Movement of scallop myosin on Nitella actin filaments: regulation by calcium

Modeling of the mussel catch muscle contractile apparatus in actomyosin suspension

In this paper, we tried to create a contractile model from proteins of the catch muscle of the Gray mussel, similar to the well-described suspension contractile model of vertebrate skeletal muscles. This model makes it possible to characterize the processes in the reconstructed contractile apparatus with the help of monitoring the two characteristics of muscle suspensions - the optical density and the particle size. Contractile model of the catch muscle we constructed was the simplest model consisting of two proteins, actin and myosin. During this work we compared the optical manifestations of the contraction and relaxation states of constructed model with earlier data on the actomyosin suspension of skeletal muscles. It appeared that the approach used in the study of skeletal muscle actomyosin relaxing - the use of an increased amount of ATP - cannot be applied to the contractile model of the molluscan catch muscle. Nevertheless we managed to reach relaxed state of this model with modifying calcium concentration. As a result, we laid the foundation for further reconstruction of the third state of the catch muscle - the catch tone.

Single-Molecule Biophysical Techniques to Study Actomyosin Force Transduction

Inside the cellular environment, molecular motors can work in concert to conduct a variety of important physiological functions and processes that are vital for the survival of a cell. However, in order to decipher the mechanism of how these molecular motors work, single-molecule microscopy techniques have been popular methods to understand the molecular basis of the emerging ensemble behavior of these motor proteins.In this chapter, we discuss various single-molecule biophysical imaging techniques that have been used to expose the mechanics and kinetics of myosins. The chapter should be taken as a general overview and introductory guide to the many existing techniques; however, since other chapters will discuss some of these techniques more thoroughly, the readership should refer to those chapters for further details and discussions. In particular, we will focus on scattering-based single-molecule microscopy methods, some of which have become more popular in the recent years and around which the work in our laboratories has been centered.

The path to visualization of walking myosin V by high-speed atomic force microscopy

The quest for understanding the mechanism of myosin-based motility started with studies on muscle contraction. From numerous studies, the basic frameworks for this mechanism were constructed and brilliant hypotheses were put forward. However, the argument about the most crucial issue of how the actin-myosin interaction generates contractile force and shortening has not been definitive. To increase the "directness of measurement", in vitro motility assays and single-molecule optical techniques were created and used. Consequently, detailed knowledge of the motility of muscle myosin evolved, which resulted in provoking more arguments to a higher level. In parallel with technical progress, advances in cell biology led to the discovery of many classes of myosins. Myosin V was discovered to be a processive motor, unlike myosin II. The processivity reduced experimental difficulties because it allowed continuous tracing of the motor action of single myosin V molecules. Extensive studies of myosin V were expected to resolve arguments and build a consensus but did not necessarily do so. The directness of measurement was further enhanced by the recent advent of high-speed atomic force microscopy capable of directly visualizing biological molecules in action at high spatiotemporal resolution. This microscopy clearly visualized myosin V molecules walking on actin filaments and at last provided irrefutable evidence for the swinging lever-arm motion propelling the molecules. However, a peculiar foot stomp behavior also appeared in the AFM movie, raising new questions of the chemo-mechanical coupling in this motor and myosin motors in general. This article reviews these changes in the research of myosin motility and proposes new ideas to resolve the newly raised questions.

Behind the movement

This year, the Albert Lasker Basic Medical Research Award will be shared by Michael Sheetz, James Spudich, and Ronald Vale for discoveries concerning the biophysical actions of cytoskeletal motor-protein machines that move cargo within cells, contract muscles, and enable cell motility.

Regulation of fission yeast myosin-II function and contractile ring dynamics by regulatory light-chain and heavy-chain phosphorylation

We investigated the role of regulatory light-chain (Rlc1p) and heavy-chain phosphorylation in controlling fission yeast myosin-II (Myo2p) motor activity and function during cytokinesis. Phosphorylation of Rlc1p leads to a fourfold increase in Myo2p's in vitro motility rate, which ensures effective contractile ring constriction and function. Surprisingly, unlike with smooth muscle and nonmuscle myosin-II, RLC phosphorylation does not influence the actin-activated ATPase activity of Myo2p. A truncated form of Rlc1p lacking its extended N-terminal regulatory region (including phosphorylation sites) supported maximal Myo2p in vitro motility rates and normal contractile ring function. Thus, the unphosphorylated N-terminal extension of Rlc1p can uncouple the ATPase and motility activities of Myo2p. We confirmed the identity of one out of two putative heavy-chain phosphorylation sites previously reported to control Myo2p function and cytokinesis. Although in vitro studies indicated that phosphorylation at Ser-1444 is not needed for Myo2p motor activity, phosphorylation at this site promotes the initiation of contractile ring constriction.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

The sliding theory of cytoplasmic streaming: fifty years of progress

Fifty years ago, an important paper appeared in Botanical Magazine Tokyo. Kamiya and Kuroda proposed a sliding theory for the mechanism of cytoplasmic streaming. This pioneering study laid the basis for elucidation of the molecular mechanism of cytoplasmic streaming--the motive force is generated by the sliding of myosin XI associated with organelles along actin filaments, using the hydrolysis energy of ATP. The role of the actin-myosin system in various plant cell functions is becoming evident. The present article reviews progress in studies on cytoplasmic streaming over the past 50 years.

Calcium regulates scallop muscle by changing myosin flexibility

Muscle myosins are molecular motors that convert the chemical free energy available from ATP hydrolysis into mechanical displacement of actin filaments, bringing about muscle contraction. Myosin cross-bridges exert force on actin filaments during a cycle of attached and detached states that are coupled to each round of ATP hydrolysis. Contraction and ATPase activity of the striated adductor muscle of scallop is controlled by calcium ion binding to myosin. This mechanism of the so-called "thick filament regulation" is quite different to vertebrate striated muscle which is switched on and off via "thin filament regulation" whereby calcium ions bind to regulatory proteins associated with the actin filaments. We have used an optically based single molecule technique to measure the angular disposition adopted by the two myosin heads whilst bound to actin in the presence and absence of calcium ions. This has allowed us to directly observe the movement of individual myosin heads in aqueous solution at room temperature in real time. We address the issue of how scallop striated muscle myosin might be regulated by calcium and have interpreted our results in terms of the structures of smooth muscle myosin that also exhibit thick filament regulation.

Regulated conformation of myosin V

We have found that myosin V, an important actin-based vesicle transporter, has a folded conformation that is coupled to inhibition of its enzymatic activity in the absence of cargo and Ca(2+). In the absence of Ca(2+) where the actin-activated MgATPase activity is low, purified brain myosin V sediments in the analytical ultracentrifuge at 14 S as opposed to 11 S in the presence of Ca(2+) where the activity is high. At high ionic strength it sediments at 10 S independent of Ca(2+), and its regulation is poor. These data are consistent with myosin V having a compact, inactive conformation in the absence of Ca(2+) and an extended conformation in the presence of Ca(2+) or high ionic strength. Electron microscopy reveals that in the absence of Ca(2+) the heads and tail are both folded to give a triangular shape, very different from the extended appearance of myosin V at high ionic strength. A recombinant myosin V heavy meromyosin fragment that is missing the distal portion of the tail domain is not regulated by calcium and has only a small change in sedimentation coefficient, which is in the opposite direction to that seen with intact myosin V. Electron microscopy shows that its heads are extended even in the absence of calcium. These data suggest that interaction between the motor and cargo binding domains may be a general mechanism for shutting down motor protein activity and thereby regulating the active movement of vesicles in cells.

Plant 115-kDa actin-filament bundling protein, P-115-ABP, is a homologue of plant villin and is widely distributed in cells

In many cases, actin filaments are arranged into bundles and serve as tracks for cytoplasmic streaming in plant cells. We have isolated an actin-filament bundling protein, which is composed of 115-kDa polypeptide (P-115-ABP), from the germinating pollen of lily, Lilium longiflorum [Nakayasu et al. (1998) BIOCHEM: Biophys. Res. Commun. 249: 61]. P-115-ABP shared similar antigenicity with a plant 135-kDa actin-filament bundling protein (P-135-ABP), a plant homologue of villin. A full-length cDNA clone (ABP115; accession no. AB097407) was isolated from an expression cDNA library of lily pollen by immuno-screening using antisera against P-115-ABP and P-135-ABP. The amino acid sequence of P-115-ABP deduced from this clone showed high homology with those of P-135-ABP and four villin isoforms of Arabidopsis thaliana (AtVLN1, AtVLN2, AtVLN3 and AtVLN4), especially AtVLN4, indicating that P-115-ABP can also be classified as a plant villin. The P-115-ABP isolated biochemically from the germinating lily pollen was able to arrange F-actin filaments with uniform polarity into bundles and this bundling activity was suppressed by Ca2+-calmodulin (CaM), similar to the actin-filament bundling properties of P-135-ABP. The P-115-ABP type of plant villin was widely distributed in plant cells, from algae to land plants. In root hair cells of Hydrocharis dubia, this type of plant villin was co-localized with actin-filament bundles in the transvacuolar strands and the sub-cortical regions. Microinjection of the antiserum against P-115-ABP into living root hair cells caused the disappearance of transvaculor strands and alteration of the route of cytoplasmic streaming. In internodal cells of Chara corallina in which the P-135-ABP type of plant villin is lacking, the P-115-ABP type showed co-localization with actin-filament cables anchored on the intracellular surface of chloroplasts. These results indicated that plant villins are widely distributed and involved in the organization of actin filaments into bundles throughout the plant kingdom.

Evaluation of the symmetric model for myosin-linked regulation: effect of site-directed mutations in the regulatory light chain on scallop myosin

Regulatory myosins are controlled through mechanisms intrinsic to their structures and can alternate between activated and inhibited states. However, the structural difference between these two states is unclear. Scallop (Pecten maximus) striated adductor myosin is activated directly by calcium. It has been proposed that the two heads of scallop myosin are symmetrically arranged and interact through their regulatory light chains [Offer and Knight (1996) J. Mol. Biol. 256, 407-416], the interface being strengthened in the inhibited state. By contrast, vertebrate smooth-muscle myosin is activated by phosphorylation. Its structure in the inhibited state has been determined from two-dimensional crystalline arrays [Wendt, Taylor, Trybus and Taylor (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 4361-4366] and is asymmetric, requiring no interaction between regulatory light chains. Using site-directed mutagenesis of the scallop regulatory light chain, we have tested the symmetric model for scallop adductor muscle myosin. Specifically, we have made myosin hybrid molecules from scallop (P. maximus) myosin, in which the normal regulatory light chains have been replaced by expressed light chains containing mutations in three residues proposed to participate in the interaction between regulatory light chains. The mutations were R126A (Arg126-->Ala), K130A and E131A; made singly, in pairs or all three together, these mutations were designed to eliminate hydrogen bonding or salt linkages between heads, which are key features of this model. Functional assays to address the competence of these hybrid myosins to bind calcium specifically, to exhibit a calcium-regulated myofibrillar Mg-ATPase and to display calcium-dependent actin sliding were performed. We conclude that the symmetrical model does not describe the inhibited state of scallop regulatory myosin and that an asymmetric structure is a plausible alternative.

Link between the enzymatic kinetics and mechanical behavior in an actomyosin motor

We have attempted to link the solution actomyosin ATPase with the mechanical properties of in vitro actin filament sliding over heavy meromyosin. To accomplish this we perturbed the system by altering the substrate with various NTPs and divalent cations, and by altering ionic strength. A wide variety of enzymatic and mechanical measurements were made under very similar solution conditions. Excellent correlations between the mechanical and enzymatic quantities were revealed. Analysis of these correlations based on a force-balance model led us to two fundamental equations, which can be described approximately as follows: the maximum sliding velocity is proportional to square root of V(max)K(m)(A), where K(m)(A) is the actin concentration at which the substrate turnover rate is half of its maximum (V(max)). The active force generated by a cross-bridge under no external load or under a small external load is proportional to square root of V(max)/K(m)(A). The equations successfully accounted for the correlations observed in the present study and observations in other laboratories.

Calcium regulation of the actin-myosin interaction of Physarum polycephalum

Plasmodia of Physarum polycephalum show vigorous cytoplasmic streaming, the motive force of which is supported by the actin-myosin interaction. Calcium is not required for the interaction but inhibits it. This calcium inhibition, a regulatory mode first discovered in Physarum, is the overwhelming mode of regulation of cytoplasmic streaming of plant cells and lower eukaryotes, and it is diametrically opposite to calcium activation of the interaction found in muscle and nonmuscle cells of the animal kingdom. Myosin, myosin II in myosin superfamily, is the most important protein for Ca2+ action. Its essential light chain, called calcium-binding light chain, is the sole protein that binds Ca2+. Although phosphorylation and dephosphorylation of myosin modify its properties, regulation of physiological significance is shown to be Ca-binding to myosin. The actin-binding protein of Physarum amplifies calcium inhibition when Ca2+ binds to calmodulin and other calcium-binding proteins. This review also includes characterization of this and other calcium-binding proteins of Physarum.

Distinct functions of calmodulin are required for the uptake step of receptor-mediated endocytosis in yeast: the type I myosin Myo5p is one of the calmodulin targets

The uptake step of receptor-mediated endocytosis in yeast is dependent on the calcium binding protein calmodulin (Cmd1p). In order to understand the role that Cmd1p plays, a search was carried out for possible targets among the genes required for the internalization process. Co-immunoprecipitation, two-hybrid and overlay assays demonstrated that Cmd1p interacts with Myo5p, a type I unconventional myosin. Analysis of the endocytic phenotype and the Cmd1p-Myo5p interaction in thermosensitive cmd1 mutants indicated that the Cmd1p-Myo5p interaction is required for endocytosis in vivo. However, the Cmd1p-Myo5p interaction requirement was partially overcome by deleting the calmodulin binding sites (IQ motifs) from Myo5p, suggesting that these motifs inhibit Myo5p function. Additionally, genetic and biochemical evidence obtained with a collection of cmd1 mutant alleles strongly suggests that Cmd1p plays an additional role in the internalization step of receptor-mediated endocytosis in yeast.

Modification of the bi-directional sliding movement of actin filaments along native thick filaments isolated from a clam

The properties of bi-directional sliding of F-actin prepared from rabbit skeletal muscle moving along clam thick filaments have been characterized in the presence of agents known to modify unloaded shortening velocity in muscle to determine if the sliding characteristics of actin are similar in the two directions of movement. Actin filaments moved at a fast velocity towards the central bare zone (11.1 +/- 0.2 microns s-1) and at a slower velocity away from the bare zone (3.9 +/- 0.3 microns s-1). Movement of filaments at the slow sliding velocity is thought to be sustained by a change in orientation of the myosin head. The Michaelis Menten constant (Km values) of approximately 0.3 mM in the presence of MgATP concentrations of 0.01-2.0 mM at an ionic strength of 43.5 mM were reduced to approximately 0.1 mM at low ionic strength (18.5 mM) although the Km values at the fast and slow sliding velocities at each ionic strength were similar. In the presence of constant concentrations of MgATP, increasing the MgADP concentrations from 0.5 to 2mM, decreased the bi-directional sliding velocity of actin. The data were well fitted with an equation described by Michaelis Menten kinetics yielding mean absolute Km and Ki values of 0.41 +/- 0.01 and 0.44 +/- 0.05 mM for the fast velocity and 0.29 +/- 0.07 and 0.45 +/- 0.02 mM for the slow velocity of sliding, respectively. The Km and Ki values were not significantly different from each other at either the fast or slow sliding velocities. The actin filament sliding velocity appeared to be controlled through the thick filament as actin was devoid of regulatory proteins and the presence of Ca2+ modified the MgATP dependent movement of actin. The pCa value for half maximal sliding velocity was 7.0 for both fast and slow velocities. The Km and Ki values and the Ca2+ sensitivity of the actin movement at the fast and slow sliding velocity are similar suggesting that no major biochemical changes have occurred in the myosin head as a result of a change in orientation.

Role of essential light chain EF hand domains in calcium binding and regulation of scallop myosin

The specific Ca2+ binding site that triggers contraction of molluscan muscle requires the presence of an essential light chain (ELC) from a Ca2+ binding myosin. Of the four EF hand-like domains in molluscan ELCs, only domain III has an amino acid sequence predicted to be capable of binding Ca2+. In this report, we have used mutant ELCs to locate the Ca2+ binding site in scallop myosin and to probe the role of the ELC in regulation. Point mutations in domain III of scallop ELC have no effect on Ca2+ binding. Interestingly, scallop and rat cardiac ELC chimeras support Ca2+ binding only if domain I is scallop. These results are nevertheless in agreement with structural studies on a proteolytic fragment of scallop myosin, the regulatory domain. Furthermore, Ca2+ sensitivity of the scallop myosin ATPase requires scallop ELC domain I: ELCs containing cardiac domain I convert scallop myosin to an unregulated molecule whose activity is no longer repressed in the absence of Ca2+. Despite its unusual EF hand domain sequence, our data indicate that the unique and required contribution of molluscan ELCs to Ca2+ binding and regulation of molluscan myosins resides exclusively in domain I.

Role of myosin light chains

All conventional myosin IIs, whether isolated from skeletal, smooth, or invertebrate muscle sources, have two heads attached to an extended 16 nm alpha-helical coiled-coil tail. The head can be divided into a globular motor domain of approximately 770 amino acids that contains the catalytic and actin binding sites, and a neck region of approximately 70 amino acids which binds one essential and one regulatory light chain (ELC and RLC). The neck region with its associated LCs plays both structural and regulatory roles. While the mechanism and extent of regulation by the LCs varies for different myosins, the structural role may be a more fundamental feature of myosin II motors. Our understanding of the neck region has advanced rapidly in recent years primarily because of two types of information: (1) the high resolution structures of the LC binding domain from the thick-filament regulated scallop myosin (Xie et al., 1994) and of the head of unregulated skeletal myosin (Rayment et al., 1993), and (2) the ability to remove and/or mutate portions of both the heavy and light chains for analysis by in vitro motility assays.

Regulation of scallop myosin by the regulatory light chain depends on a single glycine residue

Specific Ca2+ binding and Ca2+ activation of ATPase activity in scallop myosin require a regulatory light chain (RLC) from regulated (molluscan or vertebrate smooth) myosin; hybrids containing vertebrate skeletal RLCs do not bind Ca2+ and their ATPase activity is inhibited. Chimeras between scallop and chicken skeletal RLCs restore Ca2+ sensitivity to RLC-free myosin provided that residues 81-117 are derived from scallop. Six mutants (R90M, A94K, D98P, N105K, M116Q, and G117C) were generated by replacing amino acids of the scallop RLC with the corresponding skeletal RLC residues in positions conserved in either regulated or nonregulated myosins. Ca2+ binding was abolished by a G117C and a G117A mutation; however, these mutants have a decreased affinity for the heavy chain. None of the other mutations affected RLC function. Replacement of the respective cysteine with glycine in the skeletal RLC has markedly changed the regulatory properties of the molecule. The single cysteine to glycine mutation conferred to this light chain the ability to restore Ca2+ binding and regulated ATPase activity, although Ca2+ activation of the actin-activated ATPase was lower than with scallop RLC. The presence of amino acids other than glycine at this position in vertebrate skeletal myosin RLCs may explain why these are not fully functional in the scallop system. The results are in agreement with x-ray crystallography data showing the central role of G117 in stabilizing the Ca(2+)-binding site of scallop myosin.

Coupling of ATPase activity and motility in smooth muscle myosin is mediated by the regulatory light chain

Smooth muscle myosin acts as a molecular motor only if the regulatory light chain (RLC) is phosphorylated. This subunit can be removed from myosin by a novel method involving the use of trifluoperazine. The motility of RLC-deficient myosin is very slow, but native properties are restored when RLC is rebound. Truncating 6 residues from the COOH terminus of the RLC had no effect on phosphorylated myosin's motor properties, while removal of the last 12 residues reduced velocity by approximately 30%. Very slow movement was observed once 26 residues were deleted, or with myosin containing only the COOH-terminal RLC domain. These two mutants thus mimicked the behavior of RLC-deficient myosin, with the important difference that the mutant myosins were monodisperse when assayed by sedimentation velocity and electron microscopy. The decreased motility therefore cannot be caused by aggregation. A common feature of RLC-deficient myosin and the mutant myosins that moved actin slowly was an increased myosin ATPase compared with dephosphorylated myosin, and a lower actin-activated ATPase than obtained with phosphorylated myosin. These results suggest that the COOH-terminal portion of an intact RLC is involved in interactions that regulate myosin's "on-off" switch, both in terms of completely inhibiting and completely activating the molecule.

A functional recombinant myosin II lacking a regulatory light chain-binding site

Myosin II, which converts the energy of adenosine triphosphate hydrolysis into the movement of actin filaments, is a hexamer of two heavy chains, two essential light chains, and two regulatory light chains (RLCs). Dictyostelium myosin II is known to be regulated in vitro by phosphorylation of the RLC. Cells in which the wild-type myosin II heavy chain was replaced with a recombinant form that lacks the binding site for RLC carried out cytokinesis and almost normal development, processes known to be dependent on functional myosin II. Characterization of the purified recombinant protein suggests that a complex of RLC and the RLC binding site of the heavy chain plays an inhibitory role for adenosine triphosphatase activity and a structural role for the movement of myosin along actin.

Two activators of microtubule-based vesicle transport

Cytoplasmic dynein purified by nucleotide dependent microtubule affinity has significant minus end-directed vesicle motor activity that decreases with each further purification step. Highly purified dynein causes membrane vesicles to bind but not move on microtubules. We exploited these observations to develop an assay for factors that, in combination with dynein, would permit minus end-directed vesicle motility. At each step of the purification, non-dynein fractions were recombined with dynein and assayed for vesicle motility. Two activating fractions were identified by this method. One, called Activator I, copurified with 20S dynein by velocity sedimentation but could be separated from it by ion exchange chromatography. Activator I increased only the frequency of dynein-driven vesicle movements. Activator II, sedimenting at 9S, increased both the frequency and velocity of vesicle transport and also supported plus end movements. Our results suggest that dynein-based motility is controlled at multiple levels and provide a preliminary characterization of two regulatory factors.

Calmodulin dissociation regulates brush border myosin I (110-kD-calmodulin) mechanochemical activity in vitro

110-kD-calmodulin, when immobilized on nitrocellulose-coated coverslips, translocates actin filaments at a maximal rate of 0.07-0.1 micron/s at 37 degrees C. Actin activates MgATPase activity greater than 40-fold, with a Km of 40 microM and Vmax of 0.86 s-1 (323 nmol/min/mg). The rate of motility mediated by 110-kD-calmodulin is dependent on temperature and concentration of ATP, but independent of time, actin filament length, amount of enzyme, or ionic strength. Tropomyosin inhibits actin binding by 110-kD-calmodulin in MgATP and inhibits motility. Micromolar calcium slightly increases the rate of motility and increases the actin-activated MgATP hydrolysis of the intact complex. In 0.1 mM or higher calcium, motility ceases and actin-dependent MgATPase activity remains at a low rate not activated by increasing actin concentration. Correlated with these inhibitions of activity, a subset of calmodulin is dissociated from the complex. To determine if calmodulin loss is the cause of calcium inhibition, we assayed the ability of calmodulin to rescue the calcium-inactivated enzyme. Readdition of calmodulin to the nitrocellulose-bound, calcium-inactivated enzyme completely restores motility. Addition of calmodulin also restores actin activation to MgATPase activity in high calcium, but does not affect the activity of the enzyme in EGTA. These results demonstrate that in vitro 110-kD-calmodulin functions as a calcium-sensitive mechanoenzyme, a vertebrate myosin I. The properties of this enzyme suggest that despite unique structure and regulation, myosins I and II share a molecular mechanism of motility.

Microtubule translocation properties of intact and proteolytically digested dyneins from Tetrahymena cilia

Tetrahymena cilia contain a three-headed 22S (outer arm) dynein and a single-headed 14S dynein. In this study, we have employed an in vitro assay of microtubule translocation along dynein-coated glass surfaces to characterize the motile properties of 14S dynein, 22S dynein, and proteolytic fragments of 22S dynein. Microtubule translocation produced by intact 22S dynein and 14S dynein differ in a number of respects including (a) the maximal velocities of movement; (b) the ability of 22S dynein but not 14S dynein to utilize ATP gamma S to induce movement; (c) the optimal pH and ionic conditions for movement; and (d) the effects of Triton X-100 on the velocity of movement. These results indicate that 22S and 14S dyneins have distinct microtubule translocating properties and suggest that these dyneins may have specialized roles in ciliary beating. We have also explored the function of the multiple ATPase heads of 22S dynein by preparing one- and two-headed proteolytic fragments of this three-headed molecule and examining their motile activity in vitro. Unlike the single-headed 14S dynein, the single-headed fragment of 22S dynein did not induce movement, even though it was capable of binding to microtubules. The two-headed fragment, on the other hand, translocated microtubules at velocities similar to those measured for intact 22S dynein (10 microns/sec). This finding indicates that the intact three-headed structure of 22S dynein is not essential for generating microtubule movement, which raises the possibility that multiple heads may serve some regulatory function or may be required for maximal force production in the beating cilium.

Mg-ATPase activity and motility of native thick filaments isolated from the anterior byssus retractor muscle of Mytilus edulis

A method for isolating native thick filaments from the anterior byssus retractor muscle (ABRM) of Mytilus edulis is described. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the isolated thick filament preparation contained mainly paramyosin and myosin but almost no actin. Electron microscopy of negatively stained preparations showed that the isolated thick filaments were tapered at both ends and of various sizes, in the range 5-31 microns in length and 51-94nm in width in the central region. Central bare zones were observed in the smaller filaments, but were not clearly seen in the larger filaments. Mg-ATPase activity of the isolated thick filaments was activated by skeletal muscle F-actin in a Ca2+-dependent manner. The maximal activity was about 20 nmol min-1 mg-1 thick filaments (20 degrees C, pH7.0). Motility of the thick filaments attached to latex beads (diameter, 2 microns) was also studied using the native actin cables of the freshwater alga, Chara. In the presence of Mg-ATP and Ca2+, the beads moved along the actin cables at a maximal velocity of about 1 micron s-1. In the absence of Ca2+, almost no movement was observed. These results show that the isolated thick filaments are structurally intact and retain the essential mechanochemical characteristics of the ABRM myosin.

Accelerated sliding of pollen tube organelles along Characeae actin bundles regulated by Ca2+

Pollen tubes show active cytoplasmic streaming. We isolated organelles from pollen tubes and tested their ability to slide along actin bundles in characean cell models. Here, we show that sliding of organelles was ATP-dependent and that motility was lost after N-ethylmaleimide or heat treatment of organelles. On the other hand, cytoplasmic streaming in pollen tube was inhibited by either N-ethylmaleimide or heat treatment. These results strongly indicate that cytoplasmic streaming in pollen tubes is supported by the "actomyosin"-ATP system. The velocity of organelle movement along characean actin bundles was much higher than that of the native streaming in pollen tubes. We suggested that pollen tube "myosin" has a capacity to move at a velocity of the same order of magnitude as that of characean myosin. Moreover, the motility was high at Ca2+ concentrations lower than 0.18 microM (pCa 6.8) but was inhibited at concentration higher than 4.5 microM (pCa 5.4). In conclusion, cytoplasmic streaming in pollen tubes is suggested to be regulated by Ca2+ through "myosin" inactivation.

Organelle Movements along Actin Filaments and Microtubules

Organelle movements involving microtubules and actin filaments are a conspicuous and important feature of many plant cells. Movements have recently been supported in preparations of demembranated cytoplasm and reconstituted from purified proteins. The favored mechanism involves organelles carrying a force-generating ATPase moving along a track provided by either actin filaments or microtubules. Cytoplasmic free Ca(2+) concentration regulates at least some organelle movements.

Light chain phosphorylation regulates the movement of smooth muscle myosin on actin filaments

In smooth muscles there is no organized sarcomere structure wherein the relative movement of myosin filaments and actin filaments has been documented during contraction. Using the recently developed in vitro assay for myosin-coated bead movement (Sheetz, M.P., and J.A. Spudich, 1983, Nature (Lond.)., 303:31-35), we were able to quantitate the rate of movement of both phosphorylated and unphosphorylated smooth muscle myosin on ordered actin filaments derived from the giant alga, Nitella. We found that movement of turkey gizzard smooth muscle myosin on actin filaments depended upon the phosphorylation of the 20-kD myosin light chains. About 95% of the beads coated with phosphorylated myosin moved at velocities between 0.15 and 0.4 micron/s, depending upon the preparation. With unphosphorylated myosin, only 3% of the beads moved and then at a velocity of only approximately 0.01-0.04 micron/s. The effects of phosphorylation were fully reversible after dephosphorylation with a phosphatase prepared from smooth muscle. Analysis of the velocity of movement as a function of phosphorylation level indicated that phosphorylation of both heads of a myosin molecule was required for movement and that unphosphorylated myosin appears to decrease the rate of movement of phosphorylated myosin. Mixing of phosphorylated smooth muscle myosin with skeletal muscle myosin which moves at 2 microns/s resulted in a decreased rate of bead movement, suggesting that the more slowly cycling smooth muscle myosin is primarily determining the velocity of movement in such mixtures.