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

Lowfat functions downstream of Myo20 to regulate wing and leg morphogenesis in Tribolium castaneum

Myosin Myo20 plays vital roles in the morphogenesis of wings and legs among insects, but the function and signalling of Myo20 remain unclear. We show that Myo20 regulates wing cell division, ecdysteroid and amino acid metabolism, and gene expression in Tribolium castaneum. By RNA-seq, we identified 582 differentially expressed genes (DEGs) between control and ds-Myo20 larvae of T. castaneum. Of these DEGs, silencing Myo20 significantly decreased the mRNA and protein levels of lowfat. During development, lowfat has the highest expression in early pupae and the lowest level in 1-day embryos. Tissue-specific analysis indicated that lowfat was abundantly expressed in the head, fat body and epidermis of late-stage larvae and in wings and legs of 1, 2 and 5-day pupae. Likewise, knockdown of lowfat affected wing and leg morphogenesis, ecdysteroid and amino acid metabolism, and gene expression in T. castaneum. Silencing Myo20 or lowfat activated CYP18A1 to degrade ecdysteroids, stimulated amino acids catabolism to increase the transcription of 4E-BP but reduce S6K and cycE expression. These results suggest that Lowfat works downstream of Myo20 to employ target of rapamycin (TOR) signalling for wing and leg morphogenesis in insects.

Propagation of F-actin disassembly via Myosin15-Mical interactions

The F-actin cytoskeleton drives cellular form and function. However, how F-actin-based changes occur with spatiotemporal precision and specific directional orientation is poorly understood. Here, we identify that the unconventional class XV myosin [Myosin 15 (Myo15)] physically and functionally interacts with the F-actin disassembly enzyme Mical to spatiotemporally position cellular breakdown and reconstruction. Specifically, while unconventional myosins have been associated with transporting cargo along F-actin to spatially target cytoskeletal assembly, we now find they also target disassembly. Myo15 specifically positions this F-actin disassembly by associating with Mical and using its motor and MyTH4-FERM cargo-transporting functions to broaden Mical's distribution. Myo15's broadening of Mical's distribution also expands and directionally orients Mical-mediated F-actin disassembly and subsequent cellular remodeling, including in response to Semaphorin/Plexin cell surface activation signals. Thus, we identify a mechanism that spatiotemporally propagates F-actin disassembly while also proposing that other F-actin-trafficked-cargo is derailed by this disassembly to directionally orient rebuilding.

The formin inhibitor SMIFH2 inhibits members of the myosin superfamily

The small molecular inhibitor of formin FH2 domains, SMIFH2, is widely used in cell biological studies. It inhibits formin-driven actin polymerization in vitro, but not polymerization of pure actin. It is active against several types of formin from different species. Here, we found that SMIFH2 inhibits retrograde flow of myosin 2 filaments and contraction of stress fibers. We further checked the effect of SMIFH2 on non-muscle myosin 2A and skeletal muscle myosin 2 in vitro, and found that SMIFH2 inhibits activity of myosin ATPase and the ability to translocate actin filaments in the gliding actin in vitro motility assay. Inhibition of non-muscle myosin 2A in vitro required a higher concentration of SMIFH2 compared with that needed to inhibit retrograde flow and stress fiber contraction in cells. We also found that SMIFH2 inhibits several other non-muscle myosin types, including bovine myosin 10, Drosophila myosin 7a and Drosophila myosin 5, more efficiently than it inhibits formins. These off-target inhibitions demand additional careful analysis in each case when solely SMIFH2 is used to probe formin functions. This article has an associated First Person interview with Yukako Nishimura, joint first author of the paper.

A binding protein regulates myosin-7a dimerization and actin bundle assembly

Myosin-7a, despite being monomeric in isolation, plays roles in organizing actin-based cell protrusions such as filopodia, microvilli and stereocilia, as well as transporting cargoes within them. Here, we identify a binding protein for Drosophila myosin-7a termed M7BP, and describe how M7BP assembles myosin-7a into a motile complex that enables cargo translocation and actin cytoskeletal remodeling. M7BP binds to the autoinhibitory tail of myosin-7a, extending the molecule and activating its ATPase activity. Single-molecule reconstitution show that M7BP enables robust motility by complexing with myosin-7a as 2:2 translocation dimers in an actin-regulated manner. Meanwhile, M7BP tethers actin, enhancing complex’s processivity and driving actin-filament alignment during processive runs. Finally, we show that myosin-7a-M7BP complex assembles actin bundles and filopodia-like protrusions while migrating along them in living cells. Together, these findings provide insights into the mechanisms by which myosin-7a functions in actin protrusions.

Myosin-7a is found in actin bundles, microvilli and stereocilia, and plays conserved roles in hearing and vision. Here the authors identify M7BP, a myosin-7a binding protein that activates and dimerizes myosin-7a, enabling cargo transport and assembly of actin bundles and filopodia-like protrusions

Genetic Underpinnings of Host Manipulation by Ophiocordyceps as Revealed by Comparative Transcriptomics

Ant-infecting Ophiocordyceps fungi are globally distributed, host manipulating, specialist parasites that drive aberrant behaviors in infected ants, at a lethal cost to the host. An apparent increase in activity and wandering behaviors precedes a final summiting and biting behavior onto vegetation, which positions the manipulated ant in a site beneficial for fungal growth and transmission. We investigated the genetic underpinnings of host manipulation by: (i) producing a high-quality hybrid assembly and annotation of the Ophiocordyceps camponoti-floridani genome, (ii) conducting laboratory infections coupled with RNAseq of O. camponoti-floridani and its host, Camponotus floridanus, and (iii) comparing these data to RNAseq data of Ophiocordyceps kimflemingiae and Camponotus castaneus as a powerful method to identify gene expression patterns that suggest shared behavioral manipulation mechanisms across Ophiocordyceps-ant species interactions. We propose differentially expressed genes tied to ant neurobiology, odor response, circadian rhythms, and foraging behavior may result by activity of putative fungal effectors such as enterotoxins, aflatrem, and mechanisms disrupting feeding behaviors in the ant.

Myosin XVIII

Class XVIII myosins represent a branch of the myosin family tree characterized by the presence of large N- and C-terminal extensions flanking a generic myosin core. These myosins display the highest sequence similarity to conventional class II muscle myosins and are compatible with but not restricted to myosin-2 contractile structures. Instead, they fulfill their functions at diverse localities, such as lamella, actomyosin bundles, the Golgi apparatus, focal adhesions, the cell membrane, and within sarcomeres. Sequence comparison of active-site residues and biochemical data available thus far indicate that this myosin class lacks active ATPase-driven motor activity, suggesting that its members function as structural myosins. An emerging body of evidence indicates that this structural capability is essential for the organization, maturation, and regulation of the contractile machinery in both muscle and nonmuscle cells. This is supported by the clear association of myosin-18A (Myo18A) and myosin-18B (Myo18B) dysregulation with diseases such as cancer and various myopathies.

Crinkled employs wingless pathway for wing development in Tribolium castaneum

Crinkled is associated with embryonic denticle formation and auditory organ development in Drosophila melanogaster. However, the functions of Crinkled have not been fully investigated. Additionally, the genes that participate in the Crinkled pathway are unknown. Phylogenetic analysis indicates that crinkled exhibits a one-to-one orthologous relationship in insects. In Tribolium castaneum, the crinkled gene is 6,498 bp in length and consists of six exons. Crinkled expression peaked during two phases in Tribolium: late embryonic and pupal stages. High levels of crinkled mRNA were detected in the fat body, head, epidermis, ovary, and accessory gland of late adults. Knockdown of crinkled using RNA interference (RNAi) severely affected wing morphogenesis in T. castaneum. We further showed that crinkled silencing reduced forked expression through wingless and shaven-baby, and RNAi of forked phenocopied the effects of crinkled knockdown in T. castaneum. This study investigated the development role of crinkled in postembryonic stages and indicated that forked mediates the functions of crinkled during wing morphogenesis in T. castaneum.

Mammalian myosin-18A, a highly divergent myosin

The Mus musculus myosin-18A gene is expressed as two alternatively spliced isoforms, α and β, with reported roles in Golgi localization, in maintenance of cytoskeleton, and as receptors for immunological surfactant proteins. Both myosin-18A isoforms feature a myosin motor domain, a single predicted IQ motif, and a long coiled-coil reminiscent of myosin-2. The myosin-18Aα isoform, additionally, has an N-terminal PDZ domain. Recombinant heavy meromyosin- and subfragment-1 (S1)-like constructs for both myosin-18Aα and -18β species were purified from the baculovirus/Sf9 cell expression system. These constructs bound both essential and regulatory light chains, indicating an additional noncanonical light chain binding site in the neck. Myosin-18Aα-S1 and -18Aβ-S1 molecules bound actin weakly with Kd values of 4.9 and 54 μm, respectively. The actin binding data could be modeled by assuming an equilibrium between two myosin conformations, a competent and an incompetent form to bind actin. Actin binding was unchanged by presence of nucleotide. Both myosin-18A isoforms bound N-methylanthraniloyl-nucleotides, but the rate of ATP hydrolysis was very slow (<0.002 s(-1)) and not significantly enhanced by actin. Phosphorylation of the regulatory light chain had no effect on ATP hydrolysis, and neither did the addition of tropomyosin or of GOLPH3, a myosin-18A binding partner. Electron microscopy of myosin-18A-S1 showed that the lever is strongly angled with respect to the long axis of the motor domain, suggesting a pre-power stroke conformation regardless of the presence of ATP. These data lead us to conclude that myosin-18A does not operate as a traditional molecular motor in cells.

Myosin heavy chain-like localizes at cell contact sites during Drosophila myoblast fusion and interacts in vitro with Rolling pebbles 7

Besides representing the sarcomeric thick filaments, myosins are involved in many cellular transport and motility processes. Myosin heavy chains are grouped into 18 classes. Here we show that in Drosophila, the unconventional group XVIII myosin heavy chain-like (Mhcl) is transcribed in the mesoderm of embryos, most prominently in founder cells (FCs). An ectopically expressed GFP-tagged Mhcl localizes in the growing muscle at cell-cell contacts towards the attached fusion competent myoblast (FCM). We further show that Mhcl interacts in vitro with the essential fusion protein Rolling pebbles 7 (Rols7), which is part of a protein complex established at cell contact sites (Fusion-restricted Myogenic-Adhesive Structure or FuRMAS). Here, branched F-actin is likely needed to widen the fusion pore and to integrate the myoblast into the growing muscle. We show that the localization of Mhcl is dependent on the presence of Rols7, and we postulate that Mhcl acts at the FuRMAS as an actin motor protein. We further show that Mhcl deficient embryos develop a wild-type musculature. We thus propose that Mhcl functions redundantly to other myosin heavy chains in myoblasts. Lastly, we found that the protein is detectable adjacent to the sarcomeric Z-discs, suggesting an additional function in mature muscles.

Regulation of myosin 5a and myosin 7a

The myosin superfamily is diverse in its structure, kinetic mechanisms and cellular function. The enzymatic activities of most myosins are regulated by some means such as Ca2+ ion binding, phosphorylation or binding of other proteins. In the present review, we discuss the structural basis for the regulation of mammalian myosin 5a and Drosophila myosin 7a. We show that, although both myosins have a folded inactive state in which domains in the myosin tail interact with the motor domain, the details of the regulation of these two myosins differ greatly.

The kinetic mechanism of mouse myosin VIIA

Myosin VIIa is crucial in hearing and visual processes. We examined the kinetic and association properties of the baculovirus expressed, truncated mouse myosin VIIa construct containing the head, all 5IQ motifs and the putative coiled coil domain (myosin VIIa-5IQ). The construct appears to be monomeric as determined by analytical ultracentrifugation experiments, and only single headed molecules were detected by negative stain electron microscopy. The relatively high basal steady-state rate of 0.18 s(-1) is activated by actin only by ∼3.5-fold resulting in a V(max) of 0.7 s(-1) and a K(ATPase) of 11.5 μM. There is no single rate-limiting step of the ATP hydrolysis cycle. The ATP hydrolysis step (M·T M·D·P) is slow (12 s(-1)) and the equilibrium constant (K(H)) of 1 suggests significant reversal of hydrolysis. In the presence of actin ADP dissociates with a rate constant of 1.2 s(-1). Phosphate dissociation is relatively fast (>12 s(-1)), but the maximal rate could not be experimentally obtained at actin concentrations ≤ 50 μM because of the weak binding of the myosin VIIa-ADP-P(i) complex to actin. At higher actin concentrations the rate of attached hydrolysis (0.4 s(-1)) becomes significant and partially rate-limiting. Our findings suggest that the myosin VIIa is a "slow", monomeric molecular motor with a duty ratio of 0.6.

A FERM domain autoregulates Drosophila myosin 7a activity

Full-length Drosophila myosin 7a (myosin 7a-FL) has a complex tail containing a short predicted coiled coil followed by a MyTH4-FERM domain, an SH3 domain, and a C-terminal MyTH4-FERM domain. Myosin 7a-FL expressed in Sf9 cells is monomeric despite the predicted coiled coil. We showed previously that Subfragment-1 (S1) from this myosin has MgATPase of V(max) approximately 1 s(-1) and K(ATPase) approximately 1 microM actin. We find that myosin 7a-FL has V(max) similar to S1 but K(ATPase) approximately 30 microM. Thus, at low actin concentrations (5 microM), the MgATPase of S1 is fully activated, whereas that of myosin 7a-FL is low, suggesting that the tail regulates activity. Electron microscopy of myosin 7a-FL with ATP shows the tail is tightly bent back against the motor domain. Myosin 7a-FL extends at either high ionic strength or without ATP, revealing the motor domain, lever, and tail. A series of C-terminal truncations show that deletion of 99 aa (the MyTH7 subdomain of the C-terminal FERM domain) is sufficient to abolish bending, and the K(ATPase) is then similar to S1. This region is highly conserved in myosin 7a. We found that a double mutation in it, R2140A-K2143A, abolishes bending and reduces K(ATPase) to S1 levels. In addition, the expressed C-terminal FERM domain binds actin with K(d) approximately 30 microM regardless of ATP, similar to the K(ATPase) value for myosin 7a-FL. We propose that at low cellular actin concentrations, myosin 7a-FL is bent and inactive, but at high actin concentrations, it is unfolded and active because the C-terminal FERM domain binds to actin.

Epithelial morphogenesis in embryos: asymmetries, motors and brakes

Epithelial cells play a central role in many embryonic morphogenetic processes, during which they undergo highly coordinated cell shape changes. Here, we review some common principles that have recently emerged through genetic and cellular analyses performed mainly with invertebrate genetic models, focusing on morphogenetic processes involving epithelial sheets. All available data argue that myosin II is the main motor that induces cell shape changes during morphogenesis. We discuss the control of myosin II activity during epithelial morphogenesis, as well as the recently described involvement of microtubules in this process. Finally, we examine how forces unleashed by myosin II can be measured, how embryos use specific brakes to control molecular motors and the potential input of mechano-sensation in morphogenesis.

Comparative genomic analysis of the arthropod muscle myosin heavy chain genes allows ancestral gene reconstruction and reveals a new type of 'partially' processed pseudogene

BACKGROUND

Alternative splicing of mutually exclusive exons is an important mechanism for increasing protein diversity in eukaryotes. The insect Mhc (myosin heavy chain) gene produces all different muscle myosins as a result of alternative splicing in contrast to most other organisms of the Metazoa lineage, that have a family of muscle genes with each gene coding for a protein specialized for a functional niche.

RESULTS

The muscle myosin heavy chain genes of 22 species of the Arthropoda ranging from the waterflea to wasp and Drosophila have been annotated. The analysis of the gene structures allowed the reconstruction of an ancient muscle myosin heavy chain gene and showed that during evolution of the arthropods introns have mainly been lost in these genes although intron gain might have happened in a few cases. Surprisingly, the genome of Aedes aegypti contains another and that of Culex pipiens quinquefasciatus two further muscle myosin heavy chain genes, called Mhc3 and Mhc4, that contain only one variant of the corresponding alternative exons of the Mhc1 gene. Mhc3 transcription in Aedes aegypti is documented by EST data. Mhc3 and Mhc4 inserted in the Aedes and Culex genomes either by gene duplication followed by the loss of all but one variant of the alternative exons, or by incorporation of a transcript of which all other variants have been spliced out retaining the exon-intron structure. The second and more likely possibility represents a new type of a 'partially' processed pseudogene.

CONCLUSION

Based on the comparative genomic analysis of the alternatively spliced arthropod muscle myosin heavy chain genes we propose that the splicing process operates sequentially on the transcript. The process consists of the splicing of the mutually exclusive exons until one exon out of the cluster remains while retaining surrounding intronic sequence. In a second step splicing of introns takes place. A related mechanism could be responsible for the splicing of other genes containing mutually exclusive exons.

A genetic mosaic analysis with a repressible cell marker screen to identify genes involved in tracheal cell migration during Drosophila air sac morphogenesis

Branching morphogenesis of the Drosophila tracheal system relies on the fibroblast growth factor receptor (FGFR) signaling pathway. The Drosophila FGF ligand Branchless (Bnl) and the FGFR Breathless (Btl/FGFR) are required for cell migration during the establishment of the interconnected network of tracheal tubes. However, due to an important maternal contribution of members of the FGFR pathway in the oocyte, a thorough genetic dissection of the role of components of the FGFR signaling cascade in tracheal cell migration is impossible in the embryo. To bypass this shortcoming, we studied tracheal cell migration in the dorsal air sac primordium, a structure that forms during late larval development. Using a mosaic analysis with a repressible cell marker (MARCM) clone approach in mosaic animals, combined with an ethyl methanesulfonate (EMS)-mutagenesis screen of the left arm of the second chromosome, we identified novel genes implicated in cell migration. We screened 1123 mutagenized lines and identified 47 lines displaying tracheal cell migration defects in the air sac primordium. Using complementation analyses based on lethality, mutations in 20 of these lines were genetically mapped to specific genomic areas. Three of the mutants were mapped to either the Mhc or the stam complementation groups. Further experiments confirmed that these genes are required for cell migration in the tracheal air sac primordium.

Frazzled regulation of myosin II activity in the Drosophila embryonic CNS

Frazzled (Fra) is a chemoattractive guidance receptor regulating the cytoskeletal dynamics underlying growth cone steering at the Drosophila embryonic midline. Here, by genetically evaluating the role of Rho GTPases in Fra signaling in vivo, we uncover a Rho-dependent pathway apparently regulating conventional myosin II activity. Midline crossing errors induced by expressing activated Cdc42(v12) or Rac(v12) are suppressed by a heterozygous loss of fra(4) signaling but, in a Fra(wt) gain-of-function condition, no interaction is detected. In contrast, the frequency of crossovers is enhanced approximately 5-fold when Fra(wt) is co-expressed with activated Rho(v14) and this interaction specifically requires the cytoplasmic P3 motif of Fra. Expression of Rho(v14) and activated MLCK (ctMLCK) synergistically increase ectopic crossovers and both require phosphorylation of the regulatory light chain (Sqh) of myosin II. Abelson tyrosine kinase may also help regulate myosin II activity. Heterozygous abl(4) abolishes the midline crossing errors induced by ctMLCK alone or in combination with Fra(wt); suppression of Rho(v14) crossovers is not observed. Interestingly, an interaction between Fra and an activated Abl (Bcr-Abl) also specifically requires the P3 motif. Therefore, the P3 motif of Frazzled appears to initiate Rho and Abl dependent signals to directly or indirectly regulate myosin II activity in growth cones.

ORYZA SATIVA MYOSIN XI B controls pollen development by photoperiod-sensitive protein localizations

Myosins are actin-based motor proteins responsible for various motility and signal transduction. Only a small set of myosin classes is present inplants, and little is known about their functions. Here we showed how a rice myosin gene controlled pollen development by sensing changed environmental factors. The analysis is based on a gene-trapped Ds insertion mutant Oryza sativa myosin XI B (osmyoXIB). This mutant showed male sterility under short day length (SD) conditions and fertility under long day length (LD) conditions. Under both SD and LD conditions, the OSMYOXIB transcript was detected in whole anthers. However, under SD conditions, the OSMYOXIB-GUS fusion protein was localized only in the epidermal layer of anthers due to the lack of 3'-untranslated region (3'-UTR) and to dilute (DIL) domain sequences following the Ds insertion. As a result, mutant pollen development was affected, leading to male sterility. By contrast, under LD conditions, the fusion protein was localized normally in anthers. Despite normal localization, the protein was only partially functional due to the lack of DIL domain sequences, resulting in limited recovery of pollen fertility. This study also provides a case for a novel molecular aspect of gene expression, i.e., cell layer-specific translation in anthers.

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.

Thirteen is enough: the myosins of Dictyostelium discoideum and their light chains

BACKGROUND

Dictyostelium discoideum is one of the most famous model organisms for studying motile processes like cell movement, organelle transport, cytokinesis, and endocytosis. Members of the myosin superfamily, that move on actin filaments and power many of these tasks, are tripartite proteins consisting of a conserved catalytic domain followed by the neck region consisting of a different number of so-called IQ motifs for binding of light chains. The tails contain functional motifs that are responsible for the accomplishment of the different tasks in the cell. Unicellular organisms like yeasts contain three to five myosins while vertebrates express over 40 different myosin genes. Recently, the question has been raised how many myosins a simple multicellular organism like Dictyostelium would need to accomplish all the different motility-related tasks.

RESULTS

The analysis of the Dictyostelium genome revealed thirteen myosins of which three have not been described before. The phylogenetic analysis of the motor domains of the new myosins placed Myo1F to the class-I myosins and Myo5A to the class-V myosins. The third new myosin, an orphan myosin, has been named MyoG. It contains an N-terminal extension of over 400 residues, and a tail consisting of four IQ motifs and two MyTH4/FERM (myosin tail homology 4/band 4.1, ezrin, radixin, and moesin) tandem domains that are separated by a long region containing an SH3 (src homology 3) domain. In contrast to previous analyses, an extensive comparison with 126 class-VII, class-X, class-XV, and class-XXII myosins now showed that MyoI does not group into any of these classes and should not be used as a model for class-VII myosins.The search for calmodulin related proteins revealed two further potential myosin light chains. One is a close homolog of the two EF-hand motifs containing MlcB, and the other, CBP14, phylogenetically groups to the ELC/RLC/calmodulin (essential light chain/regulatory light chain) branch of the tree.

CONCLUSION

Dictyostelium contains thirteen myosins together with 6-8 MLCs (myosin light chain) to assist in a variety of actin-based processes in the cell. Although they are homologous to myosins of higher eukaryotes, the myosins of Dictyostelium should be considered with care as models for specific functions of vertebrate myosins.

Myosin VIIB from Drosophila is a high duty ratio motor

Myosin VII is an unconventional myosin widely expressed in organisms ranging from amoebae to mammals that has been shown to play vital roles in cell adhesion and phagocytosis. Here we present the first study of the mechanism of action of a myosin VII isoform. We have expressed a truncated single-headed Drosophila myosin VIIB construct in the baculovirus-Sf9 system that bound calmodulin light chains. By using steady-state and transient kinetic methods, we showed that myosin VIIB exhibits a fast release of phosphate and a slower, rate-limiting ADP release from actomyosin. As a result, myosin VIIB will be predominantly strongly bound to actin during steady-state ATP hydrolysis (its duty ratio will be at least 80%). This kinetic pattern is in many respects similar to that of the single-molecule vesicle transporters myosin V and VI. The enzymatic properties of myosin VIIB provide a kinetic basis for processivity upon possible dimerization via the C-terminal domains of the heavy chain. Our experiments also revealed conformational heterogeneity of the actomyosin VIIB complex in the absence of nucleotide.

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 from Drosophila reveals diversity of motor mechanisms within the myosin V family

Myosin V is the best characterized vesicle transporter in vertebrates, but it has been unknown as to whether all members of the myosin V family share a common, evolutionarily conserved mechanism of action. Here we show that myosin V from Drosophila has a strikingly different motor mechanism from that of vertebrate myosin Va, and it is a nonprocessive, ensemble motor. Our steady-state and transient kinetic measurements on single-headed constructs reveal that a single Drosophila myosin V molecule spends most of its mechanochemical cycle time detached from actin, therefore it has to function in processive units that comprise several molecules. Accordingly, in in vitro motility assays, double-headed Drosophila myosin V requires high surface concentrations to exhibit a continuous translocation of actin filaments. Our comparison between vertebrate and fly myosin V demonstrates that the well preserved function of myosin V motors in cytoplasmic transport can be accomplished by markedly different underlying mechanisms.

The N-terminal domain of MYO18A has an ATP-insensitive actin-binding site

Myosin XVIII is the recently identified 18th class of myosins, and its members are composed of a unique N-terminal domain, a motor domain with an unusual sequence around the ATPase site, one IQ motif, a segmented coiled-coil region for dimerization, and a C-terminal globular tail. To gain insight into the functions of this unique myosin, we characterized its human homologue, MYO18A, focusing on the functional roles of the characteristic N-terminal domain that contains a PDZ module known to mediate protein-protein interaction. GFP-tagged full-length and C-terminally truncated MYO18A molecules that were expressed in HeLa cells exhibited colocalization with actin filaments. Chemical cross-linking of these molecules showed that they form stable dimers as expected from their putative coiled-coil tails. Cosedimentation of the various types of truncated MYO18A constructs with actin filaments indicated the presence of an ATP-insensitive actin-binding site in the N-terminal domain. Further studies on truncated constructs of the N-terminal domain indicated that this actin-binding site is located outside the PDZ module, but within the middle region of this domain, which does not show any homology with the known actin-binding motifs. These results imply that this dimeric myosin might stably cross-link actin filaments by two ATP-insensitive actin-binding sites at the N-terminal domains for higher-order organization of the actin cytoskeleton.

Drosophila crinkled, mutations of which disrupt morphogenesis and cause lethality, encodes fly myosin VIIA

Myosin VIIs provide motor function for a wide range of eukaryotic processes. We demonstrate that mutations in crinkled (ck) disrupt the Drosophila myosin VIIA heavy chain. The ck/myoVIIA protein is present at a low level throughout fly development and at the same level in heads, thoraxes, and abdomens. Severe ck alleles, likely to be molecular nulls, die as embryos or larvae, but all allelic combinations tested thus far yield a small fraction of adult "escapers" that are weak and infertile. Scanning electron microscopy shows that escapers have defects in bristles and hairs, indicating that this motor protein plays a role in the structure of the actin cytoskeleton. We generate a homology model for the structure of the ck/myosin VIIA head that indicates myosin VIIAs, like myosin IIs, have a spectrin-like, SH3 subdomain fronting their N terminus. In addition, we establish that the two myosin VIIA FERM repeats share high sequence similarity with only the first two subdomains of the three-lobed structure that is typical of canonical FERM domains. Nevertheless, the approximately 100 and approximately 75 amino acids that follow the first two lobes of the first and second FERM domains are highly conserved among myosin VIIs, suggesting that they compose a conserved myosin tail homology 7 (MyTH7) domain that may be an integral part of the FERM domain or may function independently of it. Together, our data suggest a key role for ck/myoVIIA in the formation of cellular projections and other actin-based functions required for viability.

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.

Tyrosine phosphorylation of myosin heavy chain during skeletal muscle differentiation: an integrated bioinformatics approach

BACKGROUND

Previously it has been shown that insulin-mediated tyrosine phosphorylation of myosin heavy chain is concomitant with enhanced association of C-terminal SRC kinase during skeletal muscle differentiation. We sought to identify putative site(s) for this phosphorylation event.

RESULTS

A combined bioinformatics approach of motif prediction and evolutionary and structural analyses identified tyrosines163 and 1856 of the skeletal muscle heavy chain as the leading candidate for the sites of insulin-mediated tyrosine phosphorylation.

CONCLUSION

Our work is suggestive that tyrosine phosphorylation of myosin heavy chain, whether in skeletal muscle or in platelets, is a significant event that may initiate cytoskeletal reorganization of muscle cells and platelets. Our studies provide a good starting point for further functional analysis of MHC phosphor-signalling events within different cells.

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.

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.

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.

piggyBac transformation of the New World screwworm, Cochliomyia hominivorax, produces multiple distinct mutant strains

Sterile insect technique (SIT) programs are designed to eradicate pest species by releasing mass-reared, sterile insects into an infested area. The first major implementation of SIT was the New World Screwworm Eradication Program, which successfully eliminated the New World screwworm (NWS), Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), from the Continental US, Mexico and much of Central America. Ionizing radiation is currently used for sterilization, but transgenic insect techniques could replace this method, providing a safer, more cost-effective alternative. Genetic transformation methods have been demonstrated in NWS, and verified by Southern blot hybridization, PCR and sequencing of element insertion junctions. A lethal insertional mutation and enhancer detection-like phenotypic expression variations are presented and discussed. In addition to supporting the eradication efforts, transformation methods offer potential means to identify genes and examine gene function in NWS.

Characterization of the motor activity of mammalian myosin VIIA

Myosin VIIA was cloned from rat kidney, and the construct (M7IQ5) containing the motor domain, IQ domain, and the coiled-coil domain as well as the full-length myosin VIIA (M7full) was expressed. The M7IQ5 contained five calmodulins. Based upon native gel electrophoresis and gel filtration, it was found that M7IQ5 was single-headed, whereas M7full was two-headed, suggesting that the tail domain contributes to form the two-headed structure. M7IQ5 had Mg(2+)-ATPase activity that was markedly activated by actin with K(actin) of 33 microm and V(max) of 0.53 s(-1) head(-1). Myosin VIIA required an extremely high ATP concentration for ATPase activity, ATP-induced dissociation from actin, and in vitro actin-translocating activity. ADP markedly inhibited the actin-activated ATPase activity. ADP also significantly inhibited the ATP-induced dissociation of myosin VIIA from actin. Consistently, ADP decreased K(actin) of the actin-activated ATPase. ADP decreased the actin gliding velocity, although ADP did not stop the actin gliding even at high concentration. These results suggest that myosin VIIA has slow ATP binding or low affinity for ATP and relatively high affinity for ADP. The directionality of myosin VIIA was determined by using the polarity-marked dual fluorescence-labeled actin filaments. It was found that myosin VIIA is a plus-directed motor.

Human MYO18B, a novel unconventional myosin heavy chain expressed in striated muscles moves into the myonuclei upon differentiation

We have characterized a novel unconventional myosin heavy chain, named MYO18B, that appears to be expressed mainly in human cardiac and skeletal muscles and, at lower levels, in testis. MYO18B transcript is detected in all types of striated muscles but at much lower levels compared to class II sarcomeric myosins, and it is up regulated after in vitro differentiation of myoblasts into myotubes. Phylogenetic analysis shows that this myosin belongs to the recently identified class XVIII, however, unlike the other member of this class, it seems to be unique to Vertebrate since it contains two large amino acid domains of unknown function at the N and C-termini. Immunolocalization of MYO18B protein in skeletal muscle cells shows that this myosin heavy chain is located in the cytoplasm of undifferentiated myoblasts. After in vitro differentiation into myotubes, a fraction of this protein is accumulated in a subset of myonuclei. This nuclear localization was confirmed by immunofluorescence experiments on primary cardiomyocytes and adult muscle sections. In the cytoplasm MYO18B shows a punctate staining, both in cardiac and skeletal fibers. In some cases, cardiomyocytes show a partial sarcomeric pattern of MYO18B alternating that of alpha-actinin-2. In skeletal muscle the cytoplasmic MYO18B results much more evident in the fast type fibers.

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.

Association of a nonmuscle myosin II with axoplasmic organelles

Association of motor proteins with organelles is required for the motors to mediate transport. Because axoplasmic organelles move on actin filaments, they must have associated actin-based motors, most likely members of the myosin superfamily. To gain a better understanding of the roles of myosins in the axon we used the giant axon of the squid, a powerful model for studies of axonal physiology. First, a approximately 220 kDa protein was purified from squid optic lobe, using a biochemical protocol designed to isolate myosins. Peptide sequence analysis, followed by cloning and sequencing of the full-length cDNA, identified this approximately 220 kDa protein as a nonmuscle myosin II. This myosin is also present in axoplasm, as determined by two independent criteria. First, RT-PCR using sequence-specific primers detected the transcript in the stellate ganglion, which contains the cell bodies that give rise to the giant axon. Second, Western blot analysis using nonmuscle myosin II isotype-specific antibodies detected a single approximately 220 kDa band in axoplasm. Axoplasm was fractionated through a four-step sucrose gradient after 0.6 M KI treatment, which separates organelles from cytoskeletal components. Of the total nonmuscle myosin II in axoplasm, 43.2% copurified with organelles in the 15% sucrose fraction, while the remainder (56.8%) was soluble and found in the supernatant. This myosin decorates the cytoplasmic surface of 21% of the axoplasmic organelles, as demonstrated by immunogold electron-microscopy. Thus, nonmuscle myosin II is synthesized in the cell bodies of the giant axon, is present in the axon, and is associated with isolated axoplasmic organelles. Therefore, in addition to myosin V, this myosin is likely to be an axoplasmic organelle motor.

Regulation of nonmuscle myosins by heavy chain phosphorylation

Functional activities of many nonmuscle myosin isoforms are (or are postulated to be) regulated by heavy chain phosphorylation. Depending on the myosin isoform, the serine or threonine residues located within the head (myosin I or myosin VI) or within the C-terminal tail domains (myosin II or myosin V) can be phosphorylated by more or less specific endogenous kinases. In some isoforms phosphorylation can occur both in the head and tail domains, as it has been found for myosin III. There are also isoforms that can be regulated both by the heavy and regulatory light chain phosphorylation, as for the example myosin II from slide mold Dictyostelium discoideum. The goal of this review was to describe recent findings on regulation of myosin I, myosin II, myosin III, myosin V and myosin VI isoforms by their heavy chain phosphorylation including the short charcteristics of the relevant kinases. The biological aspects of the phosphorylation are also discussed.

Localization of myosin XVA in endocrine tumors of gut and pancreas

The myosin superfamily includes conventional and unconventional myosin proteins. Among unconventional myosins, myosin XVA has recently been characterized, and it has been suggested that it may be involved in cytoplasmic organelle movement, including secretory granules in pituitary cells and pituitary adenomas. In this study, we investigated the expression of myosin XVA protein and mRNA in normal endocrine cells and in a series of 53 endocrine tumors of the gut and pancreas. Myosin XVA was expressed in rare normal endocrine cells of the gut and in almost all pancreatic islet cells. In addition, myosin XVA was detected in several cells of all endocrine tumors investigated, and its expression was not related to malignancy, type, site, or functional status of tumors. These results indicate that myosin XVA protein and mRNA are widely distributed in endocrine cells of the gut and pancreas. Although the role of this protein in endocrine cells is unknown, previous studies suggest that it may have a role in secretory granule movement and/or hormone secretion.

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.

Myosin I mutants with only 1% of wild-type actin-activated MgATPase activity retain essential in vivo function(s)

The single class I myosin (MYOA) of Aspergillus nidulans is essential for hyphal growth. It is generally assumed that the functions of all myosins depend on their actin-activated MgATPase activity. Here we show that MYOA mutants with no more than 1% of the actin-activated MgATPase activity of wild-type MYOA in vitro and no detectable in vitro motility activity can support fungal cell growth, albeit with a delay in germination time and a reduction in hyphal elongation. From these and other data, we conclude that the essential role(s) of myosin I in A. nidulans is probably structural, requiring little, if any, actin-activated MgATPase or motor activity, which have long been considered the defining characteristics of the myosin family.

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.

A millennial myosin census

The past decade has seen a remarkable explosion in our knowledge of the size and diversity of the myosin superfamily. Since these actin-based motors are candidates to provide the molecular basis for many cellular movements, it is essential that motility researchers be aware of the complete set of myosins in a given organism. The availability of cDNA and/or draft genomic sequences from humans, Drosophila melanogaster, Caenorhabditis elegans, Arabidopsis thaliana, Saccharomyces cerevisiae, Schizosaccharomyces pombe, and Dictyostelium discoideum has allowed us to tentatively define and compare the sets of myosin genes in these organisms. This analysis has also led to the identification of several putative myosin genes that may be of general interest. In humans, for example, we find a total of 40 known or predicted myosin genes including two new myosins-I, three new class II (conventional) myosins, a second member of the class III/ninaC myosins, a gene similar to the class XV deafness myosin, and a novel myosin sharing at most 33% identity with other members of the superfamily. These myosins are in addition to the recently discovered class XVI myosin with N-terminal ankyrin repeats and two human genes with similarity to the class XVIII PDZ-myosin from mouse. We briefly describe these newly recognized myosins and extend our previous phylogenetic analysis of the myosin superfamily to include a comparison of the complete or nearly complete inventories of myosin genes from several experimentally important organisms.

Chimeras of Dictyostelium myosin II head and neck domains with Acanthamoeba or chicken smooth muscle myosin II tail domain have greatly increased and unregulated actin-dependent MgATPase activity

Phosphorylation of the regulatory light chain of Dictyostelium myosin II increases V(max) of its actin-dependent MgATPase activity about 5-fold under normal assay conditions. Under these assay conditions, unphosphorylated chimeric myosins in which the tail domain of the Dictyostelium myosin II heavy chain is replaced by either the tail domain of chicken gizzard smooth muscle or Acanthamoeba myosin II are 20 times more active because of a 10- to 15-fold increase in V(max) and a 2- to 7-fold decrease in apparent K(ATPase) and are only slightly activated by regulatory light chain phosphorylation. Actin-dependent MgATPase activity of the Dictyostelium/Acanthamoeba chimera is not affected by phosphorylation of serine residues in the tail whose phosphorylation completely inactivates wild-type Acanthamoeba myosin II. These results indicate that the actin-dependent MgATPase activity of these myosins involves specific, tightly coupled, interactions between head and tail domains.

Coevolution of head, neck, and tail domains of myosin heavy chains

Myosins, a large family of actin-based motors, have one or two heavy chains with one or more light chains associated with each heavy chain. The heavy chains have a (generally) N-terminal head domain with an ATPase and actin-binding site, followed by a neck domain to which the light chains bind, and a C-terminal tail domain through which the heavy chains self-associate and/or bind the myosin to its cargo. Approximately 140 members of the myosin superfamily have been grouped into 17 classes based on the sequences of their head domains. I now show that a phylogenetic tree based on the sequences of the combined neck and tail domains groups 144 myosins, with a few exceptions, into the same 17 classes. For the nine myosin classes that have multiple members, phylogenetic trees based on the head domain or the combined neck/tail domains are either identical or very similar. For class II myosins, very similar phylogenetic trees are obtained for the head, neck, and tail domains of 47 heavy chains and for 29 essential light chains and 19 regulatory light chains. These data strongly suggest that the head, neck, and tail domains of all myosin heavy chains, and light chains at least of class II myosins, have coevolved and are likely to be functionally interdependent, consistent with biochemical evidence showing that regulated actin-dependent MgATPase activity of Dictyostelium myosin II requires isoform specific interactions between the heavy chain head and tail and light chains.