Calcium can mobilize and activate myosin-VI

Motor domain phosphorylation increases nucleotide exchange and turns MYO6 into a faster and stronger motor

Myosin motors perform many fundamental functions in eukaryotic cells by providing force generation, transport or tethering capacity. Motor activity control within the cell involves on/off switches, however, few examples are known of how myosins regulate speed or processivity and fine-tune their activity to a specific cellular task. Here, we describe a phosphorylation event for myosins of class VI (MYO6) in the motor domain, which accelerates its ATPase activity leading to a 4-fold increase in motor speed determined by actin-gliding assays, single molecule mechanics and stopped flow kinetics. We demonstrate that the serine/threonine kinase DYRK2 phosphorylates MYO6 at S267 in vitro. Single-molecule optical-tweezers studies at low load reveal that S267-phosphorylation results in faster nucleotide-exchange kinetics without change in the working stroke of the motor. The selective increase in stiffness of the acto-MYO6 complex when proceeding load-dependently into the nucleotide-free rigor state demonstrates that S267-phosphorylation turns MYO6 into a stronger motor. Finally, molecular dynamic simulations of the nucleotide-free motor reveal an alternative interaction network within insert-1 upon phosphorylation, suggesting a molecular mechanism, which regulates insert-1 positioning, turning the S267-phosphorylated MYO6 into a faster motor.

Localized calcium transients in phragmoplast regulate cytokinesis of tobacco BY-2 cells

KEY MESSAGE

Plants exhibit a unique pattern of cytosolic Ca2+ dynamics to correlate with microtubules to regulate cytokinesis, which significantly differs from those observed in animal and yeast cells. Calcium (Ca2+) transients mediated signaling is known to be essential in cytokinesis across eukaryotic cells. However, the detailed spatiotemporal dynamics of Ca2+ during plant cytokinesis remain largely unexplored. In this study, we employed GCaMP5, a genetically encoded Ca2+ sensor, to investigate cytokinetic Ca2+ transients during cytokinesis in Nicotiana tabacum Bright Yellow-2 (BY-2) cells. We validated the effectiveness of GCaMP5 to capture fluctuations in intracellular free Ca2+ in transgenic BY-2 cells. Our results reveal that Ca2+ dynamics during BY-2 cell cytokinesis are distinctly different from those observed in embryonic and yeast cells. It is characterized by an initial significant Ca2+ spike within the phragmoplast region. This spike is followed by a decrease in Ca2+ concentration at the onset of cytokinesis in phragmoplast, which then remains elevated in comparison to the cytosolic Ca2+ until the completion of cell plate formation. At the end of cytokinesis, Ca2+ becomes uniformly distributed in the cytosol. This pattern contrasts with the typical dual waves of Ca2+ spikes observed during cytokinesis in animal embryonic cells and fission yeasts. Furthermore, applications of pharmaceutical inhibitors for either Ca2+ or microtubules revealed a close correlation between Ca2+ transients and microtubule organization in the regulation of cytokinesis. Collectively, our findings highlight the unique dynamics and crucial role of Ca2+ transients during plant cell cytokinesis, and provides new insights into plant cell division mechanisms.

Hydrogen/Deuterium Exchange Mass Spectrometry Provides Insights into the Role of Drosophila Testis-Specific Myosin VI Light Chain AndroCaM

In Drosophila testis, myosin VI plays a special role, distinct from its motor function, by anchoring components to the unusual actin-based structures (cones) that are required for spermatid individualization. For this, the two calmodulin (CaM) light-chain molecules of myosin VI are replaced by androcam (ACaM), a related protein with 67% identity to CaM. Although ACaM has a similar bi-lobed structure to CaM, with two EF hand-type Ca2+ binding sites per lobe, only one functional Ca2+ binding site operates in the amino-terminus. To understand this light chain substitution, we used hydrogen-deuterium exchange mass spectrometry (HDX-MS) to examine dynamic changes in ACaM and CaM upon Ca2+ binding and interaction with the two CaM binding motifs of myosin VI (insert2 and IQ motif). HDX-MS reveals that binding of Ca2+ to ACaM destabilizes its N-lobe but stabilizes the entire C-lobe, whereas for CaM, Ca2+ binding induces a pattern of alternating stabilization/destabilization throughout. The conformation of this stable holo-C-lobe of ACaM seems to be a "prefigured" version of the conformation adopted by the holo-C-lobe of CaM for binding to insert2 and the IQ motif of myosin VI. Strikingly, the interaction of holo-ACaM with either peptide converts the holo-N-lobe to its Ca2+-free, more stable, form. Thus, ACaM in vivo should bind the myosin VI light chain sites in an apo-N-lobe/holo-C-lobe state that cannot fulfill the Ca2+-related functions of holo-CaM required for myosin VI motor assembly and activity. These findings indicate that inhibition of myosin VI motor activity is a precondition for transition to an anchoring function.

Autoinhibition and activation of myosin VI revealed by its cryo-EM structure

Myosin VI is the only molecular motor that moves towards the minus end along actin filaments. Numerous cellular processes require myosin VI and tight regulations of the motor's activity. Defects in myosin VI activity are known to cause genetic diseases such as deafness and cardiomyopathy. However, the molecular mechanisms underlying the activity regulation of myosin VI remain elusive. Here, we determined the high-resolution cryo-electron microscopic structure of myosin VI in its autoinhibited state. Our structure reveals that autoinhibited myosin VI adopts a compact, monomeric conformation via extensive interactions between the head and tail domains, orchestrated by an elongated single-α-helix region resembling a "spine". This autoinhibited structure effectively blocks cargo binding sites and represses the motor's ATPase activity. Certain cargo adaptors such as GIPC can release multiple inhibitory interactions and promote motor activity, pointing to a cargo-mediated activation of the processive motor. Moreover, our structural findings allow rationalization of disease-associated mutations in myosin VI. Beyond the activity regulation mechanisms of myosin VI, our study also sheds lights on how activities of other myosin motors such as myosin VII and X might be regulated.

How myosin VI traps its off-state, is activated and dimerizes

Myosin VI (Myo6) is the only minus-end directed nanomotor on actin, allowing it to uniquely contribute to numerous cellular functions. As for other nanomotors, the proper functioning of Myo6 relies on precise spatiotemporal control of motor activity via a poorly defined off-state and interactions with partners. Our structural, functional, and cellular studies reveal key features of myosin regulation and indicate that not all partners can activate Myo6. TOM1 and Dab2 cannot bind the off-state, while GIPC1 binds Myo6, releases its auto-inhibition and triggers proximal dimerization. Myo6 partners thus differentially recruit Myo6. We solved a crystal structure of the proximal dimerization domain, and show that its disruption compromises endocytosis in HeLa cells, emphasizing the importance of Myo6 dimerization. Finally, we show that the L926Q deafness mutation disrupts Myo6 auto-inhibition and indirectly impairs proximal dimerization. Our study thus demonstrates the importance of partners in the control of Myo6 auto-inhibition, localization, and activation.

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.

Ensembles of human myosin-19 bound to calmodulin and regulatory light chain RLC12B drive multimicron transport

Myosin-19 (Myo19) controls the size, morphology, and distribution of mitochondria, but the underlying role of Myo19 motor activity is unknown. Complicating mechanistic in vitro studies, the identity of the light chains (LCs) of Myo19 remains unsettled. Here, we show by coimmunoprecipitation, reconstitution, and proteomics that the three IQ motifs of human Myo19 expressed in Expi293 human cells bind regulatory light chain (RLC12B) and calmodulin (CaM). We demonstrate that overexpression of Myo19 in HeLa cells enhances the recruitment of both Myo19 and RLC12B to mitochondria, suggesting cellular association of RLC12B with the motor. Further experiments revealed that RLC12B binds IQ2 and is flanked by two CaM molecules. In vitro, we observed that the maximal speed (∼350 nm/s) occurs when Myo19 is supplemented with CaM, but not RLC12B, suggesting maximal motility requires binding of CaM to IQ-1 and IQ-3. The addition of calcium slowed actin gliding (∼200 nm/s) without an apparent effect on CaM affinity. Furthermore, we show that small ensembles of Myo19 motors attached to quantum dots can undergo processive runs over several microns, and that calcium reduces the attachment frequency and run length of Myo19. Together, our data are consistent with a model where a few single-headed Myo19 molecules attached to a mitochondrion can sustain prolonged motile associations with actin in a CaM- and calcium-dependent manner. Based on these properties, we propose that Myo19 can function in mitochondria transport along actin filaments, tension generation on multiple randomly oriented filaments, and/or pushing against branched actin networks assembled near the membrane surface.

Binding partners regulate unfolding of myosin VI to activate the molecular motor

Myosin VI is the only minus-end actin motor and it is coupled to various cellular processes ranging from endocytosis to transcription. This multi-potent nature is achieved through alternative isoform splicing and interactions with a network of binding partners. There is a complex interplay between isoforms and binding partners to regulate myosin VI. Here, we have compared the regulation of two myosin VI splice isoforms by two different binding partners. By combining biochemical and single-molecule approaches, we propose that myosin VI regulation follows a generic mechanism, independently of the spliced isoform and the binding partner involved. We describe how myosin VI adopts an autoinhibited backfolded state which is released by binding partners. This unfolding activates the motor, enhances actin binding and can subsequently trigger dimerization. We have further expanded our study by using single-molecule imaging to investigate the impact of binding partners upon myosin VI molecular organization and dynamics.

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.

The C-terminal tail extension of myosin 16 acts as a molten globule, including intrinsically disordered regions, and interacts with the N-terminal ankyrin

The lesser-known unconventional myosin 16 protein is essential in proper neuronal functioning and has been implicated in cell cycle regulation. Its longer Myo16b isoform contains a C-terminal tail extension (Myo16Tail), which has been shown to play a role in the neuronal phosphoinositide 3-kinase signaling pathway. Myo16Tail mediates the actin cytoskeleton remodeling, downregulates the actin dynamics at the postsynaptic site of dendritic spines, and is involved in the organization of the presynaptic axon terminals. However, the functional and structural features of this C-terminal tail extension are not well known. Here, we report the purification and biophysical characterization of the Myo16Tail by bioinformatics, fluorescence spectroscopy, and CD. Our results revealed that the Myo16Tail is functionally active and interacts with the N-terminal ankyrin domain of myosin 16, suggesting an intramolecular binding between the C and N termini of Myo16 as an autoregulatory mechanism involving backfolding of the motor domain. In addition, the Myo16Tail possesses high structural flexibility and a solvent-exposed hydrophobic core, indicating the largely unstructured, intrinsically disordered nature of this protein region. Some secondary structure elements were also observed, indicating that the Myo16Tail likely adopts a molten globule-like structure. These structural features imply that the Myo16Tail may function as a flexible display site particularly relevant in post-translational modifications, regulatory functions such as backfolding, and phosphoinositide 3-kinase signaling.

Dynamic multimerization of Dab2-Myosin VI complexes regulates cargo processivity while minimizing cortical actin reorganization

Myosin VI ensembles on endocytic cargo facilitate directed transport through a dense cortical actin network. Myosin VI is recruited to clathrin-coated endosomes via the cargo adaptor Dab2. Canonically, it has been assumed that the interactions between a motor and its cargo adaptor are stable. However, it has been demonstrated that the force generated by multiple stably attached motors disrupts local cytoskeletal architecture, potentially compromising transport. In this study, we demonstrate that dynamic multimerization of myosin VI-Dab2 complexes facilitates cargo processivity without significant reorganization of cortical actin networks. Specifically, we find that Dab2 myosin interacting region (MIR) binds myosin VI with a moderate affinity (184 nM) and single-molecule kinetic measurements demonstrate a high rate of turnover (1 s⁻¹) of the Dab2 MIR–myosin VI interaction. Single-molecule motility shows that saturating Dab2-MIR concentration (2 μM) promotes myosin VI homodimerization and processivity with run lengths comparable with constitutive myosin VI dimers. Cargo-mimetic DNA origami scaffolds patterned with Dab2 MIR-myosin VI complexes are weakly processive, displaying sparse motility on single actin filaments and “stop-and-go” motion on a cellular actin network. On a minimal actin cortex assembled on lipid bilayers, unregulated processive movement by either constitutive myosin V or VI dimers results in actin remodeling and foci formation. In contrast, Dab2 MIR–myosin VI interactions preserve the integrity of a minimal cortical actin network. Taken together, our study demonstrates the importance of dynamic motor–cargo association in enabling cargo transportation without disrupting cytoskeletal organization.

Myomics: myosin VI structural and functional plasticity

Myosin VI is a minus end-directed actin motor protein that fulfils several roles in the cell. The interaction of myosin VI with its cellular cargoes is dictated by the presence of binding domains at the C-terminus of the protein. In this review, we describe how alternative splicing and structural and conformational changes modulate the plasticity of the myosin VI interactome. Recent findings highlight how the various partners can cooperate or compete for binding to allow a precise temporal and spatial regulation of myosin VI recruitment to different cellular compartments, where its motor or anchor function is needed.

Myosin XVI in the Nervous System

The myosin family is a large inventory of actin-associated motor proteins that participate in a diverse array of cellular functions. Several myosin classes are expressed in neural cells and play important roles in neural functioning. A recently discovered member of the myosin superfamily, the vertebrate-specific myosin XVI (Myo16) class is expressed predominantly in neural tissues and appears to be involved in the development and proper functioning of the nervous system. Accordingly, the alterations of MYO16 has been linked to neurological disorders. Although the role of Myo16 as a generic actin-associated motor is still enigmatic, the N-, and C-terminal extensions that flank the motor domain seem to confer unique structural features and versatile interactions to the protein. Recent biochemical and physiological examinations portray Myo16 as a signal transduction element that integrates cell signaling pathways to actin cytoskeleton reorganization. This review discusses the current knowledge of the structure-function relation of Myo16. In light of its prevalent localization, the emphasis is laid on the neural aspects.

Myosins and Disease

Myosins constitute a superfamily of actin-based molecular motor proteins that mediates a variety of cellular activities including muscle contraction, cell migration, intracellular transport, the formation of membrane projections, cell adhesion, and cell signaling. The 12 myosin classes that are expressed in humans share sequence similarities especially in the N-terminal motor domain; however, their enzymatic activities, regulation, ability to dimerize, binding partners, and cellular functions differ. It is becoming increasingly apparent that defects in myosins are associated with diseases including cardiomyopathies, colitis, glomerulosclerosis, neurological defects, cancer, blindness, and deafness. Here, we review the current state of knowledge regarding myosins and disease.

Exploiting the protein corona: coating of black phosphorus nanosheets enables macrophage polarization via calcium influx

Black phosphorus nanosheets (BPNSs) have substantially promoted biomedical nanotechnology due to their unique photothermal and chemotherapeutic properties. However, there is still a limited molecular understanding of the effects of bio-nano interfaces on BPNSs and the subsequent impacts on physiological systems. Here, it is shown that black phosphorus-corona complexes (BPCCs) could function as immune modulators to promote the polarization of macrophages. Mechanistically, BPCCs could interact with calmodulin to activate stromal interaction molecule 2 and facilitate Ca2+ influx in macrophages, which induced the activation of p38 and NF-κB and polarized M0 macrophages to the M1 phenotype. As a result, BPCC-activated macrophages show greater migration towards cancer cells, 1.3-1.9 times higher cellular cytotoxicity and effective phagocytosis of cancer cells. These findings offer insights into the development of potential and unique applications of corona on BPNSs in nanomedicine.

Approaches to Identify and Characterise MYO6-Cargo Interactions

Given the prevalence and importance of the actin cytoskeleton and the host of associated myosin motors, it comes as no surprise to find that they are linked to a plethora of cellular functions and pathologies. Although our understanding of the biophysical properties of myosin motors has been aided by the high levels of conservation in their motor domains and the extensive work on myosin in skeletal muscle contraction, our understanding of how the nonmuscle myosins participate in such a wide variety of cellular processes is less clear. It is now well established that the highly variable myosin tails are responsible for targeting these myosins to distinct cellular sites for specific functions, and although a number of adaptor proteins have been identified, our current understanding of the cellular processes involved is rather limited. Furthermore, as more adaptor proteins, cargoes and complexes are identified, the importance of elucidating the regulatory mechanisms involved is essential. Ca2+, and now phosphorylation and ubiquitination, are emerging as important regulators of cargo binding, and it is likely that other post-translational modifications are also involved. In the case of myosin VI (MYO6), a number of immediate binding partners have been identified using traditional approaches such as yeast two-hybrid screens and affinity-based pull-downs. However, these methods have only been successful in identifying the cargo adaptors, but not the cargoes themselves, which may often comprise multi-protein complexes. Furthermore, motor-adaptor-cargo interactions are dynamic by nature and often weak, transient and highly regulated and therefore difficult to capture using traditional affinity-based methods. In this chapter we will discuss the various approaches including functional proteomics that have been used to uncover and characterise novel MYO6-associated proteins and complexes and how this work contributes to a fuller understanding of the targeting and function(s) of this unique myosin motor.

Unconventional Myosins: How Regulation Meets Function

Unconventional myosins are multi-potent molecular motors that are assigned important roles in fundamental cellular processes. Depending on their mechano-enzymatic properties and structural features, myosins fulfil their roles by acting as cargo transporters along the actin cytoskeleton, molecular anchors or tension sensors. In order to perform such a wide range of roles and modes of action, myosins need to be under tight regulation in time and space. This is achieved at multiple levels through diverse regulatory mechanisms: the alternative splicing of various isoforms, the interaction with their binding partners, their phosphorylation, their applied load and the composition of their local environment, such as ions and lipids. This review summarizes our current knowledge of how unconventional myosins are regulated, how these regulatory mechanisms can adapt to the specific features of a myosin and how they can converge with each other in order to ensure the required tight control of their function.

Regulation of KIF1A-Driven Dense Core Vesicle Transport: Ca2+/CaM Controls DCV Binding and Liprin-α/TANC2 Recruits DCVs to Postsynaptic Sites

Tight regulation of neuronal transport allows for cargo binding and release at specific cellular locations. The mechanisms by which motor proteins are loaded on vesicles and how cargoes are captured at appropriate sites remain unclear. To better understand how KIF1A-driven dense core vesicle (DCV) transport is regulated, we identified the KIF1A interactome and focused on three binding partners, the calcium binding protein calmodulin (CaM) and two synaptic scaffolding proteins: liprin-α and TANC2. We showed that calcium, acting via CaM, enhances KIF1A binding to DCVs and increases vesicle motility. In contrast, liprin-α and TANC2 are not part of the KIF1A-cargo complex but capture DCVs at dendritic spines. Furthermore, we found that specific TANC2 mutations—reported in patients with different neuropsychiatric disorders—abolish the interaction with KIF1A. We propose a model in which Ca²⁺/CaM regulates cargo binding and liprin-α and TANC2 recruit KIF1A-transported vesicles.

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KIF1A directly interacts with CaM and with the scaffolds liprin-α and TANC2

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KIF1A is regulated by a Ca²⁺/CaM-dependent mechanism, which allows for DCV loading

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Liprin-α and TANC2 are static PSD proteins that are not part of the KIF1A-DCV complex

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KIF1A-driven DCVs are recruited to dendritic spines by liprin-α and TANC2

Clathrin light chain A drives selective myosin VI recruitment to clathrin-coated pits under membrane tension

Clathrin light chains (CLCa and CLCb) are major constituents of clathrin-coated vesicles. Unique functions for these evolutionary conserved paralogs remain elusive, and their role in clathrin-mediated endocytosis in mammalian cells is debated. Here, we find and structurally characterize a direct and selective interaction between CLCa and the long isoform of the actin motor protein myosin VI, which is expressed exclusively in highly polarized tissues. Using genetically-reconstituted Caco-2 cysts as proxy for polarized epithelia, we provide evidence for coordinated action of myosin VI and CLCa at the apical surface where these proteins are essential for fission of clathrin-coated pits. We further find that myosin VI and Huntingtin-interacting protein 1-related protein (Hip1R) are mutually exclusive interactors with CLCa, and suggest a model for the sequential function of myosin VI and Hip1R in actin-mediated clathrin-coated vesicle budding.

Reconstitution reveals how myosin-VI self-organises to generate a dynamic mechanism of membrane sculpting

One enigma in biology is the generation, sensing and maintenance of membrane curvature. Curvature-mediating proteins have been shown to induce specific membrane shapes by direct insertion and nanoscopic scaffolding, while the cytoskeletal motors exert forces indirectly through microtubule and actin networks. It remains unclear, whether the manifold direct motorprotein-lipid interactions themselves constitute another fundamental route to remodel the membrane shape. Here we show, combining super-resolution-fluorescence microscopy and membrane-reshaping nanoparticles, that curvature-dependent lipid interactions of myosin-VI on its own, remarkably remodel the membrane geometry into dynamic spatial patterns on the nano- to micrometer scale. We propose a quantitative theoretical model that explains this dynamic membrane sculpting mechanism. The emerging route of motorprotein-lipid interactions reshaping membrane morphology by a mechanism of feedback and instability opens up hitherto unexplored avenues of membrane remodelling and links cytoskeletal motors to early events in the sequence of membrane sculpting in eukaryotic cell biology.

The MYO6 interactome: selective motor-cargo complexes for diverse cellular processes

Myosins of class VI (MYO6) are unique actin-based motor proteins that move cargo towards the minus ends of actin filaments. As the sole myosin with this directionality, it is critically important in a number of biological processes. Indeed, loss or overexpression of MYO6 in humans is linked to a variety of pathologies including deafness, cardiomyopathy, neurodegenerative diseases as well as cancer. This myosin interacts with a wide variety of direct binding partners such as for example the selective autophagy receptors optineurin, TAX1BP1 and NDP52 and also Dab2, GIPC, TOM1 and LMTK2, which mediate distinct functions of different MYO6 isoforms along the endocytic pathway. Functional proteomics has recently been used to identify the wider MYO6 interactome including several large functionally distinct multi-protein complexes, which highlight the importance of this myosin in regulating the actin and septin cytoskeleton. Interestingly, adaptor-binding not only triggers cargo attachment, but also controls the inactive folded conformation and dimerisation of MYO6. Thus, the C-terminal tail domain mediates cargo recognition and binding, but is also crucial for modulating motor activity and regulating cytoskeletal track dynamics.

Cellular effects of diquat dibromide exposure: Interference with Wnt signaling and cytoskeletal development

The herbicidal action of diquat dibromide (DD) on plant cells is due primarily to the initiation of reactive oxygen species (ROS) formation, lipoperoxidation, and apoptotic cell death. It has been demonstrated that oxidative stress also occurs in animal cells exposed to high concentrations of DD; however, observations of DD’s effects on animal cells at concentrations below the reported ROS-initiation threshold suggest that some of these effects may not be attributable to ROS-induced cell death. Our results suggest that DD causes disruption of the Wnt pathway, calcium dysregulation, and cytoskeletal damage during development. Using embryos of the pond snail Lymnaea palustris as our model organism, we observed increased mortality, developmental delay and abnormality, altered motility, calcium dysregulation, decreased heart rate, and arrhythmia in embryos exposed to DD. Sperm extracted from adult snails that were exposed to DD exhibit altered motility, increased abundance, and high mortality. Effects were quantified via real-time imaging, heart rate assessment, flow cytometry, and mortality scoring. We propose that there are two models for the mechanism of DD’s action in animal cells: at low concentrations (≤28 µg/L), apoptotic cell death does not occur, but cytoskeletal elements, calcium regulation, and Wnt signaling are compromised, causing irreversible damage in L. palustris embryos; such damage is partially remediated with antioxidants or lithium chloride. At high concentrations of DD (≥44.4 µg/L), calcium dysregulation may be triggered, leading to the establishment of an intracellular positive feedback loop of ROS formation in the mitochondria, calcium release from the endoplasmic reticulum, calcium efflux, and apoptotic cell death. Permanent cellular damage occurring from exposure to sublethal concentrations of this widespread herbicide underscores the importance of research that elucidates the mechanism of DD on nontarget organisms.

The MYO6 interactome reveals adaptor complexes coordinating early endosome and cytoskeletal dynamics

The intracellular functions of myosin motors requires a number of adaptor molecules, which control cargo attachment, but also fine-tune motor activity in time and space. These motor-adaptor-cargo interactions are often weak, transient or highly regulated. To overcome these problems, we use a proximity labelling-based proteomics strategy to map the interactome of the unique minus end-directed actin motor MYO6. Detailed biochemical and functional analysis identified several distinct MYO6-adaptor modules including two complexes containing RhoGEFs: the LIFT (LARG-Induced F-actin for Tethering) complex that controls endosome positioning and motility through RHO-driven actin polymerisation; and the DISP (DOCK7-Induced Septin disPlacement) complex, a novel regulator of the septin cytoskeleton. These complexes emphasise the role of MYO6 in coordinating endosome dynamics and cytoskeletal architecture. This study provides the first in vivo interactome of a myosin motor protein and highlights the power of this approach in uncovering dynamic and functionally diverse myosin motor complexes.

Intracellular calcium signal at the leading edge regulates mesodermal sheet migration during Xenopus gastrulation

During the gastrulation stage in animal embryogenesis, the cells leading the axial mesoderm migrate toward the anterior side of the embryo, vigorously extending cell protrusions such as lamellipodia. It is thought that the leading cells sense gradients of chemoattractants emanating from the ectodermal cells and translate them to initiate and maintain the cell movements necessary for gastrulation. However, it is unclear how the extracellular information is converted to the intracellular chemical reactions that lead to motion. Here we demonstrated that intracellular Ca2+ levels in the protrusion-forming leading cells are markedly higher than those of the following cells and the axial mesoderm cells. We also showed that inhibiting the intracellular Ca2+ significantly retarded the gastrulation cell movements, while increasing the intracellular Ca2+ with an ionophore enhanced the migration. We further found that the ionophore treatment increased the active form of the small GTPase Rac1 in these cells. Our results suggest that transient intracellular Ca2+ signals play an essential role in the active cell migration during gastrulation.

NDP52 activates nuclear myosin VI to enhance RNA polymerase II transcription

Myosin VI (MVI) has been found to be overexpressed in ovarian, breast and prostate cancers. Moreover, it has been shown to play a role in regulating cell proliferation and migration, and to interact with RNA Polymerase II (RNAPII). Here, we find that backfolding of MVI regulates its ability to bind DNA and that a putative transcription co-activator NDP52 relieves the auto-inhibition of MVI to enable DNA binding. Additionally, we show that the MVI-NDP52 complex binds RNAPII, which is critical for transcription, and that depletion of NDP52 or MVI reduces steady-state mRNA levels. Lastly, we demonstrate that MVI directly interacts with nuclear receptors to drive expression of target genes, thereby suggesting a link to cell proliferation and migration. Overall, we suggest MVI may function as an auxiliary motor to drive transcription.

Ca2+-Induced Rigidity Change of the Myosin VIIa IQ Motif-Single α Helix Lever Arm Extension

Several unconventional myosins contain a highly charged single α helix (SAH) immediately following the calmodulin (CaM) binding IQ motifs, functioning to extend lever arms of these myosins. How such SAH is connected to the IQ motifs and whether the conformation of the IQ motifs-SAH segments are regulated by Ca²⁺ fluctuations are not known. Here, we demonstrate by solving its crystal structure that the predicted SAH of myosin VIIa (Myo7a) forms a stable SAH. The structure of Myo7a IQ5-SAH segment in complex with apo-CaM reveals that the SAH sequence can extend the length of the Myo7a lever arm. Although Ca²⁺-CaM remains bound to IQ5-SAH, the Ca²⁺-induced CaM binding mode change softens the conformation of the IQ5-SAH junction, revealing a Ca²⁺-induced lever arm flexibility change for Myo7a. We further demonstrate that the last IQ motif of several other myosins also binds to both apo- and Ca²⁺-CaM, suggesting a common Ca²⁺-induced conformational regulation mechanism.

Investigations of human myosin VI targeting using optogenetically controlled cargo loading

Myosins play countless critical roles in the cell, each requiring it to be activated at a specific location and time. To control myosin VI with this specificity, we created an optogenetic tool for activating myosin VI by fusing the light-sensitive Avena sativa phototropin1 LOV2 domain to a peptide from Dab2 (LOVDab), a myosin VI cargo protein. Our approach harnesses the native targeting and activation mechanism of myosin VI, allowing direct inferences on myosin VI function. LOVDab robustly recruits human full-length myosin VI to various organelles in vivo and hinders peroxisome motion in a light-controllable manner. LOVDab also activates myosin VI in an in vitro gliding filament assay. Our data suggest that protein and lipid cargoes cooperate to activate myosin VI, allowing myosin VI to integrate Ca²⁺, lipid, and protein cargo signals in the cell to deploy in a site-specific manner.

Filopodia formation and endosome clustering induced by mutant plus-end-directed myosin VI

Myosin VI (MYO6) is the only myosin known to move toward the minus end of actin filaments. It has roles in numerous cellular processes, including maintenance of stereocilia structure, endocytosis, and autophagosome maturation. However, the functional necessity of minus-end-directed movement along actin is unclear as the underlying architecture of the local actin network is often unknown. To address this question, we engineered a mutant of MYO6, MYO6+, which undergoes plus-end-directed movement while retaining physiological cargo interactions in the tail. Expression of this mutant motor in HeLa cells led to a dramatic reorganization of cortical actin filaments and the formation of actin-rich filopodia. MYO6 is present on peripheral adaptor protein, phosphotyrosine interacting with PH domain and leucine zipper 1 (APPL1) signaling endosomes and MYO6+ expression causes a dramatic relocalization and clustering of this endocytic compartment in the cell cortex. MYO6+ and its adaptor GAIP interacting protein, C terminus (GIPC) accumulate at the tips of these filopodia, while APPL1 endosomes accumulate at the base. A combination of MYO6+ mutagenesis and siRNA-mediated depletion of MYO6 binding partners demonstrates that motor activity and binding to endosomal membranes mediated by GIPC and PI(4,5)P₂ are crucial for filopodia formation. A similar reorganization of actin is induced by a constitutive dimer of MYO6+, indicating that multimerization of MYO6 on endosomes through binding to GIPC is required for this cellular activity and regulation of actin network structure. This unique engineered MYO6+ offers insights into both filopodia formation and MYO6 motor function at endosomes and at the plasma membrane.

Self-organization of actin networks by a monomeric myosin

The organization of actomyosin networks lies at the center of many types of cellular motility, including cell polarization and collective cell migration during development and morphogenesis. Myosin-IXa is critically involved in these processes. Using total internal reflection fluorescence microscopy, we resolved actin bundles assembled by myosin-IXa. Electron microscopic data revealed that the bundles consisted of highly ordered lattices with parallel actin polarity. The myosin-IXa motor domains aligned across the network, forming cross-links at a repeat distance of precisely 36 nm, matching the helical repeat of actin. Single-particle image processing resolved three distinct conformations of myosin-IXa in the absence of nucleotide. Using cross-correlation of a modeled actomyosin crystal structure, we identified sites of additional mass, which can only be accounted for by the large insert in loop 2 exclusively found in the motor domain of class IX myosins. We show that the large insert in loop 2 binds calmodulin and creates two coordinated actin-binding sites that constrain the actomyosin interactions generating the actin lattices. The actin lattices introduce orientated tracks at specific sites in the cell, which might install platforms allowing Rho-GTPase-activating protein (RhoGAP) activity to be focused at a definite locus. In addition, the lattices might introduce a myosin-related, force-sensing mechanism into the cytoskeleton in cell polarization and collective cell migration.

Multifaceted Roles of ALG-2 in Ca(2+)-Regulated Membrane Trafficking

ALG-2 (gene name: PDCD6) is a penta-EF-hand Ca²⁺-binding protein and interacts with a variety of proteins in a Ca²⁺-dependent fashion. ALG-2 recognizes different types of identified motifs in Pro-rich regions by using different hydrophobic pockets, but other unknown modes of binding are also used for non-Pro-rich proteins. Most ALG-2-interacting proteins associate directly or indirectly with the plasma membrane or organelle membranes involving the endosomal sorting complex required for transport (ESCRT) system, coat protein complex II (COPII)-dependent ER-to-Golgi vesicular transport, and signal transduction from membrane receptors to downstream players. Binding of ALG-2 to targets may induce conformational change of the proteins. The ALG-2 dimer may also function as a Ca²⁺-dependent adaptor to bridge different partners and connect the subnetwork of interacting proteins.

Loss of cargo binding in the human myosin VI deafness mutant (R1166X) leads to increased actin filament binding

Mutations in myosin VI have been associated with autosomal-recessive (DFNB37) and autosomal-dominant (DFNA22) deafness in humans. Here, we characterise an myosin VI nonsense mutation (R1166X) that was identified in a family with hereditary hearing loss in Pakistan. This mutation leads to the deletion of the C-terminal 120 amino acids of the myosin VI cargo-binding domain, which includes the WWY-binding motif for the adaptor proteins LMTK2, Tom1 as well as Dab2. Interestingly, compromising myosin VI vesicle-binding ability by expressing myosin VI with the R1166X mutation or with single point mutations in the adaptor-binding sites leads to increased F-actin binding of this myosin in vitro and in vivo As our results highlight the importance of cargo attachment for regulating actin binding to the motor domain, we perform a detailed characterisation of adaptor protein binding and identify single amino acids within myosin VI required for binding to cargo adaptors. We not only show that the adaptor proteins can directly interact with the cargo-binding tail of myosin VI, but our in vitro studies also suggest that multiple adaptor proteins can bind simultaneously to non-overlapping sites in the myosin VI tail. In conclusion, our characterisation of the human myosin VI deafness mutant (R1166X) suggests that defects in cargo binding may leave myosin VI in a primed/activated state with an increased actin-binding ability.

Mechanics and Activation of Unconventional Myosins

Many types of cellular motility are based on the myosin family of motor proteins ranging from muscle contraction to exo- and endocytosis, cytokinesis, cell locomotion or signal transduction in hearing. At the center of this wide range of motile processes lies the adaptation of the myosins for each specific mechanical task and the ability to coordinate the timing of motor protein mobilization and targeting. In recent years, great progress has been made in developing single molecule technology to characterize the diverse mechanical properties of the unconventional myosins. Here, we discuss the basic mechanisms and mechanical adaptations of unconventional myosins, and emerging principles regulating motor mobilization and targeting.

Interaction of myosin VI and its binding partner DOCK7 plays an important role in NGF-stimulated protrusion formation in PC12 cells

DOCK7 (dedicator of cytokinesis 7) is a guanidine nucleotide exchange factor (GEF) for Rac1 GTPase that is involved in neuronal polarity and axon generation as well in Schwann cell differentiation and myelination. Recently, we identified DOCK7 as the binding partner of unconventional myosin VI (MVI) in neuronal-lineage PC12 cells and postulated that this interaction could be important in vivo [Majewski et al. (2012) Biochem Cell Biol., 90:565-574]. Herein, we found that MVI-DOCK7 interaction takes also place in other cell lines and demonstrated that MVI cargo domain via its RRL motif binds to DOCK7 C-terminal M2 and DHR2 domains. In MVI knockdown cells, lower Rac1 activity and a decrease of DOCK7 phosphorylation on Tyr1118 were observed, indicating that MVI could contribute to DOCK7 activity. MVI and DOCK7 co-localization was maintained during NGF-stimulated PC12 cell differentiation and observed also in the outgrowths. Also, during differentiation an increase in phosphorylation of DOCK7 as well as of its downstream effector JNK kinase was detected. Interestingly, overexpression of GFP-tagged MVI cargo domain (GFP-GT) impaired protrusion formation indicating that full length protein is important for this process. Moreover, a transient increase in Rac activity observed at 5min of NGF-stimulated differentiation of PC12 cells (overexpressing either GFP or GFP-MVI) was not detected in cells overexpressing the cargo domain. These data indicate that MVI-DOCK7 interaction could have functional implications in the protrusion outgrowth, and full length MVI seems to be important for delivery and maintenance of DOCK7 along the protrusions, and exerting its GEF activity.