Myosins: a diverse superfamily

Myosin X is a high duty ratio motor

Myosin X is expressed in a variety of cell types and plays a role in cargo movement and filopodia extension, but its mechanoenzymatic characteristics are not fully understood. Here we analyzed the kinetic mechanism of the ATP hydrolysis cycle of acto-myosin X using a single-headed construct (M10IQ1). Myosin X was unique for the weak "strong actin binding state" (AMD) with a K(d) of 1.6 microm attributed to the large dissociation rate constant (2.1 s(-1)). V(max) and K(ATPase) of the actin-activated ATPase activity of M10IQ1 were 13.5 s(-1) and 17.4 mum, respectively. The ATP hydrolysis rate (>100 s(-1)) and the phosphate release rate from acto-myosin X (>100 s(-1)) were much faster than the entire ATPase cycle rate and, thus, not rate-limiting. The ADP off-rate from acto-myosin X was 23 s(-1), which was two times larger than the V(max). The P(i)-burst size was low (0.46 mol/mol), indicating that the equilibrium is significantly shifted toward the prehydrolysis intermediate. The steady-state ATPase rate can be explained by a combination of the unfavorable equilibrium constant of the hydrolysis step and the relatively slow ADP off-rate. The duty ratio calculated from our kinetic model, 0.6, was consistent with the duty ratio, 0.7, obtained from comparison of K(m ATPase) and K(m motility). Our results suggest that myosin X is a high duty ratio motor.

Muscle arm development in Caenorhabditis elegans

In several types of animals, muscle cells use membrane extensions to contact motor axons during development. To better understand the process of membrane extension in muscle cells, we investigated the development of Caenorhabditis elegans muscle arms, which extend to motor axons and form the postsynaptic element of the neuromuscular junction. We found that muscle arm development is a highly regulated process: the number of muscle arms extended by each muscle, the shape of the muscle arms and the path taken by the muscle arms to reach the motor axons are largely stereotypical. We also investigated the role of several cytoskeletal components and regulators during arm development, and found that tropomyosin (LEV-11), the actin depolymerizing activity of ADF/cofilin (UNC-60B) and, surprisingly, myosin heavy chain B (UNC-54) are each required for muscle arm extension. This is the first evidence that UNC-54, which is found in thick filaments of sarcomeres, can also play a role in membrane extension. The muscle arm phenotypes produced when these genes are mutated support a 'two-phase' model that distinguishes passive muscle arm development in embryogenesis from active muscle arm extension during larval development.

Evolution of striated muscle: Jellyfish and the origin of triploblasty

The larval and polyp stages of extant Cnidaria are bi-layered with an absence of mesoderm and its differentiation products. This anatomy originally prompted the diploblast classification of the cnidarian phylum. The medusa stage, or jellyfish, however, has a more complex anatomy characterized by a swimming bell with a well-developed striated muscle layer. Based on developmental histology of the hydrozoan medusa this muscle derives from the entocodon, a mesoderm-like third cell layer established at the onset of medusa formation. According to recent molecular studies cnidarian homologs to bilaterian mesoderm and myogenic regulators are expressed in the larval and polyp stages as well as in the entocodon and derived striated muscle. Moreover striated and smooth muscle cells may have evolved directly and independently from non-muscle cells as indicated by phylogenetic analysis of myosin heavy chain genes (MHC class II). To accommodate all evidences we propose that striated muscle-based locomotion coevolved with the nervous and digestive systems in a basic metazoan Bauplan from which the ancestors of the Ctenophora (comb jellyfish), Cnidaria (jellyfish and polyps), as well as the Bilateria are derived. We argue for a motile tri-layered cnidarian ancestor and a monophyletic descent of striated muscle in Cnidaria and Bilateria. As a consequence, diploblasty evolved secondarily in cnidarian larvae and polyps.

Altered cytoskeleton organization in platelets from patients with MYH9-related disease

MYH9-related disease (MYH9-RD) is an autosomal dominant disorder deriving from mutations in the MYH9 gene encoding for the heavy chain of non-muscle myosin IIA, and characterized by thrombocytopenia and giant platelets. Isoform IIA of myosin is the only one expressed in platelets, but the possibility that MYH9 mutations affect the organization of contractile structures in these blood elements has never been investigated. In this work we have analyzed the composition and the agonist-induced reorganization of the platelet cytoskeleton from seven MYH9-RD patients belonging to four different families. We found that an increased amount of myosin was constitutively associated with actin in the cytoskeleton of resting MYH9-RD platelets. Upon platelet stimulation, an impaired increase in the total cytoskeletal proteins was observed. Moreover, selected membrane glycoproteins, tyrosine kinases, and small GTPases failed to interact with the cytoskeleton in agonist-stimulated MYH9-RD platelets. These results demonstrate for the first time that mutations of MYH9 result in an alteration of the composition and agonist-induced reorganization of the platelet cytoskeleton. We suggest that these abnormalities may represent the biochemical basis for the previously reported functional alterations of MYH9-RD platelets, and for the abnormal platelet formation from megakaryocytes, resulting in thrombocytopenia and giant platelets.

Peroxisomal localization of a myosin XI isoform in Arabidopsis thaliana

The genome of Arabidopsis thaliana contains 13 myosin XI isoforms. Here we prepared a specific antibody against a peptide that mimics a unique C-terminal region from the myosin XI isoform, MYA2. The resulting antibody was used to demonstrate that MYA2 in Arabidopsis protein extracts co-sedimented with actin filaments and dissociated from the filaments with ATP treatment. Immunolocalization studies showed that MYA2 co-localized predominantly with actin filaments in clustered punctuate dots in leaf epidermal cells, root hair cells and suspension-cultured cells. In a transgenic plant in which peroxisomes are labeled with green fluorescent protein, some MYA2 signals were localized on peroxisomes in an actin-dependent manner. We propose that the peroxisome is one of the cargos translocated by MYA2 on actin filaments.

Myosin VI: cellular functions and motor properties

Myosin VI has been localized in membrane ruffles at the leading edge of cells, at the trans-Golgi network compartment of the Golgi complex and in clathrin-coated pits or vesicles, indicating that it functions in a wide variety of intracellular processes. Myosin VI moves along actin filaments towards their minus end, which is the opposite direction to all of the other myosins so far studied (to our knowledge), and is therefore thought to have unique properties and functions. To investigate the cellular roles of myosin VI, we identified various myosin VI binding partners and are currently characterizing their interactions within the cell. As an alternative approach, we have expressed and purified full-length myosin VI and studied its in vitro properties. Previous studies assumed that myosin VI was a dimer, but our biochemical, biophysical and electron microscopic studies reveal that myosin VI can exist as a stable monomer. We observed, using an optical tweezers force transducer, that monomeric myosin VI is a non-processive motor which, despite a relatively short lever arm, generates a large working stroke of 18 nm. Whether monomer and/or dimer forms of myosin VI exist in cells and their possible functions will be discussed.

Interaction of soluble CD163 with activated T lymphocytes involves its association with non-muscle myosin heavy chain type A

CD163 is a monocyte/macrophage-specific scavenger receptor that undergoes ectodomain shedding upon an inflammatory stimulus. Soluble CD163 (sCD163) actively inhibits lymphocyte proliferation, but to date exactly how it interacts with these cells has remained elusive. We screened T lymphocytes and endothelial cells for proteins binding to sCD163. In both cell types a high affinity binding protein was detected. Partial sequencing of the protein revealed sequence identity to a non-muscle myosin heavy chain type A. Employing labelled sCD163 we found little specific binding of sCD163 to the extracellular domains of T lymphocytes and human umbilical vein endothelial cells (HUVEC). In activated T lymphocytes we demonstrated specific binding of sCD163 to intracellular structures as well as the presence of the native protein within the cell after co-incubation with purified sCD163. Furthermore, we developed a novel ELISA for highly specific detection of sCD163-myosin complexes. These complexes were present in activated T lymphocytes after incubation with shed sCD163. Co-localization of sCD163 and cellular myosin in T lymphocytes was further confirmed by fluorescence microscopy. Our results suggest that sCD163 associates with cellular myosin, thereby possibly modulating the cells' response to an inflammatory stimulus.

Activation of myosin Va function by melanophilin, a specific docking partner of myosin Va

It is known that melanophilin is a myosin Va-targeting molecule that links myosin Va and the cargo vesicles in cells. Here we found that melanophilin directly activates the actin-activated ATPase activity of myosin Va and thus its motor activity. The actin-activated ATPase activity of the melanocyte-type myosin Va having exon-F was significantly activated by melanophilin by 4-fold. Although Rab27a binds to myosin Va/melanophilin complex, it did not affect the melanophilin-induced activation of myosin Va. Deletion of the C-terminal actin binding domain and N-terminal Rab binding domain of melanophilin resulted in no change in the activation of the ATPase by melanophilin, indicating that the myosin Va binding domain (MBD) is sufficient for the activation of myosin Va. Among MBDs, the interaction of MBD-2 with exon-F of myosin Va is critical for the binding of myosin Va and melanophilin, whereas MBD-1 interacting with the globular tail of myosin Va plays a more significant role in the activation of myosin Va ATPase activity. This is the first demonstration that the binding of the cargo molecule directly activates myosin motor activity. The present finding raises the idea that myosin motors are switched upon their binding to the cargo molecules, thus avoiding the waste of ATP consumption.

Myosin VI: cellular functions and motor properties

Myosin motor proteins use the energy derived from ATP hydrolysis to move cargo along actin tracks. Myosin VI, unlike almost all other myosins, moves toward the minus end of actin filaments and functions in a variety of intracellular processes such as vesicular membrane traffic, cell migration, and mitosis. These diverse roles of myosin VI are mediated by interaction with a number of different binding partners present in multi-protein complexes. Myosin VI can work in vitro as a processive dimeric motor and as a nonprocessive monomeric motor, each with a large working stroke. The possibility that both monomeric and dimeric forms of myosin VI operate in the cell may represent an important regulatory mechanism for controlling the multiple steps in transport pathways where nonprocessive and processive motors are required.

Targeted disruption of mouse ortholog of the human MYH9 responsible for macrothrombocytopenia with different organ involvement: hematological, nephrological, and otological studies of heterozygous KO mice

Among three different isoforms of non-muscle myosin heavy chains (NMMHCs), only NMMHCA is associated with inherited human disease, called MYH9 disorders, characterized by macrothrombocytopenia and characteristic granulocyte inclusions. Here targeted gene disruption was performed to understand fundamental as well as pathological role of the gene for NMMHCA, MYH9. Heterozygous intercrosses yielded no homozygous animals among 552 births, suggesting that MYH9 expression is required for embryonic development. In contrast, MYH9+/- mice were viable and fertile without gross anatomical, hematological, and nephrological abnormalities. Immunofluorescence analysis also showed the normal cytoplasmic distribution of NMMHCA. We further measured the auditory brainstem response and found two of six MYH9+/- mice had hearing losses, whereas the remaining four were comparable to wild-type mice. Such observation may parallel the diverse expression of Alport's manifestations of human individuals with MYH9 disorders and suggest the limited requirement of the gene for maintenance and function of specific organs.

Subcellular localization and dynamics of MysPDZ (Myo18A) in live mammalian cells

MysPDZ is an unconventional myosin belonging to the class XVIII myosin containing a KE (lysine and glutamine)-rich domain and a PDZ domain, which codistributes with actin fibers partially without any canonical actin binding sequence in its myosin head domain. Recently, we reported the identification of a novel isoform of MysPDZ lacking these domains and exhibiting subcellular localization and expression profile different from the original form of MysPDZ. In order to delineate domains directing the subcellular localization of MysPDZ, we performed co-immunoprecipitation experiments and image analyses using mutants of MysPDZ fused with enhanced yellow fluorescent protein. Co-immunoprecipitation analyses showed that MysPDZ can self-associate through its C-terminus coiled-coil domain and the KE-rich domain mediates the interaction with actin. We observed by image analyses that the codistribution with actin fibers and the localization in inner surface of cell membrane of MysPDZ are controlled by the KE-rich domain and the PDZ domain, respectively. Time lapse video microscopy showed that MysPDZ in the cytoplasm moves randomly and rapidly within short range and is allocated to a subcellular compartment without ATP hydrolysis by MysPDZ. This suggests that MysPDZ is a protein which is unlike most unconventional myosins. Our study uncovers a novel role of the KE-rich and PDZ domains in directing subcellular localization and also contributes to a better understanding of functional differences in MysPDZ isoforms.

Presence of cytoskeleton proteins in parotid glands and their roles during secretion

Amylase secretion is induced by the accumulation of cAMP in response to beta-adrenergic stimulation and by the augmentation of intracellular Ca2+ in response to muscarinic-cholinergic stimulation in rat parotid glands. The roles of cytoskeleton and motor proteins in the secretory process are not yet known. We examined the effects of cytoskeleton-modulating reagents on the amylase release induced by isoproterenol (IPR) and carbamylcholine (Cch) in rat parotid acinar cells. The amylase release induced by Cch was decreased by the microtubule-disrupting reagent colchicine (Colch) and the myosin ATPase inhibitor 2,3-butanediene monoxime (BDM), but the release induced by IPR was not. The actin filament-stabilizing reagent jasplakinolide (Jasp) and actin filament-disrupting reagent cytochalasin D (CytoD) decreased the amylase release induced by both the beta-adrenergic and the muscarinic-cholinergic stimulants. Pretreatment with CytoD affected the shape of the acinar cells, which showed an intermediate state between the fusion of the secretory granules with the apical membrane and the retrieval of the membranes only after stimulation with IPR. Myosin and Dynein/dynactin complex were detected in the secretory granule membrane fraction. We concluded from this study that the cytoskeleton played different roles in the beta-adrenergic and the muscarinic-cholinergic secretory processes.

Trifluoperazine inhibits the MgATPase activity and in vitro motility of conventional and unconventional myosins

Trifluoperazine, a calmodulin antagonist, has recently been shown to inhibit the MgATPase activity of scallop myosin in the absence of light chain dissociation (Patel et al. (2000) J Biol Chem 275: 4880-4888). To investigate the generality of this observation and the mechanism by which it occurs, we have examined the ability of trifluoperazine to inhibit the enzymatic properties of other conventional and unconventional myosins. We show that trifluoperazine can inhibit the actin-activated MgATPase activity of rabbit skeletal muscle myosin II heavy meromyosin (HMM), phosphorylated turkey gizzard smooth muscle myosin II HMM, phosphorylated human nonmuscle myosin IIA HMM and myosin V subfragment-1 (S1). In all cases half maximal inhibition occurred at 50-75 microM trifluoperazine while light chains (myosin II) or calmodulin (myosin V) remained associated with the heavy chains. In vitro motility of all myosins tested was completely inhibited by trifluoperazine. Chymotryptic digestion of baculovirus-expressed myosin V HMM possessing only two calmodulin binding sites yielded a minimal motor fragment with no bound calmodulin. The MgATPase of this fragment was inhibited by trifluoperazine over the same range of concentrations as the S1 fragment of myosin.

Phylogenetic analysis of the formin homology 2 domain

Formin proteins are key regulators of eukaryotic actin filament assembly and elongation, and many species possess multiple formin isoforms. A nomenclature system based on fundamental features would be desirable, to aid the rapid identification and characterization of novel formins. In this article, we attempt to systematize the formin family by performing phylogenetic analyses of the formin homology 2 (FH2) domain, an independently folding region common to all formins, which alone can influence actin dynamics. Through database searches, we identify 101 FH2 domains from 26 eukaryotic species, including 15 in mice. Sequence alignments reveal a highly conserved yeast-specific insert in the "knob loop" region of the FH2 domain, with unknown functional consequences. Phylogenetic analysis using minimum evolution (ME), maximum parsimony (MP), and maximum likelihood (ML) algorithms strongly supports the existence of seven metazoan groups. Yeast FH2 domains segregate from all other eukaryotes, including metazoans, other fungi, plants, and protists. Sequence comparisons of non-FH2 regions support relationships between three metazoan groups (Dia, DAAM, and FRL) and examine previously identified coiled-coil and Diaphanous auto-regulatory domain sequences. This analysis allows for a formin nomenclature system based on sequence relationships, as well as suggesting strategies for the determination of biochemical and cellular activities of these proteins.

Myosin VI: a multifunctional motor

Myosin VI moves towards the minus end of actin filaments unlike all the other myosins so far studied, suggesting that it has unique properties and functions. Myosin VI is present in clathrin-coated pits and vesicles, in membrane ruffles and in the Golgi complex, indicating that it has a wide variety of functions in the cell. To investigate the cellular roles of myosin VI, we have identified a variety of myosin VI-binding partners and characterized their interactions. As an alternative approach, we have studied the in vitro properties of intact myosin VI. Previous studies assumed that myosin VI existed as a dimer but our biochemical characterization and electron microscopy studies reveal that myosin VI is a monomer. Using an optical tweezers force transducer, we showed that monomeric myosin VI is a non-processive motor with a large working stroke of 18 nm. Potential roles for myosin VI in cells are discussed.

IRF-2 is involved in up-regulation of nonmuscle myosin heavy chain II-A gene expression during phorbol ester-induced promyelocytic HL-60 differentiation

Transcription of the nonmuscle myosin heavy chain II-A (NMHC-A) gene is regulated by various factors, including cell type, proliferation and differentiation stage, and extracellular stimuli. We have identified an intronic region (designated 32kb-150), which is located 32 kb downstream of the transcription start sites in the human NMHC-A gene, as a transcriptional regulatory region. 32kb-150 contains an interferon-stimulated response element (ISRE). By using HeLa and NIH3T3 cells, in which NMHC-A is constitutively expressed, interferon regulatory factor (IRF)-2 was found to be the only major protein, among the IRF family proteins, that bound to the ISRE in 32kb-150 both in vitro and in intact cells. IRF-2, which is known to either repress or activate target gene expression, acts as a transcriptional activator in the context of the 32kb-150 reporter gene. The carboxyl-terminal basic region of IRF-2 serves as an activation domain in this context. This is in contrast to its acting as a repressor domain in the context of the synthetic core ISRE. Furthermore, after treatment of promyelocytic HL-60 cells with 12-O-tetradecanoylphorbol-13-acetate (TPA), which triggers differentiation into macrophages, both NMHC-A expression and IRF-2 expression were found to be up-regulated with a similar time course. TPA treatment leads to recruitment of IRF-2 to 32kb-150 of the endogenous NMHC-A gene and acetylation of the core histones surrounding this region. In addition, the ISRE in the 32kb-150 reporter gene recruits IRF-2 and mediates TPA-induced activation of a reporter gene in HL-60 cells. Together, these results indicate that IRF-2 contributes to transcriptional activation of the NMHC-A gene via 32kb-150 during TPA-induced differentiation of HL-60 cells.

Molecular machines

Molecular machines are tiny energy conversion devices on the molecular-size scale. Whether naturally occurring or synthetic, these machines are generally more efficient than their macroscale counterparts. They have their own mechanochemistry, dynamics, workspace, and usability and are composed of nature's building blocks: namely proteins, DNA, and other compounds, built atom by atom. With modern scientific capabilities it has become possible to create synthetic molecular devices and interface them with each other. Countless such machines exist in nature, and it is possible to build artificial ones by mimicking nature. Here we review some of the known molecular machines, their structures, features, and characteristics. We also look at certain devices in their early development stages, as well as their future applications and challenges.

The vaa locus of Mycoplasma hominis contains a divergent genetic islet encoding a putative membrane protein

BACKGROUND

The Mycoplasma hominis vaa gene encodes a highly variable, surface antigen involved in the adhesion to host cells. We have analysed the structure of the vaa locus to elucidate the genetic basis for variation of vaa.

RESULTS

Mapping of vaa on existing physical maps of five M. hominis isolates by pulsed field gel electrophoresis revealed that vaa is located in a genomic region containing the majority of other characterized membrane protein genes of M. hominis. Sequencing of an 11 kb region containing the vaa locus of M. hominis isolate 132 showed the presence of conserved housekeeping genes at the borders of the region, uvrA upstream and the hitABL operon downstream to vaa. Analysis of 20 M. hominis isolates revealed that the vaa upstream region was conserved whereas the downstream region was highly variable. In isolate 132 this region contained an open reading frame (ORF) encoding a putative 160 kDa membrane protein. Homologous ORFs were present in half of the isolates, whereas this ORF, termed vmp (variable membrane protein), was deleted from the locus in the remaining isolates. Compellingly, the conserved upstream region and variable downstream region of vaa correlates with the genetic structure of vaa itself which consists of a conserved 5' end and a variable 3' end containing a variable number of exchangeable sequence cassettes.

CONCLUSION

Our data demonstrate that the vaa locus contains a divergent genetic islet, and indicate pronounced intraspecies recombination. The high variability level of the locus indicate that it is a chromosomal 'hot spot', presumably important for sustaining diversity and a high adaptation potential of M. hominis.

The elongation and contraction of actin bundles are induced by double-headed myosins in a motor concentration-dependent manner

Many types of myosin have been found and characterized to date, and already nearly 20 classes have been identified. However, these myosin motors can be classified more simply into two groups according to their head-structure, i.e. double- or single-headed myosins. Why do some myosin motors possess a double-headed structure? One obvious possible reason would be that the two heads improve the motor's processivity and sliding performance. Previously, to investigate the possibility that the double-headed myosins simultaneously interact with parallel arrayed two actin filaments in the presence of Mg-ATP, we developed an in vitro assay system using actin bundles formed by inert polymers. Using that system, we show here that skeletal muscle heavy meromyosin (HMM), a double-headed myosin derivative, but not subfragment-1 (S-1), a single-headed one, was able to contract or elongate actin bundles in a concentration-dependent manner. Similar elongation or contraction of actin bundles can also be induced by other double-headed myosin species isolated in the native state from Dictyostelium, from green algae Chara or from chicken brain. The results of this study confirm that double-headed myosin motors can induce sliding movements among neighboring actin filaments. The double-headed structure of myosins may also be important for generating tension or elongation in actin bundles or gels, and for organizing polarity-sorted actin networks, not just for improving their motor processivity or activity.

Non-muscle myosin II and myosin light chain kinase are downstream targets for vasopressin signaling in the renal collecting duct

We have previously demonstrated that vasopressin increases the water permeability of the inner medullary collecting duct (IMCD) by inducing trafficking of aquaporin-2 to the apical plasma membrane and that this response is dependent on intracellular calcium mobilization and calmodulin activation. Here, we address the hypothesis that this water permeability response is mediated in part through activation of the calcium/calmodulin-dependent myosin light chain kinase (MLCK) and regulation of non-muscle myosin II. Immunoblotting and immunocytochemistry demonstrated the presence of MLCK, the myosin regulatory light chain (MLC), and the IIA and IIB isoforms of the non-muscle myosin heavy chain in rat IMCD cells. Two-dimensional electrophoresis and matrix-assisted laser desorption ionization time-of-flight mass spectrometry identified two isoforms of MLC, both of which also exist in phosphorylated and non-phosphorylated forms. 32P incubation of the inner medulla followed by autoradiography of two-dimensional gels demonstrated increased 32P labeling of both isoforms in response to the V2 receptor agonist [deamino-Cys1,D-Arg8]vasopressin (DDAVP). Time course studies of MLC phosphorylation in IMCD suspensions (using immunoblotting with anti-phospho-MLC antibodies) showed that the increase in phosphorylation could be detected as early as 30 s after exposure to vasopressin. The MLCK inhibitor ML-7 blocked the DDAVP-induced MLC phosphorylation and substantially reduced [Arg8]vasopressin (AVP)-stimulated water permeability. AVP-induced MLC phosphorylation was associated with a rearrangement of actin filaments (Alexa Fluor 568-phalloidin) in primary cultures of IMCD cells. These results demonstrate that MLC phosphorylation by MLCK represents a downstream effect of AVP-activated calcium/calmodulin signaling in IMCD cells and point to a role for non-muscle myosin II in regulation of water permeability by vasopressin.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Defects in cell adhesion and the visceral endoderm following ablation of nonmuscle myosin heavy chain II-A in mice

Previous work has shown that ablation or mutation of nonmuscle myosin heavy chain II-B (NMHC II-B) in mice results in defects in the heart and brain with death occurring between embryonic day 14.5 (E14.5) and birth (Tullio, A. N., Accili, D., Ferrans, V. J., Yu, Z. X., Takeda, K., Grinberg, A., Westphal, H., Preston, Y. A., and Adelstein, R. S. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 12407-12412). Here we show that mice ablated for NMHC II-A fail to develop a normal patterned embryo with a polarized visceral endoderm by E6.5 and die by E7.5. Moreover, A(-)/A(-) embryoid bodies grown in suspension culture constantly shed cells. These defects in cell adhesion and tissue organization are explained by loss of E-cadherin and beta-catenin localization to cell adhesion sites in both cell culture and in the intact embryos. The defects can be reproduced by introducing siRNA directed against NMHC II-A into wild-type embryonic stem cells. Our results suggest an essential role for a single, specific nonmuscle myosin isoform in maintaining cell-cell adhesions in the early mammalian embryo.

The essential role of PP1beta in Drosophila is to regulate nonmuscle myosin

Reversible phosphorylation of myosin regulatory light chain (MRLC) is a key regulatory mechanism controlling myosin activity and thus regulating the actin/myosin cytoskeleton. We show that Drosophila PP1beta, a specific isoform of serine/threonine protein phosphatase 1 (PP1), regulates nonmuscle myosin and that this is the essential role of PP1beta. Loss of PP1beta leads to increased levels of phosphorylated nonmuscle MRLC (Sqh) and actin disorganisation; these phenotypes can be suppressed by reducing the amount of active myosin. Drosophila has two nonmuscle myosin targeting subunits, one of which (MYPT-75D) resembles MYPT3, binds specifically to PP1beta, and activates PP1beta's Sqh phosphatase activity. Expression of a mutant form of MYPT-75D that is unable to bind PP1 results in elevation of Sqh phosphorylation in vivo and leads to phenotypes that can also be suppressed by reducing the amount of active myosin. The similarity between fly and human PP1beta and MYPT genes suggests this role may be conserved.

Arabidopsis myosin XI mutant is defective in organelle movement and polar auxin transport

Myosins are eukaryotic molecular motors moving along actin filaments. Only a small set of myosin classes is present in plants, in which myosins have been found to play a role in cytoplasmic streaming and chloroplast movement. Whereas most studies have been done on green algae, more recent data suggest a role of higher plant myosin at the postcytokinetic cell wall. Here we characterize a loss-of-function mutation for a myosin of plant-specific class XI and demonstrate myosin functions during plant development in Arabidopsis. T-DNA insertion in MYA2 caused pleiotropic effects, including flower sterility and dwarf growth. Elongation of epidermal cells, such as in hypocotyls and anther filaments, was reduced by up to 50% of normal length. This effect on anther filaments is responsible for flower sterility. In the meristems of root tips, it was evident that cell division was delayed and that cell plates were mislocated. Like zwichel, a kinesin-related mutation causing two-branched trichomes, the mya2 knockout causes branching defects, but here the trichomes remained unbranched. Growth was also impaired in pollen tubes and root hairs, cells that are highly dependent on vesicle transport. A failure in vesicle flow could be directly confirmed, because cytoplasmic streaming of vesicles and, more so, of large endoplasmic reticulum-based organelles was slowed. The defect in vesicle trafficking was accompanied by failures in basipetal auxin transport, measured in stem segments of inflorescences. This result strongly suggests a causal link between auxin-dependent processes and the distribution of vesicles and membrane-bound molecules by plant myosin.

Smooth, slow and smart muscle motors

Smooth muscle is a slow and economical muscle with a large variability in contractile properties. This review describes results regarding the relation between expression of myosin isoforms and the contraction of smooth muscle. The focus of the review is on studies of the organised contractile system in the smooth muscle tissue. The role of the myosin heavy chain variants formed by alternative splicing in the myosin heavy chain tail (SM1, SM2 isoforms) and head (SM-A SM-B isoforms) regions, as well as the role of essential light chains (LC17a, LC17b isoforms) for the variability of contractile properties are discussed. Smooth muscle also has the ability to alter its contractile properties in response to altered functional demands in vivo, e.g. during hypertrophic growth of urinary bladder, intestine, uterus and vessels and in response to altered hormone levels. These alterations involve changes in myosin expression and altered contractile kinetics. Non-muscle myosin has been shown to have a contractile function in some smooth muscle tissues and recent data on the kinetic properties of non-muscle myosin filaments in smooth muscle tissue are described.

Myosin X transports Mena/VASP to the tip of filopodia

Myosin X (M10) is a two-headed actin based motor expressed in a variety of cell types, that is thought to play a role in cargo movement in mammalian cells, but its cellular function is unknown. Here we found that M10 binds to Mena/VASP, which facilitates actin polymerization by competing with actin capping proteins. Immunocytochemistry revealed that endogenous M10 co-localized with Mena/VASP at the tip of filopodia. Consistently, both EGFP-M10 and RFP-VASP were found at the tip of filopodia. The result raises a hypothesis that M10 transports Mena/VASP towards the tip of filopodia. Supporting this idea, the amount of VASP at the tip of filopodia was proportional to that of M10. Furthermore, we directly visualized the movement of M10 and VASP in living HeLa cells under fluorescence microscope. EGFP-M10 and RFP-VASP move together from the root to the tip of the filopodia. Interestingly, the amount of M10 at the tip of filopodia was linearly related to the length of filopodia, consistent with the actin filament extending function of VASP. These results show that M10 is a specific motor carrying Mena/VASP from the root to the tip of the filopodia where extension of actin filament takes place.

Correlation between the clinical phenotype of MYH9 -related disease and tissue distribution of class II nonmuscle myosin heavy chains

Nonmuscle myosin heavy chain II-A is responsible for MYH9-related disease, which is characterized by macrothrombocytopenia, granulocyte inclusions, deafness, cataracts, and renal failure. Since another two highly conserved nonmuscle myosins, II-B and II-C, are known, an analysis of their tissue distribution is fundamental for the understanding of their biological roles. In mouse, we found that all forms are ubiquitously expressed. However, megakaryocytic and granulocytic lineages express only II-A, suggesting that congenital features, macrothrombocytopenia, and leukocyte inclusions correlate with its exclusive presence. In kidney, eye, and ear, where clinical manifestations have a late onset, as well as in other tissues apparently not affected in patients, II-A and at least one of the other two isoforms are expressed, suggesting that II-B and II-C can partially compensate for each other. We hypothesize that cells expressing only II-A manifest the congenital defects, while tissues expressing additional myosin II isoforms show either late onset of abnormalities or no pathological sign.

Identification and Molecular Characterization of Myosin Gene Family in Oryza sativa Genome

Myosins play an important role in various developmental processes in plants. We have identified 14 myosin genes in rice (Oryza sativa cv. Nipponbare) genome using sequence information available in public databases. Phylogenetic analysis of these sequences with other plant and non-plant myosins revealed that two of the predicted sequences belonged to class VIII and the others to class XI. All of these genes were distributed on seven chromosomes in the rice genome. Domain searches on these sequences indicated that a typical rice myosin consisted of Myosin_N, head domain, neck (IQ motifs), tail, and dilute (DIL) domain. Based on the sequence information obtained from predicted myosins, we isolated and sequenced two full-length cDNAs, OsMyoVIIIA and OsMyoXIE, representing each of the two classes of myosins. These two cDNAs isolated from different organs existed in isoforms due to differential splicing and showed minor differences from the predicted myosin in exon organization. Out of 14 myosin genes 11 were expressed in three major organs: leaves, panicles, and roots, among which three myosins exhibited different expression levels. On the other hand, three of the total myosin sequences showed organ-specific expression. The existence of different myosin genes and their isoforms in different organs or tissues indicates the diversity of myosin functions in rice.

Lessons from the genome sequence of Neurospora crassa: tracing the path from genomic blueprint to multicellular organism

We present an analysis of over 1,100 of the approximately 10,000 predicted proteins encoded by the genome sequence of the filamentous fungus Neurospora crassa. Seven major areas of Neurospora genomics and biology are covered. First, the basic features of the genome, including the automated assembly, gene calls, and global gene analyses are summarized. The second section covers components of the centromere and kinetochore complexes, chromatin assembly and modification, and transcription and translation initiation factors. The third area discusses genome defense mechanisms, including repeat induced point mutation, quelling and meiotic silencing, and DNA repair and recombination. In the fourth section, topics relevant to metabolism and transport include extracellular digestion; membrane transporters; aspects of carbon, sulfur, nitrogen, and lipid metabolism; the mitochondrion and energy metabolism; the proteasome; and protein glycosylation, secretion, and endocytosis. Environmental sensing is the focus of the fifth section with a treatment of two-component systems; GTP-binding proteins; mitogen-activated protein, p21-activated, and germinal center kinases; calcium signaling; protein phosphatases; photobiology; circadian rhythms; and heat shock and stress responses. The sixth area of analysis is growth and development; it encompasses cell wall synthesis, proteins important for hyphal polarity, cytoskeletal components, the cyclin/cyclin-dependent kinase machinery, macroconidiation, meiosis, and the sexual cycle. The seventh section covers topics relevant to animal and plant pathogenesis and human disease. The results demonstrate that a large proportion of Neurospora genes do not have homologues in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. The group of unshared genes includes potential new targets for antifungals as well as loci implicated in human and plant physiology and disease.

Stabilizing and Destabilizing Clusters in the Hydrophobic Core of Long Two-stranded α-Helical Coiled-coils

Detailed sequence analyses of the hydrophobic core residues of two long two-stranded alpha-helical coiled-coils that differ dramatically in sequence, function, and length were performed (tropomyosin of 284 residues and the coiled-coil domain of the myosin rod of 1086 residues). Three types of regions were present in the hydrophobic core of both proteins: stabilizing clusters and destabilizing clusters, defined as three or more consecutive core residues of either stabilizing (Leu, Ile, Val, Met, Phe, and Tyr) or destabilizing (Gly, Ala, Cys, Ser, Thr, Asn, Gln, Asp, Glu, His, Arg, Lys, and Trp) residues, and intervening regions that consist of both stabilizing and destabilizing residues in the hydrophobic core but no clusters. Subsequently, we designed a series of two-stranded coiled-coils to determine what defines a destabilizing cluster and varied the length of the destabilizing cluster from 3 to 7 residues to determine the length effect of the destabilizing cluster on protein stability. The results showed a dramatic destabilization, caused by a single Leu to Ala substitution, on formation of a 3-residue destabilizing cluster (DeltaT(m) of 17-21 degrees C) regardless of the stability of the coiled-coil. Any further substitution of Leu to Ala that increased the size of the destabilizing cluster to 5 or 7 hydrophobic core residues in length had little effect on stability (DeltaT(m) of 1.4-2.8 degrees C). These results suggested that the contribution of Leu to protein stability is context-dependent on whether the hydrophobe is in a stabilizing cluster or its proximity to neighboring destabilizing and stabilizing clusters.

Human deafness mutation of myosin VI (C442Y) accelerates the ADP dissociation rate

The missense mutation of Cys(442) to Tyr of myosin VI causes progressive postlingual sensorineural deafness. Here we report the affects of the C442Y mutation on the kinetics of the actomyosin ATP hydrolysis mechanism and motor function of myosin VI. The largest changes in the kinetic mechanism of ATP hydrolysis produced by the C442Y mutation are about 10-fold increases in the rate of ADP dissociation from both myosin VI and actomyosin VI. The rates of ADP dissociation from acto-C442Y myosin VI-ADP and C442Y myosin VI-ADP are 20-40 times more rapid than the steady state rates and cannot be the rate-limiting steps of the hydrolysis mechanism in the presence or absence of actin. The 2-fold increase in the actin gliding velocity of C442Y compared with wild type (WT) may be explained at least in part by the more rapid rate of ADP dissociation. The C442Y myosin VI has a significant increase ( approximately 10-fold) in the steady state ATPase rate in the absence of actin relative to WT myosin VI. The steady state rate of actin-activated ATP hydrolysis is unchanged by the C442Y mutation at low (<10(-7) m) calcium but is calcium-sensitive with a 1.6-fold increase at high ( approximately 10(-4) m) calcium that does not occur with WT. The actin gliding velocity of the C442Y mutant decreases significantly at low surface density of myosin VI, suggesting that the mutation hampers the processive movement of myosin VI.

A single class II myosin modulates T cell motility and stopping, but not synapse formation

Upon encountering an antigen, motile T cells stop crawling, change morphology and ultimately form an 'immunological synapse'. Although myosin motors are thought to mediate various aspects of this process, the molecules involved and their exact roles are not defined. Here we show that nonmuscle myosin heavy chain IIA, or MyH9, is the only class II myosin expressed in T cells and is associated with the uropod during crawling. MyH9 function is required for maintenance of the uropod and for T cell motility but is dispensable for synapse formation. Phosphorylation of MyH9 in its multimerization domain by T cell receptor-generated signals indicates that inactivation of this motor may be a key step in the 'stop' response during antigen recognition.

The Melanosome as a Model to Study Organelle Motility in Mammals

Melanosomes are lysosome-related organelles within which melanin pigment is synthesized. The molecular motors that allow these organelles to move within melanocytes have been the subject of intense study in several organisms. In mammals, melanosomes travel bi-directionally along microtubule tracks. The anterograde movement, i.e., towards microtubule plus-ends at the periphery, is accomplished by proteins of the kinesin superfamily, whereas the retrograde movement, i.e., towards microtubule minus-ends at the cell center, is achieved by dynein and dynein-associated proteins. At the periphery, melanosomes interact with the actin cytoskeleton via a tripartite complex formed by the small GTPase Rab27a, melanophilin and myosin Va, an actin-based motor. This interaction is essential for the maintenance of a dispersed state of the melanosomes, as shown by the perinuclear clustering of organelles in mutants in any of the referred proteins. In the retinal pigment epithelium, a similar complex formed by Rab27a, a melanophilin homolog called MyRIP and myosin VIIa is probably responsible for the tethering of melanosomes to the actin cytoskeleton. The coordination of motor activities is still poorly characterized, although some models have emerged in recent years and are discussed here. Unraveling regulatory mechanisms responsible for melanosome motility in pigmented cells will provide general insights into organelles dynamics within eukaryotic cells.

Nonmuscle myosin heavy-chain gene MYH14 is expressed in cochlea and mutated in patients affected by autosomal dominant hearing impairment (DFNA4)

Myosins have been implicated in various motile processes, including organelle translocation, ion-channel gating, and cytoskeleton reorganization. Different members of the myosin superfamily are responsible for syndromic and nonsyndromic hearing impairment in both humans and mice. MYH14 encodes one of the heavy chains of the class II nonmuscle myosins, and it is localized within the autosomal dominant hearing impairment (DFNA4) critical region. After demonstrating that MYH14 is highly expressed in mouse cochlea, we performed a mutational screening in a large series of 300 hearing-impaired patients from Italy, Spain, and Belgium and in a German kindred linked to DFNA4. This study allowed us to identify a nonsense and two missense mutations in large pedigrees, linked to DFNA4, as well as a de novo allele in a sporadic case. Absence of these mutations in healthy individuals was tested in 200 control individuals. These findings clearly demonstrate the role of MYH14 in causing autosomal dominant hearing loss and further confirm the crucial role of the myosin superfamily in auditive functions.

Ca2+-induced activation of ATPase activity of myosin Va is accompanied with a large conformational change

We succeeded in expressing the recombinant full-length myosin Va (M5Full) and studied its regulation mechanism. The actin-activated ATPase activity of M5Full was significantly activated by Ca(2+), whereas the truncated myosin Va without C-terminal globular domain is not regulated by Ca(2+) and constitutively active. Sedimentation analysis showed that the sedimentation coefficient of M5Full undergoes a Ca(2+)-induced conformational transition from 14S to 11S. Electron microscopy revealed that at low ionic strength, M5Full showed an extended conformation in high Ca(2+) while it formed a folded shape in the presence of EGTA, in which the tail domain was folded back towards the head-neck region. Furthermore, we found that the motor domain of myosin Va folds back to the neck domain in Ca(2+) while the head-neck domain is more extended in EGTA. It is thought that the association of the motor domain to the neck inhibits the binding of the tail to the neck thus destabilizing a folded conformation in Ca(2+). This conformational transition is closely correlated to the actin-activated ATPase activity. These results suggest that the tail and neck domain play a role in the Ca(2+) dependent regulation of myosin Va.

A monomeric myosin VI with a large working stroke

Myosin VI is involved in a wide variety of intracellular processes such as endocytosis, secretion and cell migration. Unlike almost all other myosins so far studied, it moves towards the minus end of actin filaments and is therefore likely to have unique cellular properties. However, its mechanism of force production and movement is not understood. Under our experimental conditions, both expressed full-length and native myosin VI are monomeric. Electron microscopy using negative staining revealed that the addition of ATP induces a large conformational change in the neck/tail region of the expressed molecule. Using an optical tweezers-based force transducer we found that expressed myosin VI is nonprocessive and produces a large working stroke of 18 nm. Since the neck region of myosin VI is short (it contains only a single IQ motif), it is difficult to reconcile the 18 nm working stroke with the classical 'lever arm mechanism', unless other structures in the molecule contribute to the effective lever. A possible model to explain the large working stroke of myosin VI is presented.

Actin assembly and endocytosis: from yeast to mammals

Internalization of receptors, lipids, pathogens, and other cargo at the plasma membrane involves several different pathways and requires coordinated interactions between a variety of protein and lipid molecules. The actin cytoskeleton is an integral part of the cell cortex, and there is growing evidence that F-actin plays a direct role in these endocytic events. Genetic studies in yeast have firmly established a functional connection between actin and endocytosis. Identification of several proteins that may function at the interface between actin and the endocytic machinery has provided further evidence for this association in both yeast and mammalian cells. Several of these proteins are directly involved in regulating actin assembly and could thus harness forces produced during actin polymerization to facilitate specific steps in the endocytic process. Recent microscopy studies in mammalian cells provide powerful evidence that localized recruitment and polymerization of actin occurs at endocytic sites. In this review, we focus on progress made in elucidating the functions of the actin cytoskeleton in endocytosis.

Toxoplasma as a novel system for motility

Motility is a characteristic of most living organisms and often requires specialized structures like cilia or flagella. An alternative is amoeboid movement, where the polymerization/depolymerization of actin leads to the formation of pseudopodia, filopodia and/or lamellipodia that enable the cell to crawl along a surface. Despite their lack of locomotive organelles and in absence of cell deformation, members of the apicomplexan parasites employ a unique form of locomotion called gliding motility to promote their migration across biological barriers and to power host-cell invasion and egress. Detailed studies in Toxoplasma gondii and Plasmodium species have revealed that this unique mode of movement is dependent on a myosin of class XIV and necessitates actin dynamics and the concerted discharge and processing of adhesive proteins. Gliding is essential for the survival and infectivity of these obligate intracellular parasites, which cause severe disease in humans and animals.

Inherited thrombocytopenias: toward a molecular understanding of disorders of platelet production

PURPOSE OF REVIEW

To review the defined syndromes of inherited thrombocytopenia and discuss new genetic data for several disorders that shed light on the process of megakaryopoiesis.

RECENT FINDINGS

The genes responsible for several inherited thrombocytopenias have been recently discovered, including congenital amegakaryocytic leukemia, amegakaryocytic thrombocytopenia with radio-ulnar synostosis, familial platelet syndrome with predisposition to acute myelogenous leukemia, Paris-Trousseau, Wiskott-Aldrich syndrome, and the May-Hegglin, Sebastian, Epstein, and Fechner syndromes. These clinical syndromes, combined with studies in mouse and in vitro models, reveal the importance of these genes for normal hematopoiesis.

SUMMARY

Although inherited syndromes of thrombocytopenia are rare, characterization of mutations in these disorders has contributed greatly to our understanding of megakaryocyte and platelet development. A systematic registry of congenitally thrombocytopenic individuals would almost certainly lead to new genetic discoveries.

Cloning and developmental expression of nonmuscle myosin IIA (Myh9) in the mammalian inner ear

MYH9 encoding a nonmuscle myosin heavy chain has been linked to nonsyndromic and syndromic forms of autosomal dominant hereditary hearing loss, suggesting a critical biological role of this motor protein in the auditory organ. While Myh9 expression has been described in the adult mouse, critical parameters pertaining to its developmental expression remain to be characterized. The current study describes cloning of the mouse Myh9 cDNA and the temporal onset and spatial distribution of Myh9 expression in the inner ear of the developing fetus, the neonate, and the adult. The cloned Myh9 cDNA contained two single-base-pair differences from the published genomic sequence: T990C (G330G) and T5198A (L1733Q). Immunoblotting of embryonic (E15.5) and adult tissues from several organs, including the cochlea, identified a single 250-kDa anti-Myh9-immunoreactive band, supporting an absence of Myh9 splice variants in the fetus and the adult. In situ expression analysis identified Myh9 distributed within the epithelial layer of the otic vesicle at E10.5. Myh9 expression was found to persist within the epithelia surrounding the cochlear duct at E13.5 and E16.5. The sensory cells of the developing cochlea were positive for Myh9 expression at E16.5. Within the neonate and the adult cochlea, Myh9 expression was observed within the sensory hair cells and the supporting hair cells of the organ of Corti, the spiral ligament, and the spiral limbus, but not in the stria vascularis. Identification of Myh9 in the developing and mature inner ear suggests a role for this protein in the development and maintenance of auditory function.

Regulation of the Actin Cytoskeleton by PI(4,5)P2 and PI(3,4,5)P3

The actin cytoskeleton is fundamental for various motile and morphogenetic processes in cells. The structure and dynamics of the actin cytoskeleton are regulated by a wide array of actin-binding proteins, whose activities are controlled by various signal transduction pathways. Recent studies have shown that certain membrane phospholipids, especially PI(4,5)P2 and PI(3,4,5)P3, regulate actin filament assembly in cells and in cell extracts. PI(4,5)P2 appears to be a general regulator of actin polymerization at the plasma membrane or at membrane microdomains, whereas PI(3,4,5)P3 promotes the assembly of specialized actin filament structures in response to some growth factors. Biochemical studies have demonstrated that the activities of many proteins promoting actin assembly are upregulated by PI(4,5)P2, whereas proteins that inhibit actin assembly or promote filament disassembly are down-regulated by PI(4,5)P2. PI(3,4,5)P3 promotes its effects on the actin cytoskeleton mainly through activation of the Rho family of small GTPases. In addition to their effects on actin dynamics, both PI(4,5)P2 and PI(3,4,5)P3 promote the formation of specific actin filament structures through activation/inactivation of actin filament cross-linking proteins and proteins that mediate cytoskeleton-plasma membrane interactions.

Nonmuscle myosin IIA and IIB have distinct functions in the exocytosis-dependent process of cell membrane repair

Vesicle generation, recruitment, and exocytosis are essential for repairing disruptions of cell membranes. The functions of nonmuscle myosin IIA and IIB in this exocytotic process of membrane repair were studied by the antisense technique. Knockdown of myosin IIB suppressed wound-induced exocytosis and the membrane resealing process. Knockdown of myosin IIA did not suppress exocytosis at an initial wound and had no inhibitory effect on the resealing at initial wounds but did inhibit the facilitated rate of resealing normally found at repeated wounds made at the same site. COS-7 cells, which lack myosin IIA, did not show the facilitated response of membrane resealing to a repeated wound. S91 melanoma cells, a mutant cell line lacking myosin Va, showed normal membrane resealing and normal facilitated responses. We concluded that myosin IIB was required for exocytosis and therefore cell membrane repair itself and that myosin IIA was required in facilitation of cell membrane repair at repeated wounds. Myosin IIB was primarily at the subplasmalemma cortex and myosin IIA was concentrated at the trans-Golgi network consistent with their distinct roles in vesicle trafficking in cell membrane repair.

Menin, a tumor suppressor, associates with nonmuscle myosin II-A heavy chain

MEN1 is a likely tumor suppressor gene that encodes a novel protein, menin. Menin is a 610 amino-acid residue protein with as yet unknown function(s). We have used tandem affinity purification and mass spectroscopy to isolate and identify proteins associating with menin from cultured HeLa cell extracts. This strategy has resulted in the isolation and identification of nonmuscle myosin type II-A heavy chain (NMHC II-A) as a menin interacting protein. This interaction was confirmed by glutathione-S-transferase pulldown assays, by coimmunoprecipitation, and by actin selection of myosin. We have further identified the amino-terminal region of menin and the head domain of NMHC II-A to be regions required for this interaction. Moreover menin was seen to colocalize with this myosin isoform in the cleavage furrow of dividing cells by indirect immunofluoresence. These data indicate that menin through binding to NMHC II-A could participate in cell division and in other processes that involve NMHC II-A.

Myosin II is present in gastric parietal cells and required for lamellipodial dynamics associated with cell activation

Nonmuscle myosin II has been shown to participate in organizing the actin cytoskeleton in polarized epithelial cells. Vectorial acid secretion in cultured parietal cells involves translocation of proton pumps from cytoplasmic vesicular membranes to the apical plasma membrane vacuole with coordinated lamellipodial dynamics at the basolateral membrane. Here we identify nonmuscle myosin II in rabbit gastric parietal cells. Western blots with isoform-specific antibodies indicate that myosin IIA is present in both cytosolic and particulate membrane fractions whereas the IIB isoform is associated only with particulate fractions. Immunofluorescent staining demonstrates that myosin IIA is diffusely located throughout the cytoplasm of resting parietal cells. However, after stimulation, myosin IIA is rapidly redistributed to lamellipodial extensions at the cell periphery; virtually all the cytoplasmic myosin IIA joins the newly formed basolateral membrane extensions. 2,3-Butanedione monoximine (BDM), a myosin-ATPase inhibitor, greatly diminishes the lamellipodial dynamics elicited by stimulation and retains the pattern of myosin IIA cytoplasmic staining. However, BDM had no apparent effect on the stimulation associated redistribution of H,K-ATPase from a cytoplasmic membrane compartment to apical membrane vacuoles. The myosin light chain kinase inhibitor 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-7) also did not alter the stimulation-associated recruitment of H,K-ATPase to apical membrane vacuoles, but unlike BDM it had relatively minor inhibitory effects on lamellipodial dynamics. We conclude that specific disruption of the basolateral actomyosin cytoskeleton has no demonstrable effect on recruitment of H,K-ATPase-rich vesicles into the apical secretory membrane. However, myosin II plays an important role in regulating lamellipodial dynamics and cortical actomyosin associated with parietal cell activation.

Localization and characterization of the inhibitory Ca2+-binding site of Physarum polycephalum myosin II

A myosin II is thought to be the driving force of the fast cytoplasmic streaming in the plasmodium of Physarum polycephalum. This regulated myosin, unique among conventional myosins, is inhibited by direct Ca2+ binding. Here we report that Ca2+ binds to the first EF-hand of the essential light chain (ELC) subunit of Physarum myosin. Flow dialysis experiments of wild-type and mutant light chains and the regulatory domain revealed a single binding site that shows moderate specificity for Ca2+. The regulatory light chain, in contrast to regulatory light chains of higher eukaryotes, is unable to bind divalent cations. Although the Ca2+-binding loop of ELC has a canonical sequence, replacement of glutamic acid to alanine in the -z coordinating position only slightly decreased the Ca2+ affinity of the site, suggesting that the Ca2+ coordination is different from classical EF-hands; namely, the specific "closed-to-open" conformational transition does not occur in the ELC in response to Ca2+. Ca2+- and Mg2+-dependent conformational changes in the microenvironment of the binding site were detected by fluorescence experiments. Transient kinetic experiments showed that the displacement of Mg2+ by Ca2+ is faster than the change in direction of cytoplasmic streaming; therefore, we conclude that Ca2+ inhibition could operate in physiological conditions. By comparing the Physarum Ca2+ site with the well studied Ca2+ switch of scallop myosin, we surmise that despite the opposite effect of Ca2+ binding on the motor activity, the two conventional myosins could have a common structural basis for Ca2+ regulation.

Myosin IIb is unconventionally conventional

Members of the myosin II class of molecular motors have been referred to as "conventional," a term used to describe their ability to form thick filaments, their low duty ratio, the ability of individual motor-containing "heads" to operate independently of each other, and their rate-limiting phosphate release. These features ensure that those motors that have completed their power stroke dissociate rapidly enough to prevent them from interfering with those motors that are beginning theirs. However, in this study, we demonstrate that myosin IIB, a cytoplasmic myosin II particularly enriched in the central nervous system and cardiac tissue, has a number of features that it shares instead with "unconventional" myosin isoforms, including myosins V and VI. These include a high duty ratio, rate-limiting ADP release, and high ADP affinity. These features imply that myosin IIB serves a set of physiologic needs different from those served by its more conventional myosin II counterparts, and this work provides a plausible basis for explaining the physiologic role of this unconventionally conventional myosin.

A novel myosin heavy chain gene in human chromosome 19q13.3

A human myosin heavy chain gene was identified in chromosome 19q13 by computational sequence analysis, RT-PCR and DNA sequencing of the cDNA. The complete cDNA has a length of 6786 bp and comprises 41 exons (40 coding) included in 108 kb of genomic sequence. Alternative splicing variants were also identified. The gene is expressed in a multitude of tissues, but mainly in small intestine, colon and skeletal muscle. The putative protein (228 kDa) carries the common myosin domains and presents high homology with the non-muscle myosin heavy chains (MYH9 and MYH10) as well as the smooth muscle myosin heavy chain MYH11. Nevertheless, phylogenetic analysis indicated that these homologous proteins are more related among themselves than to MYH14, suggesting that possibly this myosin heavy chain should be classified in a new myosin-subfamily.

Asp1424Asn MYH9 mutation results in an unstable protein responsible for the phenotypes in May-Hegglin anomaly/Fechtner syndrome

May-Hegglin anomaly (MHA), Fechtner syndrome (FTNS), Sebastian syndrome (SBS), and Epstein syndrome (EPS) are a group of rare, autosomal dominant disorders characterized by thrombocytopenia, giant platelets, and Döhle-like inclusion bodies, together with variable manifestations of Alport-like symptoms that include high-tone sensorineural deafness, cataracts, and nephritis. These disorders result from mutations in the MYH9 gene, which encodes for the nonmuscle myosin heavy chain A protein (also known as NMMHC-A). To date 20 different mutations have been characterized for this gene, but no clear phenotype-genotype correlation has been established, and very little is known regarding the molecular pathogenesis of this group of diseases. Here, we describe 2 new families with MHA/FTNS phenotypes that have been characterized in terms of their mutations, protein localization in megakaryocytes, protein expression, and mRNA stability. Our findings suggest that, at least for the Asp1424Asn mutation in the MYH9 gene, the phenotypes result from a highly unstable protein. No abnormalities in protein localization or mRNA stability were observed. We hypothesize that haploinsufficiency of the MYH9 results in a failure to properly reorganize the cytoskeleton in megakaryocytes as required for efficient platelet production.