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

High affinity Ca2+ binding sites of calmodulin are critical for the regulation of myosin Ibeta motor function

We coexpressed myosin Ibeta heavy chain with three different calmodulin mutants in which the two Ca2+-binding sites of the two N-terminal domain (E12Q), C-terminal domain (E34Q), or all four sites (E1234Q) are mutated in order to define the importance of these Ca2+ binding sites to the regulation of myosin Ibeta. The calmodulin mutated at the two Ca2+ binding sites in N-terminal domain and C-terminal domain lost its lower affinity Ca2+ binding site and higher affinity Ca2+ binding site, respectively. We found that, based upon the change in the actin-activated ATPase activities and actin translocating activities, myosin Ibeta with E12Q calmodulin has the regulatory characteristics similar to myosin Ibeta containing wild-type calmodulin, while myosin Ibeta with E34Q or E1234Q calmodulin lose all Ca2+ regulation. While the increase in myosin Ibeta ATPase activity paralleled the dissociation of 1 mol of calmodulin from myosin Ibeta heavy chain for both wild type (above pCa 5) and E12Q calmodulin (above pCa 6), the Ca2+ level required for the inhibition of actin-translocating activity of myosin Ibeta was lower than that required for dissociation of calmodulin, suggesting that the conformational change induced by the binding of Ca2+ at the high affinity site but not the dissociation of calmodulin is critical for the inhibition of the motor activity. Our results suggest that the regulation of unconventional myosins by Ca2+ is directly mediated by the Ca2+ binding to calmodulin, and that the C-terminal pair of Ca2+-binding sites are critical for this regulation.

Amino-acid sequence of squid myosin heavy chain

This work describes the determination of the cDNA sequence encoding the myosin heavy chain (MHC) of the squid, Loligo pealei. To date, the amino-acid sequence of the MHC of calcium-regulated myosins is known only for two closely related species of scallops. We have determined the sequence of the entire coding region of the muscle MHC of squid, a cephalopod, and compared it with the MHC of scallops, which are pelecypods, and to other regulated and non-regulated myosins. Residues present in the MHC of only regulated myosins have been identified. The 6504 base pair (bp) sequence contains an open reading frame of 5805 nucleotides, which encodes 1935 amino acids. The sequence includes 697 bps of 3' untranslated sequence and 2 bps of 5' untranslated sequence. The deduced amino-acid sequence shows the squid MHC to be 72-73% identical and 86-87% similar to the calcium-regulated scallop MHCs cloned previously. In contrast, the squid MHC sequence is only 54-55% identical and 74% similar to skeletal MHCs of non-regulated myosins such as human fast skeletal embryonic and human perinatal skeletal muscle, and 39-40% identical and 60-62% similar to smooth muscle MHC of rabbit uterus muscle and chicken gizzard muscle, respectively. We have also detected two isoforms of the MHC in squid that appear to be spliced variants of a single myosin gene. These isoforms differ in the sequence encoding the surface loop at the nucleotide binding site. Taken together, our data may help to identify more precisely the residues that are crucial in regulated myosins.

A myosin III from Limulus eyes is a clock-regulated phosphoprotein

The lateral eyes of the horseshoe crab Limulus polyphemus undergo dramatic daily changes in structure and function that lead to enhanced retinal sensitivity and responsiveness to light at night. These changes are controlled by a circadian neural input that alters photoreceptor and pigment cell shape, pigment migration, and phototransduction. Clock input to the eyes also regulates photomechanical movements within photoreceptors, including membrane shedding. The biochemical mechanisms underlying these diverse effects of the clock on the retina are unknown, but a major biochemical consequence of activating clock input to the eyes is a rise in the concentration of cAMP in photoreceptors and the phosphorylation of a 122 kDa visual system-specific protein. We have cloned and sequenced cDNA encoding the clock-regulated 122 kDa phosphoprotein and show here that it is a new member of the myosin III family. We report that Limulus myosin III is similar to other unconventional myosins in that it binds to calmodulin in the absence of Ca2+; it is novel in that it is phosphorylated within its myosin globular head, probably by cAMP-dependent protein kinase. The protein is present throughout the photoreceptor, including the region occupied by the photosensitive rhabdom. We propose that the phosphorylation of Limulus myosin III is involved in one or more of the structural and functional changes that occur in Limulus eyes in response to clock input.

Actomyosin motor in the merozoite of the malaria parasite, Plasmodium falciparum: implications for red cell invasion

The genome of the malaria parasite, Plasmodium falciparum, contains a myosin gene sequence, which bears a close homology to one of the myosin genes found in another apicomplexan parasite, Toxoplasma gondii. A polyclonal antibody was generated against an expressed polypeptide of molecular mass 27,000, based on part of the deduced sequence of this myosin. The antibody reacted with the cognate antigen and with a component of the total parasite protein on immunoblots, but not with vertebrate striated or smooth muscle myosins. It did, however, recognise two components in the cellular protein of Toxoplasma gondii. The antibody was used to investigate stage-specificity of expression of the myosin (here designated Pf-myo1) in P. falciparum. The results showed that the protein is synthesised in mature schizonts and is present in merozoites, but vanishes after the parasite enters the red cell. Pf-myo1 was found to be largely, though not entirely, associated with the particulate parasite cell fraction and is thus presumably mainly membrane bound. It was not solubilised by media that would be expected to dissociate actomyosin or myosin filaments, or by non-ionic detergent. Immunofluorescence revealed that in the merozoite and mature schizont Pf-myo1 is predominantly located around the periphery of the cell. Immuno-gold electron microscopy also showed the presence of the myosin around almost the entire parasite periphery, and especially in the region surrounding the apical prominence. Labelling was concentrated under the plasma membrane but was not seen in the apical prominence itself. This suggests that Pf-myo1 is associated with the plasma membrane or with the outer membrane of the subplasmalemmal cisterna, which forms a lining to the plasma membrane, with a gap at the apical prominence. The results lead to a conjectural model of the invasion mechanism.

A PCR screen identifies a novel, unconventional myosin heavy chain gene (MYO1) in Tetrahymena thermophila

Degenerate primers for two regions of sequence homology in the myosin head domain were used in a polymerase chain reaction screen of Tetrahymena thermophila genomic DNA to amplify a 765 bp fragment that was cloned and sequenced. Based on the presence of conserved, myosin-specific sequences, the 765 bp PCR product was identified as a fragment of a myosin gene, the first to be discovered in ciliated protozoa and herein referred to as MYO1. An inverse polymerase chain reaction strategy was used to obtain additional sequence data that included the entire head domain of MYO1. Alignment of the predicted amino acid sequence of the MYO1 head domain with known myosin sequences identified the ATP-binding site, a phosphorylation site, and other myosin-specific consensus regions. In a northern blot analysis, a 765 bp MYO1-specific probe detected a 6.6 kb transcript under highly stringent hybridization conditions. Phylogenetic analysis revealed that the predicted protein encoded by MYO1 is not a member of any of the previously defined myosin classes and therefore represents a presumptive new myosin class.

Molecular genetic dissection of mouse unconventional myosin-VA: head region mutations

The mouse dilute (d) locus encodes unconventional myosin-VA (MyoVA). Mice carrying null alleles of dilute have a lightened coat color and die from a neurological disorder resembling ataxia and opisthotonus within three weeks of birth. Immunological and ultrastructural studies suggest that MyoVA is involved in the transport of melanosomes in melanocytes and smooth endoplasmic reticulum in cerebellar Purkinje cells. In studies described here, we have used an RT-PCR-based sequencing approach to identify the mutations responsible for 17 viable dilute alleles that vary in their effects on coat color and the nervous system. Seven of these mutations mapped to the MyoVA motor domain and are reported here. Crystallographic modeling and mutant expression studies were used to predict how these mutations might affect motor domain function and to attempt to correlate these effects with the mutant phenotype.

Human myosin-IXb is a mechanochemically active motor and a GAP for rho

The heavy chains of the class IX myosins, rat myr5 and human myosin-IXb, contain within their tail domains a region with sequence homology to GTPase activating proteins for the rho family of G proteins. Because low levels of myosin-IXb expression preclude purification by conventional means, we have employed an immunoadsorption strategy to purify myosin-IXb, enabling us to characterize the mechanochemical and rho-GTPase activation properties of the native protein. In this report we have examined the light chain content, actin binding properties, in vitro motility and rho-GTPase activity of human myosin-IXb purified from leukocytes. The results presented here indicate that myosin-IXb contains calmodulin as a light chain and that it binds to actin with high affinity in both the absence and presence of ATP. Myosin-IXb is an active motor which, like other calmodulin-containing myosins, exhibits maximal velocity of actin filaments (15 nm/second) in the absence of Ca2+. Native myosin-IXb exhibits GAP activity on rho. Class IX myosins may be an important link between rho and rho-dependent remodeling of the actin cytoskeleton.

Myosins: matching functions with motors

It is an exciting time to be studying myosins and their roles in the function of cells and organisms. Past efforts aimed at finding new members of this family have now given way to a focus on identifying individual functions for each motor protein. These actin-based motors are now known to be intimately involved in the following processes: neurosensory function; vesicle trafficking; determinant partitioning; and cortical function. The following article reviews the inroads made into the functions of myosins in these processes over the past several years.

Molecular characterization of myosin V from Drosophila melanogaster

Recent studies have revealed unconventional myosin V to be an important actin-based molecular motor involved in vesicular movement. In this paper we report the molecular characterization of the Drosophila myosin V, identified by reverse genetics. The gene encodes a 1792-residue, 207 kDa heavy chain polypeptide which possesses a typical head or motor domain of 771 residues, a region of six IQ motifs (139 residues) which serve as potential calmodulin/light chain binding sites at the head/tail junction, and a tail domain of 882 residues containing sequences of putative alpha-helical coiled-coils required for dimerization of the molecule and sequences of non-helical structure at the C-terminal end. Based on Southern blot analyses and chromosomal localization, evidence is presented for a single Drosophila myosin V gene. RNA analyses revealed a doublet of transcripts of about 6 kb, expressed throughout the lifetime of a fly but particularly abundant in the early stages of embryonic development (maternally contributed), in the ectodermic tissue of the hindgut starting at stage 16, and in the adult head. These results suggest that myosin V may be involved in processes required in a variety of cell types in Drosophila. We have also mapped the Drosophila myosin V locus to chromosome 2 at the position 43C-D, and we are currently searching for known mutations in this region. Finally, phylogenetic analysis of the head domain reveals that Drosophila myosin V is more closely related to mammalian myosin Va and Vb than to other invertebrate class-V myosins; nevertheless, it is not significantly more related to myosin Va than to myosin Vb. While vertebrates would need two different myosin V isoforms to accomplish specific functions, we speculate that Drosophila myosin V might provide the equivalent functions by itself.

Unconventional myosins in cell movement, membrane traffic, and signal transduction

In the past few years genetic, biochemical, and cytolocalization data have implicated members of the myosin superfamily of actin-based molecular motors in a variety of cellular functions including membrane trafficking, cell movements, and signal transduction. The importance of myosins is illustrated by the identification of myosin genes as targets for disease-causing mutations. The task at hand is to decipher how the multitude of myosins function at both the molecular and cellular level-a task facilitated by our understanding of myosin structure and function in muscle.

The myosin I SH3 domain and TEDS rule phosphorylation site are required for in vivo function

The class I myosins play important roles in controlling many different types of actin-based cell movements. Dictyostelium cells either lacking or overexpressing amoeboid myosin Is have significant defects in cortical activities such as pseudopod extension, cell migration, and macropinocytosis. The existence of Dictyostelium null mutants with strong phenotypic defects permits complementation analysis as a means of exploring important functional features of the myosin I heavy chain. Mutant Dictyostelium cells lacking two myosin Is exhibit profound defects in growth, endocytosis, and rearrangement of F-actin. Expression of the full-length myoB heavy chain in these cells fully rescues the double mutant defects. However, mutant forms of the myoB heavy chain in which a serine at the consensus phosphorylation site has been altered to an alanine or in which the C-terminal SH3 domain has been removed fail to complement the null phenotype. The wild-type and mutant forms of the myoB heavy chain appeared to be properly localized when they were expressed in the myosin I null mutants. These results suggest that the amoeboid myosin I consensus phosphorylation site and SH3 domains do not play a role in the localization of myosin I, but are absolutely required for in vivo function.

A family of unconventional myosins from the nematode Caenorhabditis elegans

The unconventional myosins are a superfamily of actin-based motor proteins that are expressed in a wide range of cell types and organisms. Thirteen classes of unconventional myosin have been defined, and current efforts are focused on elucidating their individual functions in vivo. Here, we report the identification of a family of unconventional myosin genes in Caenorhabditis elegans. The hum-1, hum-2, hum-3 and hum-6 (heavy chain of an unconventional myosin) genes encode members of myosin classes I, V, VI and VII, respectively. The hum-4 gene encodes a high molecular mass myosin (ca 307 kDa) that is one of the most highly divergent myosins, and is the founding and only known member of class XII. The physical position of each hum gene has been determined. The hum-1, hum-2 and hum-3 genes have been mapped by extrapolation near previously uncharacterized mutations, several of which are lethal, identifying potentially essential unconventional myosin genes in C. elegans.

The growing family of myosin motors and their role in neurons and sensory cells

Biochemical and physiological evidence has suggested that myosins, both conventional and unconventional, are critical for neurosensory activities. In the past few years, this premise has been supported by genetic evidence that has shown that unconventional myosins are essential for the proper functioning of neurons, retina and the sensory cells of the inner ear.

A novel class of unconventional myosins from Toxoplasma gondii

Here, we describe the complete deduced amino acid sequence of three unconventional myosins identified in the protozoan parasite Toxoplasma gondii. Phylogenetic analysis reveals that the three myosins represent a novel, highly-divergent class addition to the myosin superfamily. Toxoplasma gondii myosin-A (TgM-A) is a remarkably small approximately 93 kDa myosin that shows a striking departure from typical myosin heavy chain structure in having a head and tail domain but no discernible neck domain. In other myosins, the neck is defined by one or more IQ motifs that serve as potential light chain binding domains. No IQ motifs are apparent in TgM-A. The tail domain of TgM-A encompasses only 57 amino acid residues and is characterized by its highly basic charge (pI = 10.8). The other two Toxoplasma myosins, TgM-B and TgM-C appear to be the product of differential RNA splicing with TgM-B yielding a protein of approximately 114 kDa and TgM-C a protein of approximately 125 kDa. These two myosins are identical throughout their head domain and neck domain which contains a single IQ motif. TgM-B and C share the proximal 245 residues of their tail domain and then diverge in their tail structure distally. The tails, like that of TgM-A, share no homology to any other myosin tails apart from a highly basic charge. The identification of yet another class of unconventional myosins, including a myosin as novel in structure as the 93 kDa TgM-A, continues to underscore the diversity of this family of molecular motors.

The structure of the acto-myosin subfragment 1 complex: results of searches using data from electron microscopy and x-ray crystallography

Surmises of how myosin subfragment 1 (S1) interacts with actin filaments in muscle contraction rest upon knowing the relative arrangement of the two proteins. Although there exist crystallographic structures for both S1 and actin, as well as electron microscopy data for the acto-S1 complex (AS1), modeling of this arrangement has so far only been done "by eye." Here we report fitted AS1 structures obtained using a quantitative method that is both more objective and makes more complete use of the data. Using undistorted crystallographic results, the best-fit AS1 structure shows significant differences from that obtained by visual fitting. The best fit is produced using the F-actin model of Holmes et al. [Holmes, K. C., Popp, D., Gebhard, W. & Kabsch, W. (1990) Nature (London) 347, 44-49]. S1 residues at the AS1 interface are now found at a higher radius as well as being translated axially and rotated azimuthally. Fits using S1 plus loops missing from the crystal structure were achieved using a homology search method to predict loop structures. These improved fits favor an arrangement in which the loop at the 50- to 20-kDa domain junction of S1 is located near the N terminus of actin. Rigid-body movements of the lower 50-kDa domain, which further improve the fit, produce closure of the large 50-kDa domain cleft and bring conserved residues in the lower 50-kDa domain into an apparently appropriate orientation for close interaction with actin. This finding supports the idea that binding of ATP to AS1 at the end of the ATPase cycle disrupts the actin binding site by changing the conformation of the 50-kDa cleft of S1.

Structural studies on myosin II: communication between distant protein domains

Understanding how chemical energy is converted into directed movement is a fundamental problem in biology. In higher organisms this is accomplished through the hydrolysis of ATP by three families of motor proteins: myosin, dynein and kinesin. The most abundant of these is myosin, which operates against actin and plays a central role in muscle contraction. As summarized here, great progress has been made towards understanding the molecular basis of movement through the determination of the three-dimensional structures of myosin and actin and through the establishment of systems for site-directed mutagenesis of this motor protein. It now appears that the generation of movement is coupled to ATP hydrolysis by a series of domain movements within myosin.

Mutations in the myosin VIIA gene cause non-syndromic recessive deafness

Genetic hearing impairment affects around 1 in every 2,000 births. The bulk (approximately 70%) of genetic deafness is non-syndromic, in which hearing impairment is not associated with any other abnormalities. Over 25 loci involved in non-syndromic deafness have been mapped and mutations in connexin 26 have been identified as a cause of non-sydromic deafness. One locus for non-syndromic recessive deafness, DFNB2 (ref. 4), has been localized to the same chromosomal region, 11q14, as one of the loci, USH1B, underlying the recessive deaf-blind syndrome. Usher syndrome type 1b, which is characterized by profound congenital sensorineural deafness, constant vestibular dysfunction and prepubertal onset of retinitis pigmentosa. Recently, it has been shown that a gene encoding an unconventional myosin, myosin VIIA, underlies the mouse recessive deafness mutation, shaker-1 (ref. 5) as well as Usher syndrome type 1b. Mice with shaker-1 demonstrate typical neuroepithelial defects manifested by hearing loss and vestibular dysfunction but no retinal pathology. Differences in retinal patterns of expression may account for the variance in phenotype between shaker-1 mice and Usher type 1 syndrome. Nevertheless, the expression of MYO7A in the neuroepithelium suggests that it should be considered a candidate for non-syndromic deafness in the human population. By screening families with non-syndromic deafness from China, we have identified two families carrying MYO7A mutations.

Structure and dynamics of molecular motors

The structures of the oppositely directed microtubule motors kinesin and ncd have been solved to atomic resolution. The two structures are very similar and are also homologous to myosin. Myosins and kinesins differ kinetically but, tantalizingly, cryoelectron microscopy has recently revealed that both structures may tilt during ADP release. Such evidence suggests that the two motor families use common structural mechanisms.

Unconventional myosins: new frontiers in actin-based motors

The unconventional myosins are a superfamily of actin-based motors responsible for a rich array of intracellular motility events. Recent evidence suggests that these motors play important roles in cell migration, endocytosis and intracellular transport. Several genetic mutants have been identified whose abnormalities are the result of the loss of a specific myosin. This article describes how analysis of these mutants, coupled with basic studies of the intracellular localization and biochemical properties of individual myosins, is leading to a clearer understanding of the in vivo function of a number of these interesting motor proteins.

The swinging lever-arm hypothesis of muscle contraction

The molecular mechanism of muscle contraction is a problem that has exercised biophysicists and biochemists for many years. The common view of the mechanism is embodied in the 'cross-bridge hypothesis', in which the relative sliding of thick (myosin) and thin (actin) filaments in cross-striated muscle is brought about by the 'cross-bridges', parts of the myosin molecules which protrude from the thick filaments and interact cyclically with the actin filaments, transporting them by a rowing action that is powered by the hydrolysis of ATP. This hypothesis is, however, rather vague on the molecular details of cross-bridge movement and, in the light of the recently determined crystal structures of myosin and actin, it has evolved into the more precise 'swinging lever-arm hypothesis'.

Muscle proteins--their actions and interactions

Muscle contracts by the myosin cross-bridges "rowing' the actin filaments past the myosin filaments. In the past year many structural details of this mechanism have become clear. Structural studies indicate distinct states for myosin S1 in the rigor, ATP or "down' conformation and in the products complex (ADP.Pi) or "up' to state. Crystallographic studies substantiate this classification and yield details of the transformation. The isomerization "up' to "down' is the power stroke of muscle. This consists in the main of large changes of angle of the "lever arm' (at the distal part of the myosin head) which can account for an 11 nm power stroke.