Conservation within the myosin motor domain: implications for structure and function

Human Myo19 is a novel myosin that associates with mitochondria

Mitochondria are pleomorphic organelles that have central roles in cell physiology. Defects in their localization and dynamics lead to human disease. Myosins are actin-based motors that power processes such as muscle contraction, cytokinesis, and organelle transport. Here we report the initial characterization of myosin-XIX (Myo19), the founding member of a novel class of myosin that associates with mitochondria. The 970 aa heavy chain consists of a motor domain, three IQ motifs, and a short tail. Myo19 mRNA is expressed in multiple tissues, and antibodies to human Myo19 detect an approximately 109 kDa band in multiple cell lines. Both endogenous Myo19 and GFP-Myo19 exhibit striking localization to mitochondria. Deletion analysis reveals that the Myo19 tail is necessary and sufficient for mitochondrial localization. Expressing full-length GFP-Myo19 in A549 cells reveals a remarkable gain of function where the majority of the mitochondria move continuously. Moving mitochondria travel for many micrometers with an obvious leading end and distorted shape. The motility and shape change are sensitive to latrunculin B, indicating that both are actin dependent. Expressing the GFP-Myo19 tail in CAD cells resulted in decreased mitochondrial run lengths in neurites. These results suggest that this novel myosin functions as an actin-based motor for mitochondrial movement in vertebrate cells.

Myosin XI is required for actin-associated movement of plastid stromules

Stromules are highly dynamic stroma-filled tubules extending from the surface of plastids and occasionally interconnecting individual plastids, allowing the movement of complex biological molecules between the interconnected plastids. Experiments with inhibitors of cytoskeleton assembly have indicated the involvement of an actin-based system in stromule movement. However, the motor protein associated with the system had not been identified. Here, we present direct evidence that myosin XI is involved in the formation and movement of stromules in tobacco leaves. Application of 2,3-butanedione 2-monoxime, an inhibitor of myosin ATPase activity, resulted in the loss of stromules from tobacco leaf epidermal cells. Transient RNA interference of myosin XI in leaves of Nicotiana benthamiana also resulted in the loss of stromules from epidermal cells, without any effect on transcripts for actin or myosin VIII. Transient expression of a GFP-tagged myosin XI tail domain in tobacco leaf epidermal cells showed that the fusion protein localized to the chloroplast envelope, as well as to mitochondria and other organelles. Our findings identify myosin XI as a key protein involved in the formation and movement of stromules.

Alternative exon 9-encoded relay domains affect more than one communication pathway in the Drosophila myosin head

We investigated the biochemical and biophysical properties of one of the four alternative regions within the Drosophila myosin catalytic domain: the relay domain encoded by exon 9. This domain of the myosin head transmits conformational changes in the nucleotide-binding pocket to the converter domain, which is crucial to coupling catalytic activity with mechanical movement of the lever arm. To study the function of this region, we used chimeric myosins (IFI-9b and EMB-9a), which were generated by exchange of the exon 9-encoded domains between the native embryonic body wall (EMB) and indirect flight muscle isoforms (IFI). Kinetic measurements show that exchange of the exon 9-encoded region alters the kinetic properties of the myosin S1 head. This is reflected in reduced values for ATP-induced actomyosin dissociation rate constant (K(1)k(+2)) and ADP affinity (K(AD)), measured for the chimeric constructs IFI-9b and EMB-9a, compared to wild-type IFI and EMB values. Homology models indicate that, in addition to affecting the communication pathway between the nucleotide-binding pocket and the converter domain, exchange of the relay domains between IFI and EMB affects the communication pathway between the nucleotide-binding pocket and the actin-binding site in the lower 50-kDa domain (loop 2). These results suggest an important role of the relay domain in the regulation of actomyosin cross-bridge kinetics.

Analysis of tarantula skeletal muscle protein sequences and identification of transcriptional isoforms

Background

Tarantula has been used as a model system for studying skeletal muscle structure and function, yet data on the genes expressed in tarantula muscle are lacking.

Results

We constructed a cDNA library from Aphonopelma sp. (Tarantula) skeletal muscle and got 2507 high-quality 5'ESTs (expressed sequence tags) from randomly picked clones. EST analysis showed 305 unigenes, among which 81 had more than 2 ESTs. Twenty abundant unigenes had matches to skeletal muscle-related genes including actin, myosin, tropomyosin, troponin-I, T and C, paramyosin, muscle LIM protein, muscle protein 20, a-actinin and tandem Ig/Fn motifs (found in giant sarcomere-related proteins). Matches to myosin light chain kinase and calponin were also identified. These results support the existence of both actin-linked and myosin-linked regulation in tarantula skeletal muscle.

We have predicted full-length as well as partial cDNA sequences both experimentally and computationally for myosin heavy and light chains, actin, tropomyosin, and troponin-I, T and C, and have deduced the putative peptides. A preliminary analysis of the structural and functional properties was also carried out. Sequence similarities suggested multiple isoforms of most myofibrillar proteins, supporting the generality of multiple isoforms known from previous muscle sequence studies. This may be related to a mix of muscle fiber types.

Conclusion

The present study serves as a basis for defining the transcriptome of tarantula skeletal muscle, for future in vitro expression of tarantula proteins, and for interpreting structural and functional observations in this model species.

Expression of multiple slow myosin heavy chain genes reveals a diversity of zebrafish slow twitch muscle fibres with differing requirements for Hedgehog and Prdm1 activity

The zebrafish embryo develops a series of anatomically distinct slow twitch muscle fibres that characteristically express genes encoding lineage-specific isoforms of sarcomeric proteins such as MyHC and troponin. We show here that different subsets of these slow fibres express distinct members of a tandem array of slow MyHC genes. The first slow twitch muscle fibres to differentiate, which are specified by the activity of the transcription factor Prdm1 (also called Ubo or Blimp1) in response to Hedgehog (Hh) signalling, express the smyhc1 gene. Subsequently, secondary slow twitch fibres differentiate in most cases independently of Hh activity. We find that although some of these later-forming fibres also express smyhc1, others express smyhc2 or smyhc3. We show that the smyhc1-positive fibres express the ubo (prdm1) gene and adopt fast twitch fibre characteristics in the absence of Prdm1 activity, whereas those that do not express smyhc1 can differentiate independently of Prdm1 function. Conversely, some smyhc2-expressing fibres, although independent of Prdm1 function, require Hh activity to form. The adult trunk slow fibres express smyhc2 and smyhc3, but lack smyhc1 expression. The different slow fibres in the craniofacial muscles variously express smyhc1, smyhc2 and smyhc3, and all differentiate independently of Prdm1.

The small molecule tool (S)-(-)-blebbistatin: novel insights of relevance to myosin inhibitor design

The small molecule blebbistatin is now a front line tool in the study of myosin function. Chemical modification of the tricyclic core of blebbistatin could deliver the next generation of myosin inhibitors and to help address this we report here on the impact of structural changes in the methyl-substituted aromatic ring of blebbistatin on its biological activity. Chemical methods for the preparation of isomeric methyl-containing analogues are reported and a series of co-crystal structures are used to rationalise the observed variations in their biological activity. These studies further support the view that the previously identified binding mode of blebbistatin to Dictyostelium discoideum myosin II is of relevance to its mode of action. A discussion of the role that these observations have on planning the synthesis of focused libraries of blebbistatin analogues is also provided including an assessment of possibilities by computational methods. These studies are ultimately directed at the development of novel myosin inhibitors with improved affinity and different selectivity profiles from blebbistatin itself.

How are the cellular functions of myosin VI regulated within the cell?

This review, dedicated to the memory of Professor Setsuro Ebashi, focuses on our current work investigating the cellular functions and regulation of the unique unconventional motor, myosin VI. This myosin, unlike all the other myosins so far studied, moves towards the minus end of actin filaments and has been implicated in a wide range of cellular processes such as endocytosis, exocytosis, cell migration, cell division and cytokinesis. Myosin VI’s involvement in these cellular pathways is mediated by its interaction with specific adaptor proteins and is regulated by multiple regulatory signals and modifications such as calcium ions, PtdIns(4,5)P₂ (PIP₂) and phosphorylation. Understanding the functions of myosin VI within the cell and how it is regulated is now of utmost importance given the recent observations that it is associated with a number of human disorders such as deafness and cancers.

Human deafness mutation E385D disrupts the mechanochemical coupling and subcellular targeting of myosin-1a

Missense mutations in the membrane-binding actin-based motor protein, myosin-1a (Myo1a), have recently been linked to sensorineural deafness in humans. One of these mutations, E385D, impacts a residue in the switch II region of the motor domain that is present in virtually all members of the myosin superfamily. We sought to examine the impact of E385D on the function of Myo1a, both in terms of mechanochemical activity and ability to target to actin-rich microvilli in polarized epithelial cells. While E385D-Myo1a demonstrated actin-activated ATPase activity, the V(MAX) was reduced threefold relative to wild-type. Despite maintaining an active mechanochemical cycle, E385D-Myo1a was unable to move actin in the sliding filament assay. Intriguingly, when an enhanced-green-fluorescent-protein-tagged form of E385D-Myo1a was stably expressed in polarized epithelial cells, this mutation abolished the microvillar targeting normally demonstrated by wild-type Myo1a. Notably, these data are the first to suggest that mechanical activity is essential for proper localization of Myo1a in microvilli. These studies also provide a unique example of how even the most mild substitution of invariant switch II residues can effectively uncouple enzymatic and mechanical activity of the myosin motor domain.

Predicting allosteric communication in myosin via a pathway of conserved residues

We present a computational method that predicts a pathway of residues that mediate protein allosteric communication. The pathway is predicted using only a combination of distance constraints between contiguous residues and evolutionary data. We applied this analysis to find pathways of conserved residues connecting the myosin ATP binding site to the lever arm. These pathway residues may mediate the allosteric communication that couples ATP hydrolysis to the lever arm recovery stroke. Having examined pre-stroke conformations of Dictyostelium, scallop, and chicken myosin II as well as Dictyostelium myosin I, we observed a conserved pathway traversing switch II and the relay helix, which is consistent with the understood need for allosteric communication in this conformation. We also examined post-rigor and rigor conformations across several myosin species. Although initial residues of these paths are more heterogeneous, all but one of these paths traverse a consistent set of relay helix residues to reach the beginning of the lever arm. We discuss our results in the context of structural elements and reported mutational experiments, which substantiate the significance of the pre-stroke pathways. Our method provides a simple, computationally efficient means of predicting a set of residues that mediate allosteric communication. We provide a refined, downloadable application and source code (on https://simtk.org) to share this tool with the wider community (https://simtk.org/home/allopathfinder).

The Structural Coupling between ATPase Activation and Recovery Stroke in the Myosin II Motor

Before the myosin motor head can perform the next power stroke, it undergoes a large conformational transition in which the converter domain, bearing the lever arm, rotates approximately 65 degrees . Simultaneous with this "recovery stroke," myosin activates its ATPase function by closing the Switch-2 loop over the bound ATP. This coupling between the motions of the converter domain and of the 40 A-distant Switch-2 loop is essential to avoid unproductive ATP hydrolysis. The coupling mechanism is determined here by finding a series of optimized intermediates between crystallographic end structures of the recovery stroke (Dictyostelium discoideum), yielding movies of the transition at atomic detail. The successive formation of two hydrogen bonds by the Switch-2 loop is correlated with the successive see-saw motions of the relay and SH1 helices that hold the converter domain. SH1 helix and Switch-2 loop communicate via a highly conserved loop that wedges against the SH1-helix upon Switch-2 closing.

Evidence for an interaction between the SH3 domain and the N-terminal extension of the essential light chain in class II myosins

The function of the src-homology 3 (SH3) domain in class II myosins, a distinct beta-barrel structure, remains unknown. Here, we provide evidence, using electron cryomicroscopy, in conjunction with light-scattering, fluorescence and kinetic analyses, that the SH3 domain facilitates the binding of the N-terminal extension of the essential light chain isoform (ELC-1) to actin. The 41 residue extension contains four conserved lysine residues followed by a repeating sequence of seven Pro/Ala residues. It is widely believed that the highly charged region interacts with actin, while the Pro/Ala-rich sequence forms a rigid tether that bridges the approximately 9 nm distance between the myosin lever arm and the thin filament. In order to localize the N terminus of ELC in the actomyosin complex, an engineered Cys was reacted with undecagold-maleimide, and the labeled ELC was exchanged into myosin subfragment-1 (S1). Electron cryomicroscopy of S1-bound actin filaments, together with computer-based docking of the skeletal S1 crystal structure into 3D reconstructions, showed a well-defined peak for the gold cluster near the SH3 domain. Given that SH3 domains are known to bind proline-rich ligands, we suggest that the N-terminal extension of ELC interacts with actin and modulates myosin kinetics by binding to the SH3 domain during the ATPase cycle.

A variable domain near the ATP-binding site in Drosophila muscle myosin is part of the communication pathway between the nucleotide and actin-binding sites

Drosophila expresses several muscle myosin isoforms from a single gene by alternatively splicing six of the 19 exons. Here we investigate exon 7, which codes for a region in the upper 50 kDa domain near the nucleotide-binding pocket. This region is of interest because it is also the place where a large insert is found in myosin VI and where several cardiomyopathy mutations have been identified in human cardiac myosin. We expressed and purified chimeric muscle myosins from Drosophila, each varying at exon 7. Two chimeras exchanged the entire exon 7 domain between the indirect flight muscle (IFI, normally containing exon 7d) and embryonic body wall muscle (EMB, normally containing exon 7a) isoforms to create IFI-7a and EMB-7d. The second two chimeras replaced each half of the exon 7a domain in EMB with the corresponding portion of exon 7d to create EMB-7a/7d and EMB-7d/7a. Transient kinetic studies of the motor domain from these myosin isoforms revealed changes in several kinetic parameters between the IFI or EMB isoforms and the chimeras. Of significance were changes in nucleotide binding, which differed in the presence and absence of actin, consistent with a model in which the exon 7 domain is part of the communication pathway between the nucleotide and actin-binding sites. Homology models of the structures suggest how the exon 7 domain might modulate this pathway.

The Slow Skeletal Muscle Isoform of Myosin Shows Kinetic Features Common to Smooth and Non-muscle Myosins

Fast and slow mammalian muscle myosins differ in the heavy chain sequences (MHC-2, MHC-1) and muscles expressing the two isoforms contract at markedly different velocities. One role of slow skeletal muscles is to maintain posture with low ATP turnover, and MHC-1 expressed in these muscles is identical to heavy chain of the beta-myosin of cardiac muscle. Few studies have addressed the biochemical kinetic properties of the slow MHC-1 isoform. We report here a detailed analysis of the MHC-1 isoform of the rabbit compared with MHC-2 and focus on the mechanism of ADP release. We show that MHC-1, like some non-muscle myosins, shows a biphasic dissociation of actin-myosin by ATP. Most of the actin-myosin dissociates at up to approximately 1000 s(-1), a very similar rate constant to MHC-2, but 10-15% of the complex must go through a slow isomerization (approximately 20 s(-1)) before ATP can dissociate it. Similar slow isomerizations were seen in the displacement of ADP from actin-myosin.ADP and provide evidence of three closely related actin-myosin.ADP complexes, a complex in rapid equilibrium with free ADP, a complex from which ADP is released at the rate required to define the maximum shortening velocity of slow muscle fibers (approximately 20 s(-1)), and a third complex that releases ADP too slowly (approximately 6 s(-1)) to be on the main ATPase pathway. The role of these actin-myosin.ADP complexes in the mechanochemistry of slow muscle contraction is discussed in relation to the load dependence of ADP release.

Structure determination of the motor domain of yeast kinesin kar3 by x-ray crystallography

Kinesins are molecular motors that share a common structural core with myosins and G proteins and play diverse roles in organelle transport and cell division and movement. Kinesin motors use the chemical energy derived from ATP hydrolysis to generate force for moving on the microtubule track. The mechanism by which kinesin motors capture the energy from ATP hydrolysis and convert it to a force is not completely known. Structural elements that undergo movement and the force-producing conformational changes of the motor must be identified to elucidate this mechanism. X-ray crystallography is the method of choice for elucidating the structural changes of kinesin motors during ATP hydrolysis.

Arabidopsis thaliana myosin XIK is involved in root hair as well as trichome morphogenesis on stems and leaves

Myosins form a large superfamily of molecular motors that move along actin filaments. The functions of myosins in plant cells are thought to be related to various processes: cell division, movement of mitochondria and chloroplasts, cytoplasmic streaming, rearrangement of transvacuolar strands, and statolith positioning. Class VIII and XI myosins are represented in the Arabidopsis thaliana genome by 4 and 13 potential genes, respectively. The roles of individual class XI myosins and their cellular targets in A. thaliana are still unclear. In this work we implemented a reverse genetic approach to analyse the loss-of-function mutants of XIK, a representative of class XI myosins in A. thaliana. Three different T-DNA insertion mutants in the myosin XIK gene showed similar phenotypes: impaired growth of root hair cells, twisted shape of stem trichomes, and irregular size, branch positioning, and branch expansion of leaf trichomes. Morphometric analysis of mutant seedlings showed that the average length of root hairs was reduced up to 50% in comparison with wild-type root hairs, suggesting an involvement of the class XI myosin XIK in tip growth. On leaves, the proportion of trichomes with short branches was doubleed in mutant plants, and the mutant trichomes possessed a mildly twisted shape. Therefore, we concluded that myosin XIK is involved also in the elongation of stalks and branches of trichomes.

The principal motions involved in the coupling mechanism of the recovery stroke of the myosin motor

Muscle contraction is driven by a cycle of conformational changes in the myosin II head. After myosin binds ATP and releases from the actin fibril, myosin prepares for the next power stroke by rotating back the converter domain that carries the lever arm by 60 degrees . This recovery stroke is coupled to the activation of myosin ATPase by a mechanism that is essential for an efficient motor cycle. The mechanics of this coupling have been proposed to occur via two distinct and successive motions of the two helices that hold the converter domain: in a first phase a seesaw motion of the relay helix, followed by a piston-like motion of the SH1 helix in a second phase. To test this model, we have determined the principal motions of these structural elements during equilibrium molecular dynamics simulations of the crystallographic end states of the recovery-stroke by using principal component analysis. This reveals that the only principal motions of these two helices that make a large-amplitude contribution towards the conformational change of the recovery stroke are indeed the predicted seesaw and piston motions. Moreover, the results demonstrate that the seesaw motion of the relay helix dominates in the dynamics of the pre-recovery stroke structure, but not in the dynamics of the post-recovery stroke structure, and vice versa for the piston motion of the SH1 helix. This is consistent with the order of the proposed two-phase model for the coupling mechanism of the recovery stroke. Molecular movies of these principal motions are available at http://www.iwr.uni-heidelberg.de/groups/biocomp/fischer.

The cargo-binding domain regulates structure and activity of myosin 5

Myosin 5 is a two-headed motor protein that moves cargoes along actin filaments. Its tail ends in paired globular tail domains (GTDs) thought to bind cargo. At nanomolar calcium levels, actin-activated ATPase is low and the molecule is folded. Micromolar calcium concentrations activate ATPase and the molecule unfolds. Here we describe the structure of folded myosin and the GTD's role in regulating activity. Electron microscopy shows that the two heads lie either side of the tail, contacting the GTDs at a lobe of the motor domain (approximately Pro 117-Pro 137) that contains conserved acidic side chains, suggesting ionic interactions between motor domain and GTD. Myosin 5 heavy meromyosin, a constitutively active fragment lacking the GTDs, is inhibited and folded by a dimeric GST-GTD fusion protein. Motility assays reveal that at nanomolar calcium levels heavy meromyosin moves robustly on actin filaments whereas few myosins bind or move. These results combine to show that with no cargo, the GTDs bind in an intramolecular manner to the motor domains, producing an inhibited and compact structure that binds weakly to actin and allows the molecule to recycle towards new cargoes.

Simulations of the myosin II motor reveal a nucleotide-state sensing element that controls the recovery stroke

During the recovery stroke, the myosin motor is primed for the next power stroke by a 60 degree rotation of its lever arm. This reversible motion is coupled to the activation of the ATPase function of myosin through conformational changes along the relay helix, which runs from the Switch-2 loop near the ATP to the converter domain carrying the lever arm. Via a hydrogen bond between the side-chain of Asn475 on the relay helix and the Gly457/Ser456 peptide group on the Switch-2, the rotation of the converter domain is coupled to the formation of a hydrogen bond between Gly457 and gamma-phosphate that is essential for ATP hydrolysis. Here, molecular dynamics simulations of Dictyostelium discoideum myosin II in the two end conformations of the recovery stroke with different nucleotide states (ATP, ADP x Pi, ADP) reveal that the side-chain of Asn475 breaks away from Switch-2 upon ATP hydrolysis to make a hydrogen bond with Tyr573. This sensing of the nucleotide state is achieved by a small displacement of the cleaved gamma-phosphate towards Gly457 which in turn pushes Asn475 away. The sensing plays a dual role by (i) preventing the wasteful reversal of the recovery stroke while the nucleotide is in the ADP x Pi state, and (ii) decoupling the relay helix from Switch-2, thus allowing the power stroke to start upon initial binding to actin while Gly457 of Switch-2 keeps interacting with the Pi (known to be released only later after tight actin binding). A catalytically important salt bridge between Arg238 (on Switch-1) and Glu459 (on Switch-2), which covers the hydrolysis site, is seen to form rapidly when ATP is added to the pre-recovery stroke conformer and remains stable after the recovery stroke, indicating that it has a role in shaping the ATP binding site by induced fit.

Class II myosins in nematodes — genetic relationships, fundamental and applied implications

Myosins are represented by a wide range of different classes of molecule, of which the most extensively studied are the class II myosins which drive muscle contraction and cell organization; the functional unit of class II myosins comprises two myosin heavy chains (MHCs). This minireview gives an update on class II MHCs of nematodes and describes a comparative analysis of MHC genes from nematodes and other organismal groups. Genetic analyses of sequence data for the four functional domains of MHCs (i.e., the SH3-like N-terminal, head, neck and tail domains) reveal a delineation between both the nematode and non-nematode myosins and between muscle and non-muscle myosins. The distinctiveness of the MHCs of nematodes suggests functional and tissue specialization. The elucidation of the functional roles of myosins and other molecules in specific signaling pathways in nematodes has the potential to lead to new intervention strategies for parasites via the specific disruption or interruption of key developmental processes, having biotechnological implications in the longer term.

Plasmodium falciparum myosins: transcription and translation during asexual parasite development

Six myosins genes are now annotated in the Plasmodium falciparum Genome Project. Malaria myosins have been named alphabetically; accordingly, we refer to the two latest additions as Pfmyo-E and Pfmyo-F. Both new myosins contain regions characteristic of the functional motor domain of "true" myosins and, unusually for P. falciparum myosins, Pfmyo-F encodes two consensus IQ light chain-binding motifs. Phylogenetic analysis of the 17 currently known apicomplexan myosins together with one representative of each myosin class clusters all but one of the apicomplexan sequences together in Class XIV. This refines the earlier definition of the Class XIV Subclasses XIVa and XIVb. RT-PCR on blood stage parasite mRNA amplifies a specific product for all six myosins and each shows developmentally regulated transcription. Thus: Pfmyo-A and Pfmyo-B genes are transcribed throughout development; Pfmyo-C is predominant in trophozoites; Pfmyo-D occurs in trophozoites and schizonts; Pfmyo-E though barely present in earlier stages is abundant in schizonts; Pfmyo-F increases steadily throughout development and maturation. It is known that Pfmyo-A and Pfmyo-B are synthesised during late schizogony and we now show that Pfmyo-D expression is also temporally regulated to late trophozoites and schizonts where it distributes close to segregating nuclei. Thus, in asexual stages myosin synthesis does not always parallel transcript accumulation, showing that translation is also regulated. The implication is that the mRNAs are either subjected to turnover, synthesised and degraded, or that they are sequestered in an inactivate form until required for protein synthesis.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

Myosin V attachment to cargo requires the tight association of two functional subdomains

The myosin V carboxyl-terminal globular tail domain is essential for the attachment of myosin V to all known cargoes. Previously, the globular tail was viewed as a single, functional entity. Here, we show that the globular tail of the yeast myosin Va homologue, Myo2p, contains two structural subdomains that have distinct functions, namely, vacuole-specific and secretory vesicle–specific movement. Biochemical and genetic analyses demonstrate that subdomain I tightly associates with subdomain II, and that the interaction does not require additional proteins. Importantly, although neither subdomain alone is functional, simultaneous expression of the separate subdomains produces a functional complex in vivo. Our results suggest a model whereby intramolecular interactions between the globular tail subdomains help to coordinate the transport of multiple distinct cargoes by myosin V.

Molecular dynamics analysis of structural factors influencing back door pi release in myosin

The back door has been proposed to be an exit pathway from the myosin active site for phosphate (P(i)) generated by adenosine 5'-triphosphate hydrolysis. We used molecular dynamics simulations to investigate the interaction of P(i) with the back door and the plausibility of P(i) release via this route. Molecular dynamics simulations were performed on the Dictyostelium motor domain with bound Mg.adenosine 5'-diphosphate (ADP) and P(i), modeled upon the Mg.ADP.BeF(x) and Mg.ADP.V(i) structures. Simulations revealed that the relaxation of ADP and free P(i) from their initial positions reduced the diameter of the back door via motions of switch 1 and switch 2 located in the upper and lower 50-kDa subdomains, respectively. In neither simulation could P(i) freely diffuse out the back door. Water molecules, however, could flux through the back door in the Mg.ADP.BeF(x)-based simulation but not in the Mg.ADP.V(i)-based simulation. In neither structure was water observed fluxing through the main (front door) entrance. These observations suggest that the ability of P(i) to leave via the back door is linked tightly to conformational changes between the upper and lower 50-kDa subdomains. The simulations offer structural explanations for (18)O-exchange with P(i) at the active site, and P(i) release being the rate-limiting step in the myosin adenosine 5'-triphosphatase.

The “Roll and Lock” Mechanism of Force Generation in Muscle

Muscle force results from the interaction of the globular heads of myosin-II with actin filaments. We studied the structure-function relationship in the myosin motor in contracting muscle fibers by using temperature jumps or length steps combined with time-resolved, low-angle X-ray diffraction. Both perturbations induced simultaneous changes in the active muscle force and in the extent of labeling of the actin helix by stereo-specifically bound myosin heads at a constant total number of attached heads. The generally accepted hypothesis assumes that muscle force is generated solely by tilting of the lever arm, or the light chain domain of the myosin head, about its catalytic domain firmly bound to actin. Data obtained suggest an additional force-generating step: the "roll and lock" transition of catalytic domains of non-stereo-specifically attached heads to a stereo-specifically bound state. A model based on this scheme is described to quantitatively explain the data.

Myosin head mechanical performance under different conformational change mechanisms

The present paper puts forward a mathematical approach to model the conformational changes of the myosin head due to ATP hydrolysis, which determine the head swinging and consequent sliding of the actin filament. Our aim is to provide a simple but effective model simulating myosin head performance to be integrated into the overall model of sarcomere mechanics under development at our Laboratory (J. Biomech. 34 (2001) 1607). We began by exploring myosin head mechanics in recent findings about myosin ultrastructure, morphology and energetics in order to calculate the working stroke distance (WS) and the force transmitted to the actin filament during muscle contraction. Two different working stroke mechanisms were investigated, assuming that the swinging of the myosin head occurs either as a consequence of purely conformational changes (Science 261 (1993a) 58) or by thermally driven motion (ratchet mechanism) followed by conformational changes (Cell 99 (1999) 421). Our results show that force and WS values vary markedly between the two models. The maximum force generated is about 10 pN for the first model and 31 pN for the second model, and the WSs are about 13 and 4 nm, respectively. These results are then discussed and compared with published data. The experimental data used for comparison are scarce and non-homogeneous; hence, the final remarks do not lead to definite conclusions. In any event, relatively speaking, the first model is more coherent with experimental findings.

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.

cDNA cloning and characterization of a novel gene encoding the MLF1-interacting protein MLF1IP

Myelodysplasia/acute myeloid leukemia (MDS/AML) is characterized by a t(3;5)(q25.1;q34) chromosomal translocation that forms a fusion gene between nucleophosmin (NPM) and MDS/myeloid leukemia factor 1 (MLF1). We identified a novel protein, MLF1-interacting protein (MLF1IP), that specifically associates with MLF1 by yeast two-hybrid analysis and in pulldown assays, and colocalizes with it in both the nuclei and cytoplasm of cells. The MLF1IP gene locus is at chromosome 4q35.1 and is composed of 14 exons spanning 75.8 kb of genomic DNA. The MLF1IP cDNA encodes a 46-kDa protein that contains two bipartite and two classical nuclear localization signals, two nuclear receptor-binding motifs (LXXLL), two leucine zippers, two PEST residues and several potential phosphorylation sites. MLF1IP transcripts are expressed in a variety of tissues (e.g. fetal liver, bone marrow, thymus and testis). MLF1IP appears to be a lineage-specific gene whose expression is confined exclusively to the CFU-E erythroid precursor cells, but not in mature erythrocytes. These observations, together with previous data demonstrating a role for MLF1 in suppressing red cell maturation, suggest a possible role for MLF1IP and MLF1 deregulation in the genesis of erythroleukemias.

A subclass of myosin XI is associated with mitochondria, plastids, and the molecular chaperone subunit TCP-1? in maize

The role and regulation of specific plant myosins in cyclosis is not well understood. In the present report, an affinity-purified antibody generated against a conserved tail region of some class XI plant myosin isoforms was used for biochemical and immunofluorescence studies of Zea mays. Myosin XI co-localized with plastids and mitochondria but not with nuclei, the Golgi apparatus, endoplasmic reticulum, or peroxisomes. This suggests that myosin XI is involved in the motility of specific organelles. Myosin XI was more than 50% co-localized with tailless complex polypeptide-1alpha (TCP-1alpha) in tissue sections of mature tissues located more than 1.0 mm from the apex, and the two proteins co-eluted from gel filtration and ion exchange columns. On Western blots, TCP-1alpha isoforms showed a developmental shift from the youngest 5.0 mm of the root to more mature regions that were more than 10.0 mm from the apex. This developmental shift coincided with a higher percentage of myosin XI /TCP-1alpha co-localization, and faster degradation of myosin XI by serine protease. Our results suggest that class XI plant myosin requires TCP-1alpha for regulating folding or providing protection against denaturation.

Time-resolved fluorescence anisotropy studies show domain-specific interactions of calmodulin with IQ target sequences of myosin V

Single cysteine mutants of calmodulin, Cam(S38C) and Cam(N111C), have been specifically labelled with Alexa488 maleimide to study the effects of calcium on the structural dynamics of calmodulin complexed with IQ3, IQ4 and IQ34 target peptide motifs of mouse unconventional myosin-V. Using phase fluorometry, the time-resolved anisotropy shows well-separated global and segmental correlation times. The calcium-sensitive global motion of either calmodulin domain can be independently monitored in domain-specific interactions of either apo- or Ca(4).calmodulin with IQ3 or IQ4 peptides. C-domain interactions predominate, and apo-N-domain interactions are unexpectedly weak. The 1:1 complex of Ca(4).calmodulin with IQ34 behaves as a compact globular species. The results demonstrate novel dynamic aspects of calmodulin-IQ interactions relating to the calcium regulation of motility of unconventional myosin.

Myo3A, one of two class III myosin genes expressed in vertebrate retina, is localized to the calycal processes of rod and cone photoreceptors and is expressed in the sacculus

The striped bass has two retina-expressed class III myosin genes, each composed of a kinase, motor, and tail domain. We report the cloning, sequence analysis, and expression patterns of the long (Myo3A) and short (Myo3B) class III myosins, as well as cellular localization and biochemical characterization of the long isoform, Myo3A. Myo3A (209 kDa) is expressed in the retina, brain, testis, and sacculus, and Myo3B (155 kDa) is expressed in the retina, intestine, and testis. The tails of these two isoforms contain two highly conserved domains, 3THDI and 3THDII. Whereas Myo3B has three IQ motifs, Myo3A has nine IQ motifs, four in its neck and five in its tail domain. Myo3A localizes to actin filament bundles of photoreceptors and is concentrated in the calycal processes. An anti-Myo3A antibody decorates the actin cytoskeleton of rod inner/outer segments, and this labeling is reduced by the presence of ATP. The ATP-sensitive actin association is a feature characteristic of myosin motors. The numerous IQ motifs may play a structural or signaling role in the Myo3A, and its localization to calycal processes indicates that this myosin mediates a local function at this site in vertebrate photoreceptors.

Regulatory implications of a novel mode of interaction of calmodulin with a double IQ-motif target sequence from murine dilute myosin V

Apo-Calmodulin acts as the light chain for unconventional myosin V, and treatment with Ca(2+) can cause dissociation of calmodulin from the 6IQ region of the myosin heavy chain. The effects of Ca(2+) on the stoichiometry and affinity of interactions of calmodulin and its two domains with two myosin-V peptides (IQ3 and IQ4) have therefore been quantified in vitro, using fluorescence and near- and far-UV CD. The results with separate domains show their differential affinity in interactions with the IQ motif, with the apo-N domain interacting surprisingly weakly. Contrary to expectations, the effect of Ca(2+) on the interactions of either peptide with either isolated domain is to increase affinity, reducing the K(d) at physiological ionic strengths by >200-fold to approximately 75 nM for the N domain, and approximately 10-fold to approximately 15 nM for the C domain. Under suitable conditions, intact (holo- or apo-) calmodulin can bind up to two IQ-target sequences. Interactions of apo- and holo-calmodulin with the double-length, concatenated sequence (IQ34) can result in complex stoichiometries. Strikingly, holo-calmodulin forms a high-affinity 1:1 complex with IQ34 in a novel mode of interaction, as a "bridged" structure wherein two calmodulin domains interact with adjacent IQ motifs. This apparently imposes a steric requirement for the alpha-helical target sequence to be discontinuous, possibly in the central region, and a model structure is illustrated. Such a mode of interaction could account for the Ca(2+)-dependent regulation of myosin V in vitro motility, by changing the structure of the regulatory complex, and paradoxically causing calmodulin dissociation through a change in stoichiometry, rather than a Ca(2+)-dependent reduction in affinity.

Mutations in human nonmuscle myosin IIA found in patients with May-Hegglin anomaly and Fechtner syndrome result in impaired enzymatic function

A family of autosomal-dominant diseases including May-Hegglin anomaly, Fechtner syndrome, Sebastian syndrome, Alport syndrome, and Epstein syndrome are commonly characterized by giant platelets and thrombocytopenia. In addition, there may be leukocyte inclusions, deafness, cataracts, and nephritis, depending on the syndrome. Mutations in the human nonmuscle myosin IIA heavy chain gene (MYH9) have been linked to these diseases. Two of the recently described mutations, N93K and R702C, are conserved in smooth and nonmuscle myosins from vertebrates and lie in the head domain of myosin. Interestingly, the two mutations lie within close proximity in the three-dimensional structure of myosin. These two mutations were engineered into a heavy meromyosin-like recombinant fragment of nonmuscle myosin IIA, which was expressed in baculovirus along with the appropriate light chains. The R702C mutant displays 25% of the maximal MgATPase activity of wild type heavy meromyosin and moves actin filaments at half the wild type rate. The effects of the N93K mutation are more dramatic. This heavy meromyosin has only 4% of the maximal MgATPase activity of wild type and does not translocate actin filaments in an in vitro motility assay. Biochemical characterization of the mutant is consistent with this mutant being unable to fully adopt the "on" conformation.

Differential scanning calorimetric study of myosin subfragment 1 with tryptic cleavage at the N-terminal region of the heavy chain

The thermal unfolding of myosin subfragment 1 (S1) cleaved by trypsin was studied by differential scanning calorimetry. In the absence of nucleotides, trypsin splits the S1 heavy chain into three fragments (25, 50, and 20 kDa). This cleavage has no appreciable influence on the thermal unfolding of S1 examined in the presence of ADP, in the ternary complexes of S1 with ADP and phosphate analogs, such as orthovanadate (Vi) or beryllium fluoride (BeFx), and in the presence of F-actin. In the presence of ATP and in the complexes S1.ADP.Vi or S1.ADP.BeFx, trypsin produces two additional cleavages in the S1 heavy chain: a faster cleavage in the N-terminal region between Arg23 and Ile24, and a slower cleavage at the 50 kDa fragment. It has been shown that the N-terminal cleavage strongly decreases the thermal stability of S1 by shifting the maximum of its thermal transition by about 7 degrees C to a lower temperature, from 50 degrees C to 42.4 degrees C, whereas the cleavage at both these sites causes dramatic destabilization of the S1 molecule leading to total loss of its thermal transition. Our results show that S1 with ATP-induced N-terminal cleavage is able, like uncleaved S1, to undergo global structural changes in forming the stable ternary complexes with ADP and Pi analogs (Vi, BeFx). These changes are reflected in a pronounced increase of S1 thermal stability. However, S1 cleaved by trypsin in the N-terminal region is unable, unlike S1, to undergo structural changes induced by interaction with F-actin that are expressed in a 4-5 degrees C shift of the S1 thermal transition to higher temperature. Thus, the cleavage between Arg23 and Ile24 does not significantly affect nucleotide-induced structural changes in the S1, but it prevents structural changes that occur when S1 is bound to F-actin. The results suggest that the N-terminal region of the S1 heavy chain plays an important role in structural stabilization of the entire motor domain of the myosin head, and a long-distance communication pathway may exist between this region and the actin-binding sites.

Sequence landmark patterns identify and characterize protein families

The three-dimensional structures of homologous proteins are usually conserved during evolution, as are critical residues in a few short sequence motifs that often constitute the active site in enzymes. The precise spatial organization of such sites depends on the lengths and positions of the secondary structural elements connecting the motifs. We show how members of protein superfamilies, such as kinesins, myosins, and G(alpha) subunits of trimeric G proteins, are identified and classed by simply counting the number of amino acid residues between important sequence motifs in their nucleotide triphosphate-hydrolyzing domains. Subfamily-specific landmark patterns (motif to motif scores) are principally due to inserts and gaps in surface loops. Unusual protein sequences and possible sequence prediction errors are detected.

Identification and phylogenetic analysis of Drosophila melanogaster myosins

Myosins constitute a superfamily of motor proteins that convert energy from ATP hydrolysis into mechanical movement along the actin filaments. Phylogenetic analysis currently places myosins into 17 classes based on class-specific features of their conserved motor domain. Traditionally, the myosins have been divided into two classes depending on whether they form monomers or dimers. The conventional myosin of muscle and nonmuscle cells forms class II myosins. They are complex molecules of four light chains bound to two heavy chains that form bipolar filaments via interactions between their coiled-coil tails (type II). Class I myosins are smaller monomeric myosins referred to as unconventional myosins. Now, at least 15 other classes of unconventional myosins are known. How many myosins are needed to ensure the proper development and function of eukaryotic organisms? Thus far, three types of myosins were found in budding yeast, six in the nematode Caenorhabditis elegans, and at least 12 in human. Here, we report on the identification and classification of Drosophila melanogaster myosins. Analysis of the Drosophila genome sequence identified 13 myosin genes. Phylogenetic analysis based on the sequence comparison of the myosin motor domains, as well as the presence of the class-specific domains, suggests that Drosophila myosins can be divided into nine major classes. Myosins belonging to previously described classes I, II, III, V, VI, and VII are present. Molecular and phylogenetic analysis indicates that the fruitfly genome contains at least five new myosins. Three of them fall into previously described myosin classes I, VII, and XV. Another myosin is a homolog of the mouse and human PDZ-containing myosins, forming the recently defined class XVIII myosins. PDZ domains are named after the postsynaptic density, disc-large, ZO-1 proteins in which they were first described. The fifth myosin shows a unique domain composition and a low homology to any of the existing classes. We propose that this is classified when similar myosins are identified in other species.

Human myosin XVBP is a transcribed pseudogene

A novel human myosin gene located at 17q25 was identified through evaluation of genomic DNA sequence and designated myosin XVBP since it resembled human myosin XVA. In humans, myosin XVBP along with an adjacent gene, Lethal Giant Larvae 2 (LLGL2) appears to have arisen from a genomic duplication of a chromosomal interval that included LLGL and an ancestral myosin XV. Inspection of human myosin XVBP predicted amino acid sequence from genomic DNA revealed that 36 of the 131 conserved amino acid residues of the motor domain are substituted or deleted, including sequence changes within the regions involved in the binding of ATP and actin. Twelve myosin XVBP overlapping cDNAs from kidney and stomach mRNA samples were cloned and sequenced. Analyses of these myosin XVBP cDNAs revealed numerous additional disablements including translational reading frame shifts resulting in stop codons. From these data we conclude that myosin XVBP is a transcribed, unprocessed pseudogene.

Evolutionary implications of three novel members of the human sarcomeric myosin heavy chain gene family

Sarcomeric myosin heavy chain (MyHC) is the major contractile protein of striated muscle. Six tandemly linked skeletal MyHC genes on chromosome 17 and two cardiac MyHC genes on chromosome 14 have been previously described in the human genome. We report the identification of three novel human sarcomeric MyHC genes on chromosomes 3, 7, and 20, which are notable for their atypical size and intron-exon structure. Two of the encoded proteins are structurally most like the slow-beta MyHC, whereas the third one is closest to the adult fast IIb isoform. Data from pairwise comparisons of aligned coding sequences imply the existence of ancestral genomes with four sarcomeric genes before the emergence of a dedicated smooth muscle MyHC gene. To further address the evolutionary relationships of the distinct sarcomeric and nonsarcomeric rod sequences, we have identified and further annotated human genomic DNA sequences corresponding to 14 class-II MyHCs. An extensive analysis provides a timeline for intron gain and loss, gene contraction and expansion, and gene conversion among genes encoding class-II myosins. One of the novel human genes is found to have introns at positions shared only with the molluscan catchin/MyHC gene, providing evidence for the structure of a pre-Cambrian ancestral gene.

Hamy3, a novel type 100 kDa myosin from sunflower

Hamy3, a novel type myosin heavy chain from sunflower is the smallest myosin described so far, with only 900 amino acid residues. One interesting finding in Hamy3 is the glycine to glutamine alteration at residue 741, which corresponds to chicken skeletal muscle myosin glycine 699 (G699). G699 is found in 125 out of 129 myosin sequences and is interpreted in terms of its role as a pivot point for motion in the myosin "lever arm hypothesis". Changes in this crucial part of myosin might indicate a role that is different from the generation of intracellular motility.

Myosins in protists

This review focuses on selected papers that illustrate an historical perspective and the current knowledge of myosin structure and function in protists. The review contains a general description of myosin structure, a phylogenetic tree of the myosin classes, and descriptions of myosin isoforms identified in protists. Each myosin is discussed within the context of the taxonomic group of the organism in which the myosin has been identified. Domain structure, cellular location, function, and regulation are described for each myosin.

Myosin XVA expression in the pituitary and in other neuroendocrine tissues and tumors

The myosin superfamily of molecular motor proteins includes conventional myosins and several classes of unconventional myosins. Recent studies have characterized the human and mouse unconventional myosin XVA, which has a role in the formation and/or maintenance of the unique actin-rich structures of inner ear sensory hair cells. Myosin XVA is also highly expressed in human anterior pituitary cells. In this study we examined the distribution of myosin XVA protein and mRNA in normal and neoplastic human pituitaries and other neuroendocrine cells and tumors. Myosin XVA was expressed in all types of normal anterior pituitary cells and pituitary tumors and in other neuroendocrine cells and tumors including those of the adrenal medulla, parathyroid, and pancreatic islets. Most nonneuroendocrine tissues examined including liver cells were negative for myosin XVA protein and mRNA, although the distal and proximal tubules of normal kidneys showed moderate immunoreactivity for myosin XVA. Ultrastructural immunohistochemistry localized myosin XVA in association with secretory granules of human anterior pituitary cells and human pituitary tumors. These data suggest that in neuroendocrine cells myosin XVA may have a role in secretory granule movement and/or secretion.

Analysis of the myosins encoded in the recently completed Arabidopsis thaliana genome sequence

BACKGROUND

Three types of molecular motors play an important role in the organization, dynamics and transport processes associated with the cytoskeleton. The myosin family of molecular motors move cargo on actin filaments, whereas kinesin and dynein motors move cargo along microtubules. These motors have been highly characterized in non-plant systems and information is becoming available about plant motors. The actin cytoskeleton in plants has been shown to be involved in processes such as transportation, signaling, cell division, cytoplasmic streaming and morphogenesis. The role of myosin in these processes has been established in a few cases but many questions remain to be answered about the number, types and roles of myosins in plants.

RESULTS

Using the motor domain of an Arabidopsis myosin we identified 17 myosin sequences in the Arabidopsis genome. Phylogenetic analysis of the Arabidopsis myosins with non-plant and plant myosins revealed that all the Arabidopsis myosins and other plant myosins fall into two groups - class VIII and class XI. These groups contain exclusively plant or algal myosins with no animal or fungal myosins. Exon/intron data suggest that the myosins are highly conserved and that some may be a result of gene duplication.

CONCLUSIONS

Plant myosins are unlike myosins from any other organisms except algae. As a percentage of the total gene number, the number of myosins is small overall in Arabidopsis compared with the other sequenced eukaryotic genomes. There are, however, a large number of class XI myosins. The function of each myosin has yet to be determined.

Pulse electrophoresis of muscle myosin heavy chains in sodium dodecyl sulfate-polyacrylamide gels

We have developed a new method that provides enhanced resolution of myosin heavy chain (MHC) isoforms by sodium dodecyl sulfate--polyacrylamide gel electrophoresis (SDS-PAGE). The key feature of this protocol involves the application of current to slab SDS gels in a pulsatile, repetitive manner rather than continuously as in standard gel systems. This protocol, designated pulse electrophoresis, was achieved by means of a device that intermittently gates the output of a conventional power supply. When used in long (32 cm) separating gels, pulse electrophoresis not only significantly improves the resolution of MHC isoforms compared to conventional systems, but also reduces common artifacts associated with long running times, such as blurred bands and comingling of closely spaced bands. In addition to the increased resolution of protein bands, pulse electrophoresis also allows detection of bands corresponding to previously unidentified MHC isoforms in mammalian and avian tissue. In rat myocardium, for example, pulse electrophoresis revealed three MHC isoform bands, two of which appeared to correspond to two alpha-MHC subspecies. Alternative splicing of the rat alpha-MHC gene is known to generate two isoform species differing by inclusion (or exclusion) of a single glutamine residue, whose relative levels of expression correspond nicely with the amounts of each band identified in this study. Therefore, we cannot rule out that the system presented here may be sufficiently sensitive to differentiate between high molecular weight proteins differing in a single amino acid.

Identification and analysis of the myosin superfamily in Drosophila: a database approach

The recent sequencing of the genome of Drosophila melanogaster has provided a valuable resource for mining the database for genes of interest. We took advantage of this opportunity in an attempt to identify novel myosins in Drosophila and confirm the presence of the previously identified myosins from classes I, II, III, V, VI, and VII. The Drosophila database annotators predicted the structure of three additional proteins which we identified as novel unconventional myosins, two of which fell into classes XV and XVIII, respectively. Our own efforts predicted the presence of four additional partial sequences that appear to be myosin proteins which did not fall into any specific class. In the future comparative genomics will hopefully lead to the placement of these myosins into new classes.

Regulation of Dictyostelium myosin I and II

Dictyostelium expresses 12 different myosins, including seven single-headed myosins I and one conventional two-headed myosin II. In this review we focus on the signaling pathways that regulate Dictyostelium myosin I and myosin II. Activation of myosin I is catalyzed by a Cdc42/Rac-stimulated myosin I heavy chain kinase that is a member of the p21-activated kinase (PAK) family. Evidence that myosin I is linked to the Arp2/3 complex suggests that pathways that regulate myosin I may also influence actin filament assembly. Myosin II activity is stimulated by a cGMP-activated myosin light chain kinase and inhibited by myosin heavy chain kinases (MHCKs) that block bipolar filament assembly. Known MHCKs include MHCK A and MHCK B, which have a novel type of kinase catalytic domain joined to a WD repeat domain, and MHC-protein kinase C (PKC), which contains both diacylglycerol kinase and PKC-related protein kinase catalytic domains. A Dictyostelium PAK (PAKa) acts indirectly to promote myosin II filament formation, suggesting that the MHCKs may be indirectly regulated by Rac GTPases.

Myosin-VIIb, a novel unconventional myosin, is a constituent of microvilli in transporting epithelia

Mouse myosin-VIIb, a novel unconventional myosin, was cloned from the inner ear and kidney. The human myosin-VIIb (HGMW-approved symbol MYO7B) sequence and exon structure were then deduced from a human BAC clone. The mouse gene was mapped to chromosome 18, approximately 0.5 cM proximal to D18Mit12. The human gene location at 2q21.1 was deduced from the map location of the BAC and confirmed by fluorescence in situ hybridization. Myosin-VIIb has a conserved myosin head domain, five IQ domains, two MyTH4 domains coupled to two FERM domains, and an SH3 domain. A phylogenetic analysis based on the MyTH4 domains suggests that the coupled MyTH and FERM domains were duplicated in myosin evolution before separation into different classes. Myosin-VIIb is expressed primarily in kidney and intestine, as shown by Northern and immunoblot analyses. An antibody to myosin-VIIb labeled proximal tubule cells of the kidney and enterocytes of the intestine, specifically the distal tips of apical microvilli on these transporting epithelial cells.

A flash photolysis fluorescence/light scattering apparatus for use with sub microgram quantities of muscle proteins

Transient kinetic methods such as stopped flow and quenched flow have been used to elucidate many of the fundamental features of the molecular interactions which underlie muscle contraction. However, these methods traditionally require relatively large amounts of protein (10(-3) g) and so have been used most effectively for the proteins purified from bulk muscle tissue of large animals or where the proteins can be expressed in large amounts (e.g.. Dictyostelium). We have investigated the use of flash photolysis of an inert precursor of ATP (cATP) to initiate the dissociation of acto.S1 and acto.myosin and the subsequent ATP turnover reaction. Using a sample volume of 10 microl we show that a significant amount of information on the transient and steady-state kinetics of the system can be obtained from a sample containing just 50 nM of acto.myosin or acto.S1 complex in solution. Therefore in presence of excess of one protein component the measurements require only 250 ng myosin, 62 ng S1 or 25 ng actin. This is therefore the method of choice for kinetic analysis of acto.myosins which are only available in microgram quantities. We report for the first time the determination of the second order rate constant of ATP-induced dissociation of actin from the myosin extracted from a single fibre from a rabbit psoas muscle.

Myosin VIIA is specifically associated with calmodulin and microtubule-associated protein-2B (MAP-2B)

Myosin VIIA is a motor molecule with a conserved head domain and tail region unique to myosin VIIA, which probably defines its unique function in vivo. In an attempt to further characterize myosin VIIA function we set out to identify molecule(s) that specifically associate with it. We demonstrate that 17 and 55 kDa proteins from mouse kidney and cochlea co-purify with myosin VIIA on affinity columns carrying immobilized anti-myosin VIIA antibody. N-terminal sequencing and immunoblotting analysis identified the 17 kDa protein as calmodulin, whereas MS and immunoblotting analysis identified the 55 kDa protein as microtubule-associated protein-2B (MAP-2B). Myosin VIIA can also be co-immunoprecipitated from kidney homogenate using anti-calmodulin or anti-MAP2 (recognizing isoforms 2A and 2B) antibodies, confirming the strong association between calmodulin and myosin VIIA and between MAP-2B and myosin VIIA. Myosin VIIA binds to calmodulin with an apparent K(d) of 10(-9) M. Scatchard analysis of the binding of myosin VIIA to MAP-2B provided evidence for two binding sites, with K(d) values of 10(-10) and 10(-9) M, which have been mapped to medial and C-terminal tail domains of myosin VIIA. The characterization of the interaction of calmodulin and MAP-2B with myosin VIIA provides new insights into the function of myosin VIIA.