MHC-IIB filament assembly and cellular localization are governed by the rod net charge

Synaptopodin stress fiber and contractomere at the epithelial junction

The apical junction of epithelial cells can generate force to control cell geometry and perform contractile processes while maintaining barrier function and adhesion. Yet, the structural basis for force generation at the apical junction is not fully understood. Here, we describe two synaptopodin-dependent actomyosin structures that are spatially, temporally, and structurally distinct. The first structure is formed by the retrograde flow of synaptopodin initiated at the apical junction, creating a sarcomeric stress fiber that lies parallel to the apical junction. Contraction of the apical stress fiber is associated with either clustering of membrane components or shortening of junctional length. Upon junction maturation, apical stress fibers are disassembled. In mature epithelial monolayer, a motorized “contractomere” capable of “walking the junction” is formed at the junctional vertex. Actomyosin activities at the contractomere produce a compressive force evident by actin filament buckling and measurement with a new α-actinin-4 force sensor. The motility of contractomeres can adjust junctional length and change cell packing geometry during cell extrusion and intercellular movement. We propose a model of epithelial homeostasis that utilizes contractomere motility to support junction rearrangement while preserving the permeability barrier.

Aurora-B phosphorylates the myosin II heavy chain to promote cytokinesis

Cytokinesis, the final step of mitosis, is mediated by an actomyosin contractile ring, the formation of which is temporally and spatially regulated following anaphase onset. Aurora-B is a member of the chromosomal passenger complex, which regulates various processes during mitosis; it is not understood, however, how Aurora-B is involved in cytokinesis. Here, we show that Aurora-B and myosin-IIB form a complex in vivo during telophase. Aurora-B phosphorylates the myosin-IIB rod domain at threonine 1847 (T¹⁸⁴⁷), abrogating the ability of myosin-IIB monomers to form filaments. Furthermore, phosphorylation of myosin-IIB filaments by Aurora-B also promotes filament disassembly. We show that myosin-IIB possessing a phosphomimetic mutation at T¹⁸⁴⁷ was unable to rescue cytokinesis failure caused by myosin-IIB depletion. Cells expressing a phosphoresistant mutation at T¹⁸⁴⁷ had significantly longer intercellular bridges, implying that Aurora-B-mediated phosphorylation of myosin-IIB is important for abscission. We propose that myosin-IIB is a substrate of Aurora-B and reveal a new mechanism of myosin-IIB regulation by Aurora-B in the late stages of mitosis.

Two Motors and One Spring: Hypothetic Roles of Non-Muscle Myosin II and Submembrane Actin-Based Cytoskeleton in Cell Volume Sensing

Changes in plasma membrane curvature and intracellular ionic strength are two key features of cell volume perturbations. In this hypothesis we present a model of the responsible molecular apparatus which is assembled of two molecular motors [non-muscle myosin II (NMMII) and protrusive actin polymerization], a spring [a complex between the plasma membrane (PM) and the submembrane actin-based cytoskeleton (smACSK) which behaves like a viscoelastic solid] and the associated signaling proteins. We hypothesize that this apparatus senses changes in both the plasma membrane curvature and the ionic strength and in turn activates signaling pathways responsible for regulatory volume increase (RVI) and regulatory volume decrease (RVD). During cell volume changes hydrostatic pressure (HP) changes drive alterations in the cell membrane curvature. HP difference has opposite directions in swelling versus shrinkage, thus allowing distinction between them. By analogy with actomyosin contractility that appears to sense stiffness of the extracellular matrix we propose that NMMII and actin polymerization can actively probe the transmembrane gradient in HP. Furthermore, NMMII and protein-protein interactions in the actin cortex are sensitive to ionic strength. Emerging data on direct binding to and regulating activities of transmembrane mechanosensors by NMMII and actin cortex provide routes for signal transduction from transmembrane mechanosensors to cell volume regulatory mechanisms.

Electrostatic and bending energies predict staggering and splaying in nonmuscle myosin II minifilaments

Recent experiments with super-resolution live cell microscopy revealed that nonmuscle myosin II minifilaments are much more dynamic than formerly appreciated, often showing plastic processes such as splitting, concatenation and stacking. Here we combine sequence information, electrostatics and elasticity theory to demonstrate that the parallel staggers at 14.3, 43.2 and 72 nm have a strong tendency to splay their heads away from the minifilament, thus potentially initiating the diverse processes seen in live cells. In contrast, the straight antiparallel stagger with an overlap of 43 nm is very stable and likely initiates minifilament nucleation. Using stochastic dynamics in a newly defined energy landscape, we predict that the optimal parallel staggers between the myosin rods are obtained by a trial-and-error process in which two rods attach and re-attach at different staggers by rolling and zipping motion. The experimentally observed staggers emerge as the configurations with the largest contact times. We find that contact times increase from isoforms C to B to A, that A-B-heterodimers are surprisingly stable and that myosin 18A should incorporate into mixed filaments with a small stagger. Our findings suggest that nonmuscle myosin II minifilaments in the cell are first formed by isoform A and then convert to mixed A-B-filaments, as observed experimentally.

Nonmuscle myosin II (NM2) is a non-processive molecular motor that assembles into minifilaments with a typical size of 300 nm to generate force and motion in the actin cytoskeleton. This process is essential for many cellular processes such as adhesion, migration, division and mechanosensing. Due to their small size below the resolution limit, minifilaments are a challenge for imaging with traditional light microscopy. With the advent of super-resolution microscopy, however, it has become apparent that the formation of NM2-minifilaments is much more dynamic than formerly appreciated. Modelling the electrostatic interaction between the rigid rods of the myosin monomers has confirmed the main staggers observed in experiments, but cannot explain these high dynamics. Here we complement electrostatics by elasticity theory and stochastic dynamics to show that the parallel staggers are likely to splay away from the main axis of the minifilament and that monomers attach and detach with rolling and zipping motions. Based on the sequences of the different NM2-isoforms, we predict that isoform A forms the most stable homofilaments and that A-B-heterofilaments are also very stable.

Common and Specific Functions of Nonmuscle Myosin II Paralogs in Cells

Various forms of cell motility critically depend on pushing, pulling, and resistance forces generated by the actin cytoskeleton. Whereas pushing forces largely depend on actin polymerization, pulling forces responsible for cell contractility and resistance forces maintaining the cell shape require interaction of actin filaments with the multivalent molecular motor myosin II. In contrast to muscle-specific myosin II paralogs, nonmuscle myosin II (NMII) functions in virtually all mammalian cells, where it executes numerous mechanical tasks. NMII is expressed in mammalian cells as a tissue-specific combination of three paralogs, NMIIA, NMIIB, and NMIIC. Despite overall similarity, these paralogs differ in their molecular properties, which allow them to play both unique and common roles. Importantly, the three paralogs can also cooperate with each other by mixing and matching their unique capabilities. Through specialization and cooperation, NMII paralogs together execute a great variety of tasks in many different cell types. Here, we focus on mammalian NMII paralogs and review novel aspects of their kinetics, regulation, and functions in cells from the perspective of how distinct features of the three myosin II paralogs adapt them to perform specialized and joint tasks in the cells.

Multiple S100 protein isoforms and C-terminal phosphorylation contribute to the paralog-selective regulation of nonmuscle myosin 2 filaments

Nonmuscle myosin 2 (NM2) has three paralogs in mammals, NM2A, NM2B, and NM2C, which have both unique and overlapping functions in cell migration, formation of cell-cell adhesions, and cell polarity. Their assembly into homo- and heterotypic bipolar filaments in living cells is primarily regulated by phosphorylation of the N-terminally bound regulatory light chain. Here, we present evidence that the equilibrium between these filaments and single NM2A and NM2B molecules can be controlled via S100 calcium-binding protein interactions and phosphorylation at the C-terminal end of the heavy chains. Furthermore, we show that in addition to S100A4, other members of the S100 family can also mediate disassembly of homotypic NM2A filaments. Importantly, these proteins can selectively remove NM2A molecules from heterotypic filaments. We also found that tail phosphorylation (at Ser-1956 and Ser-1975) of NM2B by casein kinase 2, as well as phosphomimetic substitutions at sites targeted by protein kinase C (PKC) and transient receptor potential cation channel subfamily M member 7 (TRPM7), down-regulates filament assembly in an additive fashion. Tail phosphorylation of NM2A had a comparatively minor effect on filament stability. S100 binding and tail phosphorylation therefore preferentially disassemble NM2A and NM2B, respectively. These two distinct mechanisms are likely to contribute to the temporal and spatial sorting of the two NM2 paralogs within heterotypic filaments. The existence of multiple NM2A-depolymerizing S100 paralogs offers the potential for diverse regulatory inputs modulating NM2A filament disassembly in cells and provides functional redundancy under both physiological and pathological conditions.

Mammalian nonmuscle myosin II comes in three flavors

Nonmuscle myosin II is an actin-based motor that executes numerous mechanical tasks in cells including spatiotemporal organization of the actin cytoskeleton, adhesion, migration, cytokinesis, tissue remodeling, and membrane trafficking. Nonmuscle myosin II is ubiquitously expressed in mammalian cells as a tissue-specific combination of three paralogs. Recent studies reveal novel specific aspects of their kinetics, intracellular regulation and functions. On the other hand, the three paralogs also can copolymerize and cooperate in cells. Here we review the recent advances from the prospective of how distinct features of the three myosin II paralogs adapt them to perform specialized and joint tasks in the cell.

Effect of ATP and regulatory light-chain phosphorylation on the polymerization of mammalian nonmuscle myosin II

Addition of 1 mM ATP substantially reduces the light scattering of solutions of polymerized unphosphorylated nonmuscle myosin IIs (NM2s), and this is reversed by phosphorylation of the regulatory light chain (RLC). It has been proposed that these changes result from substantial depolymerization of unphosphorylated NM2 filaments to monomers upon addition of ATP, and filament repolymerization upon RLC-phosphorylation. We now show that the differences in myosin monomer concentration of RLC-unphosphorylated and -phosphorylated recombinant mammalian NM2A, NM2B, and NM2C polymerized in the presence of ATP are much too small to explain their substantial differences in light scattering. Rather, we find that the decrease in light scattering upon addition of ATP to polymerized unphosphorylated NM2s correlates with the formation of dimers, tetramers, and hexamers, in addition to monomers, an increase in length, and decrease in width of the bare zones of RLC-unphosphorylated filaments. Both effects of ATP addition are reversed by phosphorylation of the RLC. Our data also suggest that, contrary to previous models, assembly of RLC-phosphorylated NM2s at physiological ionic strength proceeds from folded monomers to folded antiparallel dimers, tetramers, and hexamers that unfold and polymerize into antiparallel filaments. This model could explain the dynamic relocalization of NM2 filaments in vivo by dephosphorylation of RLC-phosphorylated filaments, disassembly of the dephosphorylated filaments to folded monomers, dimers, and small oligomers, followed by diffusion of these species, and reassembly of filaments at the new location following rephosphorylation of the RLC.

Myosin II isoform co-assembly and differential regulation in mammalian systems

Non-muscle myosin 2 (NM2) is a major force-producing, actin-based motor in mammalian non-muscle cells, where it plays important roles in a broad range of fundamental biological processes, including cytokinesis, cell migration, and epithelial barrier function. This breadth of function at the tissue and cellular levels suggests extensive diversity and differential regulation of NM2 bipolar filaments, the major, if not sole, functional form of NM2s in vivo. Previous in vitro, cellular and animal studies indicate that some of this diversity is supported by the existence of multiple NM2 isoforms. Moreover, two recent studies have shown that these isoforms can co-assemble to form heterotypic filaments, further expanding functional diversity. In addition to isoform co-assembly, cells may differentially regulate NM2 function via isoform-specific expression, RLC phosphorylation, MHC phosphorylation or regulation via binding partners. Here, we provide a brief summary of NM2 filament assembly, summarize the recent findings regarding NM2 isoform co-assembly, consider the mechanisms cells might utilize to differentially regulate NM2 isoforms, and review the data available to support these mechanisms.

The heavy chain has its day: regulation of myosin-II assembly

Nonmuscle myosin-II is an actin-based motor that converts chemical energy into force and movement, and thus functions as a key regulator of the eukaryotic cytoskeleton. Although it is established that phosphorylation on the regulatory light chain increases the actin-activated MgATPase activity of the motor and promotes myosin-II filament assembly, studies have begun to characterize alternative mechanisms that regulate filament assembly and disassembly. These investigations have revealed that all three nonmuscle myosin-II isoforms are subject to additional regulatory controls, which impact diverse cellular processes. In this review, we discuss current knowledge on mechanisms that regulate the oligomerization state of nonmuscle myosin-II filaments by targeting the myosin heavy chain.

Nonmuscle myosin II isoforms coassemble in living cells

Nonmuscle myosin II (NM II) powers myriad developmental and cellular processes, including embryogenesis, cell migration, and cytokinesis [1]. To exert its functions, monomers of NM II assemble into bipolar filaments that produce a contractile force on the actin cytoskeleton. Mammalian cells express up to three isoforms of NM II (NM IIA, IIB, and IIC), each of which possesses distinct biophysical properties and supports unique as well as redundant cellular functions [2-8]. Despite previous efforts [9-13], it remains unclear whether NM II isoforms assemble in living cells to produce mixed (heterotypic) bipolar filaments or whether filaments consist entirely of a single isoform (homotypic). We addressed this question using fluorescently tagged versions of NM IIA, IIB, and IIC, isoform-specific immunostaining of the endogenous proteins, and two-color total internal reflection fluorescence structured-illumination microscopy, or TIRF-SIM, to visualize individual myosin II bipolar filaments inside cells. We show that NM II isoforms coassemble into heterotypic filaments in a variety of settings, including various types of stress fibers, individual filaments throughout the cell, and the contractile ring. We also show that the differential distribution of NM IIA and NM IIB typically seen in confocal micrographs of well-polarized cells is reflected in the composition of individual bipolar filaments. Interestingly, this differential distribution is less pronounced in freshly spread cells, arguing for the existence of a sorting mechanism acting over time. Together, our work argues that individual NM II isoforms are potentially performing both isoform-specific and isoform-redundant functions while coassembled with other NM II isoforms.

Characterization of three full-length human nonmuscle myosin II paralogs

Nonmuscle myosin IIs (NM IIs) are a group of molecular motors involved in a wide variety of cellular processes including cytokinesis, migration, and control of cell morphology. There are three paralogs of the NM II heavy chain in humans (IIA, IIB, and IIC), each encoded by a separate gene. These paralogs are expressed at different levels according to cell type and have different roles and intracellular distributions in vivo. Most previous studies on NM II used tissue-purified protein or expressed fragments of the molecule, which presents potential drawbacks for characterizing individual paralogs of the intact protein in vitro. To circumvent current limitations and approach their native properties, we have successfully expressed and purified the three full-length human NM II proteins with their light chains, using the baculovirus/Sf9 system. The enzymatic and structural properties of the three paralogs were characterized. Although each NM II is capable of forming bipolar filaments, those formed by IIC tend to contain fewer constituent molecules than those of IIA and IIB. All paralogs adopt the compact conformation in the presence of ATP. Phosphorylation of the regulatory light chain leads to assembly into filaments, which bind to actin in the presence of ATP. The nature of interactions with actin filaments is shown with different paralogs exhibiting different actin binding behaviors under equivalent conditions. The data show that although NM IIA and IIB form filaments with similar properties, NM IIC forms filaments that are less well suited to roles such as tension maintenance within the cell.

High resolution characterization of myosin IIC protein tailpiece and its effect on filament assembly

The motor protein nonmuscle myosin II (NMII) must undergo dynamic oligomerization into filaments to perform its cellular functions. A small nonhelical region at the tail of the long coiled-coil region (tailpiece) is a common feature of all dynamically assembling myosin II proteins. This tailpiece is a key regulatory domain affecting NMII filament assembly properties and is subject to phosphorylation in vivo. We previously demonstrated that the positively charged region of the tailpiece binds to assembly-incompetent NMII-C fragments, inducing filament assembly. In the current study, we investigated the molecular mechanisms by which the tailpiece regulates NMII-C self-assembly. Using alanine scan, we found that specific positive and aromatic residues within the positively charged region of the tailpiece are important for inducing NMII-C filament assembly and for filament elongation. Combining peptide arrays with deletion studies allowed us to identify the tailpiece binding sites in the coiled-coil rod. Elucidation of the mechanism by which the tailpiece induces filament assembly permitted us further investigation into the role of tailpiece phosphorylation. Sedimentation and CD spectroscopy identified that phosphorylation of Thr(1957) or Thr(1960) inhibited the ability of the tailpiece to bind the coiled-coil rod and to induce NMII-C filament formation. This study provides molecular insight into the role of specific residues within the NMII-C tailpiece that are responsible for shifting the oligomeric equilibrium of NMII-C toward filament assembly and determining its morphology.

Nonmuscle myosin II folds into a 10S form via two portions of tail for dynamic subcellular localization

Nonmuscle myosin II forms a folded conformation (10S form) in the inactivated state; however, the physiological importance of the 10S form is still unclear. To investigate the role of 10S form, we generated a chimeric mutant of nonmuscle myosin IIB (IIB-SK1·2), in which S1462-R1490 and L1551-E1577 were replaced with the corresponding portions of skeletal muscle myosin heavy chain. The IIB-SK1·2 mutant did not fold into a 10S form under physiological condition in vitro. IIB-SK1·2 was less dynamic by stabilizing the filamentous form and accumulated in the posterior region of migrating cells. IIB-SK1·2 functioned properly in cytokinesis but altered migratory properties; the rate and directional persistence were increased by IIB-SK1·2 expression. Surprisingly, endogenous nonmuscle myosin IIA was excluded from the posterior region of migrating cells expressing IIB-SK1·2, which may underlie the change of the cellular migratory properties. These results suggest that the 10S form is necessary for maintaining nonmuscle myosin II in an unassembled state and for recruitment of nonmuscle myosin II to a specific region of the cell.

The tumor suppressor Lgl1 regulates NMII-A cellular distribution and focal adhesion morphology to optimize cell migration

The Drosophila tumor suppressor Lethal (2) giant larvae (Lgl) regulates the apical-basal polarity in epithelia and asymmetric cell division. However, little is known about the role of Lgl in cell polarity in migrating cells. In this study we show direct physiological interactions between the mammalian homologue of Lgl (Lgl1) and the nonmuscle myosin II isoform A (NMII-A). We demonstrate that Lgl1 and NMII-A form a complex in vivo and provide data that Lgl1 inhibits NMII-A filament assembly in vitro. Furthermore, depletion of Lgl1 results in the unexpected presence of NMII-A in the cell leading edge, a region that is not usually occupied by this protein, suggesting that Lgl1 regulates the cellular localization of NMII-A. Finally, we show that depletion of Lgl1 affects the size and number of focal adhesions, as well as cell polarity, membrane dynamics, and the rate of migrating cells. Collectively these findings indicate that Lgl1 regulates the polarity of migrating cells by controlling the assembly state of NMII-A, its cellular localization, and focal adhesion assembly.

Dynamic assembly properties of nonmuscle myosin II isoforms revealed by combination of fluorescence correlation spectroscopy and fluorescence cross-correlation spectroscopy

Myosin II molecules assemble into filaments through their C-terminal rod region, and are responsible for several cellular motile activities. Three isoforms of nonmuscle myosin II (IIA, IIB and IIC) are expressed in mammalian cells. However, little is known regarding the isoform composition in filaments. To obtain new insight into the assembly properties of myosin II isoforms, especially regarding the isoform composition in filaments, we performed a combination analysis of fluorescence correlation spectroscopy (FCS) and fluorescence cross-correlation spectroscopy (FCCS), which enables us to acquire information on both the interaction and the size of each molecule simultaneously. Using C-terminal rod fragments of IIA and IIB (ARF296 and BRF305) labelled with different fluorescent probes, we demonstrated that hetero-assemblies were formed from a mixture of ARF296 and BRF305, and that dynamic exchange of rod fragments occurred between preformed homo-assemblies of each isoform in an isoform-independent manner. We also showed that Mts1 (S100A4) specifically stripped ARF296 away from the hetero-assemblies, and consequently, homo-assemblies of BRF305 were formed. These results suggest that IIA and IIB can form hetero-filaments in an isoform-independent manner, and that a factor like Mts1 can remove one isoform from the hetero-filament, resulting in a formation of homo-filaments consisting of another isoform.

Myosin II motor proteins with different functions determine the fate of lamellipodia extension during cell spreading

Non-muscle cells express multiple myosin-II motor proteins myosin IIA, myosin IIB and myosin IIC transcribed from different loci in the human genome. Due to a significant homology in their sequences, these ubiquitously expressed myosin II motor proteins are believed to have overlapping cellular functions, but the mechanistic details are not elucidated. The present study uncovered a mechanism that coordinates the distinctly localized myosin IIA and myosin IIB with unexpected opposite mechanical roles in maneuvering lamellipodia extension, a critical step in the initiation of cell invasion, spreading, and migration. Myosin IIB motor protein by localizing at the front drives lamellipodia extension during cell spreading. On the other hand, myosin IIA localizes next to myosin IIB and attenuates or retracts lamellipodia extension. Myosin IIA and IIB increase cell adhesion by regulating focal contacts formation in the spreading margins and central part of the spreading cell, respectively. Spreading cells expressing both myosin IIA and myosin IIB motor proteins display an organized actin network consisting of retrograde filaments, arcs and central filaments attached to focal contacts. This organized actin network especially arcs and focal contacts formation in the spreading margins were lost in myosin IIÂ cells. Surprisingly, myosin IIB̂ cells displayed long parallel actin filaments connected to focal contacts in the spreading margins. Thus, with different roles in the regulation of the actin network and focal contacts formation, both myosin IIA and IIB determine the fate of lamellipodia extension during cell spreading.

The positively charged region of the myosin IIC non-helical tailpiece promotes filament assembly

The motor protein, non-muscle myosin II (NMII), must undergo dynamic oligomerization into filaments to participate in cellular processes such as cell migration and cytokinesis. A small non-helical region at the tail of the long coiled-coil region (tailpiece) is a common feature of all dynamically assembling myosin II proteins. In this study, we investigated the role of the tailpiece in NMII-C self-assembly. We show that the tailpiece is natively unfolded, as seen by circular dichroism and NMR experiments, and is divided into two regions of opposite charge. The positively charged region (Tailpiece(1946-1967)) starts at residue 1946 and is extended by seven amino acids at its N terminus from the traditional coiled-coil ending proline (Tailpiece(1953-1967)). Pull-down and sedimentation assays showed that the positive Tailpiece(1946-1967) binds to assembly incompetent NMII-C fragments inducing filament assembly. The negative region, residues 1968-2000, is responsible for NMII paracrystal morphology as determined by chimeras in which the negative region was swapped between the NMII isoforms. Mixing the positive and negative peptides had no effect on the ability of the positive peptide to bind and induce filament assembly. This study provides molecular insight into the role of the structurally disordered tailpiece of NMII-C in shifting the oligomeric equilibrium of NMII-C toward filament assembly and determining its morphology.

Myosin II tailpiece determines its paracrystal structure, filament assembly properties, and cellular localization

Non muscle myosin II (NMII) is a major motor protein present in all cell types. The three known vertebrate NMII isoforms share high sequence homology but play different cellular roles. The main difference in sequence resides in the C-terminal non-helical tailpiece (tailpiece). In this study we demonstrate that the tailpiece is crucial for proper filament size, overcoming the intrinsic properties of the coiled-coil rod. Furthermore, we show that the tailpiece by itself determines the NMII filament structure in an isoform-specific manner, thus providing a possible mechanism by which each NMII isoform carries out its unique cellular functions. We further show that the tailpiece determines the cellular localization of NMII-A and NMII-B and is important for NMII-C role in focal adhesion complexes. We mapped NMII-C sites phosphorylated by protein kinase C and casein kinase II and showed that these phosphorylations affect its solubility properties and cellular localization. Thus phosphorylation fine-tunes the tailpiece effects on the coiled-coil rod, enabling dynamic regulation of NMII-C assembly. We thus show that the small tailpiece of NMII is a distinct domain playing a role in isoform-specific filament assembly and cellular functions.

The C-terminal tail region of nonmuscle myosin II directs isoform-specific distribution in migrating cells

Nonmuscle myosin II isoforms A and B (hereafter, IIA and IIB) perform unique roles in cell migration, even though both isoforms share the same basic molecular functions. That IIA and IIB assume distinct subcellular distribution in migrating cells suggests that discrete spatiotemporal regulation of each isoform's activity may provide a basis for its unique migratory functions. Here, we make the surprising finding that swapping a small C-terminal portion of the tail between IIA and IIB inverts the distinct distribution of these isoforms in migrating cells. Moreover, swapping this region between isoforms also inverts their specific turnover properties, as assessed by fluorescence recovery after photobleaching and Triton solubility. These data, acquired through the use of chimeras of IIA and IIB, suggest that the C-terminal region of the myosin heavy chain supersedes the distinct motor properties of the two isoforms as the predominant factor directing isoform-specific distribution. Furthermore, our results reveal a correlation between isoform solubility and distribution, leading to the proposal that the C-terminal region regulates isoform distribution by tightly controlling the amount of each isoform that is soluble and therefore available for redistribution into new protrusions.

TRPM7 regulates myosin IIA filament stability and protein localization by heavy chain phosphorylation

Deregulation of myosin II-based contractility contributes to the pathogenesis of human diseases, such as cancer, which underscores the necessity for tight spatial and temporal control of myosin II activity. Recently, we demonstrated that activation of the mammalian alpha-kinase TRPM7 inhibits myosin II-based contractility in a Ca(2+)- and kinase-dependent manner. However, the molecular mechanism is poorly defined. Here, we demonstrate that TRPM7 phosphorylates the COOH-termini of both mouse and human myosin IIA heavy chains--the COOH-terminus being a region that is critical for filament stability. Phosphorylated residues were mapped to Thr1800, Ser1803 and Ser1808. Mutation of these residues to alanine and that to aspartic acid lead to an increase and a decrease, respectively, in myosin IIA incorporation into the actomyosin cytoskeleton and accordingly affect subcellular localization. In conclusion, our data demonstrate that TRPM7 regulates myosin IIA filament stability and localization by phosphorylating a short stretch of amino acids within the alpha-helical tail of the myosin IIA heavy chain.