterization. We illustrate these capabilities in areas as diverse as mechanistic studies of motor protein function, viral capsid assembly, and single-molecule mass measurement in solution. We anticipate that iSCAT will become a widely used approach to unravel previously hidden details of biomolecular dynamics and interactions.

Myosin Va transport of liposomes in three-dimensional actin networks is modulated by actin filament density, position, and polarity

The cell's dense 3D actin filament network presents numerous challenges to vesicular transport by teams of myosin Va (MyoVa) molecular motors. These teams must navigate their cargo through diverse actin structures ranging from Arp2/3-branched lamellipodial networks to the dense, unbranched cortical networks. To define how actin filament network organization affects MyoVa cargo transport, we created two different 3D actin networks in vitro. One network was comprised of randomly oriented, unbranched actin filaments; the other was comprised of Arp2/3-branched actin filaments, which effectively polarized the network by aligning the actin filament plus-ends. Within both networks, we defined each actin filament's 3D spatial position using superresolution stochastic optical reconstruction microscopy (STORM) and its polarity by observing the movement of single fluorescent reporter MyoVa. We then characterized the 3D trajectories of fluorescent, 350-nm fluid-like liposomes transported by MyoVa teams (∼10 motors) moving within each of the two networks. Compared with the unbranched network, we observed more liposomes with directed and fewer with stationary motion on the Arp2/3-branched network. This suggests that the modes of liposome transport by MyoVa motors are influenced by changes in the local actin filament polarity alignment within the network. This mechanism was supported by an in silico 3D model that provides a broader platform to understand how cellular regulation of the actin cytoskeletal architecture may fine tune MyoVa-based intracellular cargo transport.

Myosin Va interacts with the exosomal protein spermine synthase

Myosin Va (MyoVa) is an actin-based molecular motor that plays key roles in the final stages of secretory pathways, including neurotransmitter release. Several studies have addressed how MyoVa coordinates the trafficking of secretory vesicles, but why this molecular motor is found in exosomes is still unclear. In this work, using a yeast two-hybrid screening system, we identified the direct interaction between the globular tail domain (GTD) of MyoVa and four protein components of exosomes: the WD repeat-containing protein 48 (WDR48), the cold shock domain-containing protein E1 (CSDE1), the tandem C2 domain-containing protein 1 (TC2N), and the enzyme spermine synthase (SMS). The interaction between the GTD of MyoVa and SMS was further validated in vitro and displayed a K_d in the low micromolar range (3.5 ± 0.5 µM). SMS localized together with MyoVa in cytoplasmic vesicles of breast cancer MCF-7 and neuroblastoma SH-SY5Y cell lines, known to produce exosomes. Moreover, MYO5A knockdown decreased the expression of SMS gene and rendered the distribution of SMS protein diffuse, supporting a role for MyoVa in SMS expression and targeting.

Duplicated Myosin V Genes in Teleosts Show Evolutionary Rate Variations among the Motor and Cargo-Binding Domains

We analyzed evolutionary rates of conserved, duplicated myosin V (myo5) genes in nine teleost species to examine the outcomes of duplication events. Syntenic analysis and ancestral chromosome mapping suggest one tandem gene duplication event leading to the appearance of myo5a and myo5c, two rounds of whole genome duplication for vertebrates, and an additional round of whole genome duplication for teleosts account for the presence and location of the myo5 genes and their duplicates in teleosts and other vertebrates and the timing of the duplication events. Phylogenetic analyses reveal a previously unidentified myo5 clade that we refer to now as myo5bb. Analysis using dN/dS rate comparisons revealed large regions within duplicated myo5 genes that are highly conserved. Codons identified in other studies as encoding functionally important portions of the Myo5a and Myo5b proteins are shown to be highly conserved within the newly identified myo5bb clade and in other myo5 duplicates. As much as 30% of 319 codons encoding the cargo-binding domain in the myo5aa genes are conserved in all three codon positions in nine teleost species. For the myo5bb cargo-binding domain, 6.6% of 336 codons have zero substitutions in all nine teleost species. Using molecular evolution assays, we identify the myo5bb branch as being subject to evolutionary rate variation with the cargo-binding domain, having 20% of the sites under positive selection and the motor domain having 8% of its sites under positive selection. The high number of invariant codons coupled with relatively high dN/dS values in the region of the myo5 genes encoding the ATP-binding domain suggests the encoded proteins retain function and may have acquired novel functions associated with changes to the cargo-binding domain.

The adaptor protein melanophilin regulates dynamic myosin-Va:cargo interaction and dendrite development in melanocytes

The regulation of organelle transport by the cytoskeleton is fundamental for eukaryotic survival. Cytoskeleton motors are typically modular proteins with conserved motor and diverse cargo-binding domains. Motor:cargo interactions are often indirect and mediated by adaptor proteins, for example, Rab GTPases. Rab27a, via effector melanophilin (Mlph), recruits myosin-Va (MyoVa) to melanosomes and thereby disperses them into melanocyte dendrites. To better understand how adaptors regulate motor:cargo interaction, we used single melanosome fluorescence recovery after photobleaching (smFRAP) to characterize the association kinetics among MyoVa, its adaptors, and melanosomes. We found that MyoVa and Mlph rapidly recovered after smFRAP, whereas Rab27a did not, indicating that MyoVa and Mlph dynamically associate with melanosomes and Rab27a does not. This suggests that dynamic Rab27a:effector interaction rather than Rab27a melanosome:cytosol cycling regulates MyoVa:melanosome association. Accordingly, a Mlph-Rab27a fusion protein reduced MyoVa smFRAP, indicating that it stabilized melanosomal MyoVa. Finally, we tested the functional importance of dynamic MyoVa:melanosome interaction. We found that whereas a MyoVa-Rab27a fusion protein dispersed melanosomes in MyoVa-deficient cells, dendrites were significantly less elongated than in wild-type cells. Given that dendrites are the prime sites of melanosome transfer from melanocytes to keratinocytes, we suggest that dynamic MyoVa:melanosome interaction is important for pigmentation in vivo.

Regulation of Myosin-5b by Rab11a and the Rab11 family interacting protein 2

Mammalian myosin-5b (Myo5b) plays a critical role in the recycling of endosomes to the plasma membrane via the interactions with Rab11a and the Rab11 family interacting protein 2 (FIP2). However, it remains unclear on how Rab11a and FIP2 are coordinated in tethering Myo5b with the vesicles and activating the motor function of Myo5b. In the present study, we show that Rab11a binds to the globular tail domain (GTD) of Myo5b and this binding abolishes the head–GTD interaction of Myo5b, thus activating the motor function of Myo5b. On the other hand, FIP2 directly interacts with both Rab11a and the tail of Myo5b, and the binding of FIP2 to Myo5b does not affect Myo5b motor function. Moreover, Rab11a displays higher affinity to FIP2 than to Myo5b, suggesting that Rab11a binds preferentially to FIP2 than to Myo5b. Based on the current findings, we propose that the association of Myo5b with vesicles is mediated by FIP2, which bridges Myo5b and the membrane-bound Rab11a, whereas the motor function of Myo5b is regulated by Rab11a.

Converter domain mutations in myosin alter structural kinetics and motor function

Myosins are molecular motors that use a conserved ATPase cycle to generate force. We investigated two mutations in the converter domain of myosin V (R712G and F750L) to examine how altering specific structural transitions in the motor ATPase cycle can impair myosin mechanochemistry. The corresponding mutations in the human β-cardiac myosin gene are associated with hypertrophic and dilated cardiomyopathy, respectively. Despite similar steady-state actin-activated ATPase and unloaded in vitro motility-sliding velocities, both R712G and F750L were less able to overcome frictional loads measured in the loaded motility assay. Transient kinetic analysis and stopped-flow FRET demonstrated that the R712G mutation slowed the maximum ATP hydrolysis and recovery-stroke rate constants, whereas the F750L mutation enhanced these steps. In both mutants, the fast and slow power-stroke as well as actin-activated phosphate release rate constants were not significantly different from WT. Time-resolved FRET experiments revealed that R712G and F750L populate the pre- and post-power-stroke states with similar FRET distance and distance distribution profiles. The R712G mutant increased the mole fraction in the post-power-stroke conformation in the strong actin-binding states, whereas the F750L decreased this population in the actomyosin ADP state. We conclude that mutations in key allosteric pathways can shift the equilibrium and/or alter the activation energy associated with key structural transitions without altering the overall conformation of the pre- and post-power-stroke states. Thus, therapies designed to alter the transition between structural states may be able to rescue the impaired motor function induced by disease mutations.

Molecular mechanisms of the interhead coordination by interhead tension in cytoplasmic dyneins

Cytoplasmic dyneins play a major role in retrograde cellular transport by moving vesicles and organelles along microtubule filaments. Dyneins are multidomain motor proteins with two heads that coordinate their motion via their interhead tension. Compared with the leading head, the trailing head has a higher detachment rate from microtubules, facilitating the movement. However, the molecular mechanism of such coordination is unknown. To elucidate this mechanism, we performed molecular dynamics simulations on a cytoplasmic dynein with a structure-based coarse-grained model that probes the effect of the interhead tension on the structure. The tension creates a torque that influences the head rotating about its stalk. The conformation of the stalk switches from the α registry to the β registry during the rotation, weakening the binding affinity to microtubules. The directions of the tension and the torque of the leading head are opposite to those of the trailing head, breaking the structural symmetry between the heads. The leading head transitions less often to the β registry than the trailing head. The former thus has a greater binding affinity to the microtubule than the latter. We measured the moment arm of the torque from a dynein structure in the simulations to develop a phenomenological model that captures the influence of the head rotating about its stalk on the differential detachment rates of the two heads. Our study provides a consistent molecular picture for interhead coordination via interhead tension.

Allosteric binding sites in Rab11 for potential drug candidates

Rab11 is an important protein subfamily in the RabGTPase family. These proteins physiologically function as key regulators of intracellular membrane trafficking processes. Pathologically, Rab11 proteins are implicated in many diseases including cancers, neurodegenerative diseases and type 2 diabetes. Although they are medically important, no previous study has found Rab11 allosteric binding sites where potential drug candidates can bind to. In this study, by employing multiple clustering approaches integrating principal component analysis, independent component analysis and locally linear embedding, we performed structural analyses of Rab11 and identified eight representative structures. Using these representatives to perform binding site mapping and virtual screening, we identified two novel binding sites in Rab11 and small molecules that can preferentially bind to different conformations of these sites with high affinities. After identifying the binding sites and the residue interaction networks in the representatives, we computationally showed that these binding sites may allosterically regulate Rab11, as these sites communicate with switch 2 region that binds to GTP/GDP. These two allosteric binding sites in Rab11 are also similar to two allosteric pockets in Ras that we discovered previously.

Bilobal architecture is a requirement for calmodulin signaling to CaV1.3 channels

Calmodulin (CaM) regulation of voltage-gated calcium (Ca_V) channels is a powerful Ca²⁺ feedback mechanism that adjusts Ca²⁺ influx, affording rich mechanistic insights into Ca²⁺ decoding. CaM possesses a dual-lobed architecture, a salient feature of the myriad Ca²⁺-sensing proteins, where two homologous lobes that recognize similar targets hint at redundant signaling mechanisms. Here, by tethering CaM lobes, we demonstrate that bilobal architecture is obligatory for signaling to Ca_V channels. With one lobe bound, Ca_V carboxy tail rearranges itself, resulting in a preinhibited configuration precluded from Ca²⁺ feedback. Reconstitution of two lobes, even as separate molecules, relieves preinhibition and restores Ca²⁺ feedback. Ca_V channels thus detect the coincident binding of two Ca²⁺-free lobes to promote channel opening, a molecular implementation of a logical NOR operation that processes spatiotemporal Ca²⁺ signals bifurcated by CaM lobes. Overall, a unified scheme of Ca_V channel regulation by CaM now emerges, and our findings highlight the versatility of CaM to perform exquisite Ca²⁺ computations.

Interacting-heads motif has been conserved as a mechanism of myosin II inhibition since before the origin of animals

Electron microscope studies have shown that the switched-off state of myosin II in muscle involves intramolecular interaction between the two heads of myosin and between one head and the tail. The interaction, seen in both myosin filaments and isolated molecules, inhibits activity by blocking actin-binding and ATPase sites on myosin. This interacting-heads motif is highly conserved, occurring in invertebrates and vertebrates, in striated, smooth, and nonmuscle myosin IIs, and in myosins regulated by both Ca²⁺ binding and regulatory light-chain phosphorylation. Our goal was to determine how early this motif arose by studying the structure of inhibited myosin II molecules from primitive animals and from earlier, unicellular species that predate animals. Myosin II from Cnidaria (sea anemones, jellyfish), the most primitive animals with muscles, and Porifera (sponges), the most primitive of all animals (lacking muscle tissue) showed the same interacting-heads structure as myosins from higher animals, confirming the early origin of the motif. The social amoeba Dictyostelium discoideum showed a similar, but modified, version of the motif, while the amoeba Acanthamoeba castellanii and fission yeast (Schizosaccharomyces pombe) showed no head-head interaction, consistent with the different sequences and regulatory mechanisms of these myosins compared with animal myosin IIs. Our results suggest that head-head/head-tail interactions have been conserved, with slight modifications, as a mechanism for regulating myosin II activity from the emergence of the first animals and before. The early origins of these interactions highlight their importance in generating the inhibited (relaxed) state of myosin in muscle and nonmuscle cells.

MYO5B, STX3, and STXBP2 mutations reveal a common disease mechanism that unifies a subset of congenital diarrheal disorders: A mutation update

Microvillus inclusion disease (MVID) is a rare but fatal autosomal recessive congenital diarrheal disorder caused by MYO5B mutations. In 2013, we launched an open‐access registry for MVID patients and their MYO5B mutations (www.mvid-central.org). Since then, additional unique MYO5B mutations have been identified in MVID patients, but also in non‐MVID patients. Animal models have been generated that formally prove the causality between MYO5B and MVID. Importantly, mutations in two other genes, STXBP2 and STX3, have since been associated with variants of MVID, shedding new light on the pathogenesis of this congenital diarrheal disorder. Here, we review these additional genes and their mutations. Furthermore, we discuss recent data from cell studies that indicate that the three genes are functionally linked and, therefore, may constitute a common disease mechanism that unifies a subset of phenotypically linked congenital diarrheal disorders. We present new data based on patient material to support this. To congregate existing and future information on MVID geno‐/phenotypes, we have updated and expanded the MVID registry to include all currently known MVID‐associated gene mutations, their demonstrated or predicted functional consequences, and associated clinical information.

Regulation of class V myosin

Class V myosin (myosin-5) is a molecular motor that functions as an organelle transporter. The activation of myosin-5's motor function has long been known to be associated with a transition from the folded conformation in the off-state to the extended conformation in the on-state, but only recently have we begun to understand the underlying mechanism. The globular tail domain (GTD) of myosin-5 has been identified as the inhibitory domain and has recently been shown to function as a dimer in regulating the motor function. The folded off-state of myosin-5 is stabilized by multiple intramolecular interactions, including head-GTD interactions, GTD-GTD interactions, and interactions between the GTD and the C-terminus of the first coiled-coil segment. Any cellular factor that affects these intramolecular interactions and thus the stability of the folded conformation of myosin-5 would be expected to regulate myosin-5 motor function. Both the adaptor proteins of myosin-5 and Ca2+ are potential regulators of myosin-5 motor function, because they can destabilize its folded conformation. A combination of these regulators provides a versatile scheme in regulating myosin-5 motor function in the cell.

Loss of Myosin Vb in colorectal cancer is a strong prognostic factor for disease recurrence

Background:

Selecting the most beneficial treatment regimens for colorectal cancer (CRC) patients remains challenging due to a lack of prognostic markers. Members of the Myosin family, proteins recognised to have a major role in trafficking and polarisation of cells, have recently been reported to be closely associated with several types of cancer and might thus serve as potential prognostic markers in the context of CRC.

Methods:

We used a previously established meta-analysis of publicly available gene expression data to analyse the expression of different members of the Myosin V family, namely MYO5A, 5B, and 5C, in CRC. Using laser-microdissected material as well as tissue microarrays from paired human CRC samples, we validated both RNA and protein expression of Myosin Vb (MYO5B) and its known adapter proteins (RAB8A and RAB25) in an independent patient cohort. Finally, we assessed the prognostic value of both MYO5B and its adapter-coupled combinatorial gene expression signatures.

Results:

The meta-analysis as well as an independent patient cohort study revealed a methylation-independent loss of MYO5B expression in CRC that matched disease progression. Although MYO5B mutations were identified in a small number of patients, these cannot be solely responsible for the common downregulation observed in CRC patients. Significantly, CRC patients with low MYO5B expression displayed shorter overall, disease-, and metastasis-free survival, a trend that was further reinforced when RAB8A expression was also taken into account.

Conclusions:

Our data identify MYO5B as a powerful prognostic biomarker in CRC, especially in early stages (stages I and II), which might help stratifying patients with stage II for adjuvant chemotherapy.

Hypertrophic cardiomyopathy and the myosin mesa: viewing an old disease in a new light

The sarcomere is an exquisitely designed apparatus that is capable of generating force, which in the case of the heart results in the pumping of blood throughout the body. At the molecular level, an ATP-dependent interaction of myosin with actin drives the contraction and force generation of the sarcomere. Over the past six decades, work on muscle has yielded tremendous insights into the workings of the sarcomeric system. We now stand on the cusp where the acquired knowledge of how the sarcomere contracts and how that contraction is regulated can be extended to an understanding of the molecular mechanisms of sarcomeric diseases, such as hypertrophic cardiomyopathy (HCM). In this review we present a picture that combines current knowledge of the myosin mesa, the sequestered state of myosin heads on the thick filament, known as the interacting-heads motif (IHM), their possible interaction with myosin binding protein C (MyBP-C) and how these interactions can be abrogated leading to hyper-contractility, a key clinical manifestation of HCM. We discuss the structural and functional basis of the IHM state of the myosin heads and identify HCM-causing mutations that can directly impact the equilibrium between the 'on state' of the myosin heads (the open state) and the IHM 'off state'. We also hypothesize a role of MyBP-C in helping to maintain myosin heads in the IHM state on the thick filament, allowing release in a graded manner upon adrenergic stimulation. By viewing clinical hyper-contractility as the result of the destabilization of the IHM state, our aim is to view an old disease in a new light.