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

Go with the flow - bulk transport by molecular motors

Cells are the smallest building blocks of all living eukaryotic organisms, usually ranging from a couple of micrometers (for example, platelets) to hundreds of micrometers (for example, neurons and oocytes) in size. In eukaryotic cells that are more than 100 µm in diameter, very often a self-organized large-scale movement of cytoplasmic contents, known as cytoplasmic streaming, occurs to compensate for the physical constraints of large cells. In this Review, we discuss cytoplasmic streaming in multiple cell types and the mechanisms driving this event. We particularly focus on the molecular motors responsible for cytoplasmic movements and the biological roles of cytoplasmic streaming in cells. Finally, we describe bulk intercellular flow that transports cytoplasmic materials to the oocyte from its sister germline cells to drive rapid oocyte growth.

Competition between kinesin-1 and myosin-V defines Drosophila posterior determination

Local accumulation of oskar (osk) mRNA in the Drosophila oocyte determines the posterior pole of the future embryo. Two major cytoskeletal components, microtubules and actin filaments, together with a microtubule motor, kinesin-1, and an actin motor, myosin-V, are essential for osk mRNA posterior localization. In this study, we use Staufen, an RNA-binding protein that colocalizes with osk mRNA, as a proxy for osk mRNA. We demonstrate that posterior localization of osk/Staufen is determined by competition between kinesin-1 and myosin-V. While kinesin-1 removes osk/Staufen from the cortex along microtubules, myosin-V anchors osk/Staufen at the cortex. Myosin-V wins over kinesin-1 at the posterior pole due to low microtubule density at this site, while kinesin-1 wins at anterior and lateral positions because they have high density of cortically-anchored microtubules. As a result, posterior determinants are removed from the anterior and lateral cortex but retained at the posterior pole. Thus, posterior determination of Drosophila oocytes is defined by kinesin-myosin competition, whose outcome is primarily determined by cortical microtubule density.

One of the most fundamental steps of embryonic development is deciding which end of the body should be the head, and which should be the tail. Known as 'axis specification', this process depends on the location of genetic material called mRNAs. In fruit flies, for example, the tail-end of the embryo accumulates an mRNA called oskar. If this mRNA is missing, the embryo will not develop an abdomen.

The build-up of oskar mRNA happens before the egg is even fertilized and depends on two types of scaffold proteins in the egg cell called microtubules and microfilaments. These scaffolds act like ‘train tracks’ in the cell and have associated protein motors, which work a bit like trains, carrying cargo as they travel up and down along the scaffolds. For microtubules, one of the motors is a protein called kinesin-1, whereas for microfilaments, the motors are called myosins.

Most microtubules in the egg cell are pointing away from the membrane, while microfilament tracks form a dense network of randomly oriented filaments just underneath the membrane. It was already known that kinesin-1 and a myosin called myosin-V are important for localizing oskar mRNA to the posterior of the egg. However, it was not clear why the mRNA only builds up in that area.

To find out, Lu et al. used a probe to track oskar mRNA, while genetically manipulating each of the motors so that their ability to transport cargo changed. Modulating the balance of activity between the two motors revealed that kinesin-1 and myosin-V engage in a tug-of-war inside the egg: myosin-V tries to keep oskar mRNA underneath the membrane of the cell, while kinesin-1 tries to pull it away from the membrane along microtubules. The winner of this molecular battle depends on the number of microtubule tracks available in the local area of the cell. In most parts of the cell, there are abundant microtubules, so kinesin-1 wins and pulls oskar mRNA away from the membrane. But at the posterior end of the cell there are fewer microtubules, so myosin-V wins, allowing oskar mRNA to localize in this area. Artificially 'shaving' some microtubules in a local area immediately changed the outcome of this tug-of-war creating a build-up of oskar mRNA in the 'shaved' patch.

This is the first time a molecular tug-of-war has been shown in an egg cell, but in other types of cell, such as neurons and pigment cells, myosins compete with kinesins to position other molecular cargoes. Understanding these processes more clearly sheds light not only on embryo development, but also on cell biology in general.

Kinetic signatures of myosin-5B, the motor involved in microvillus inclusion disease

Myosin-5B is a ubiquitous molecular motor that transports cargo vesicles of the endomembrane system in intracellular recycling pathways. Myosin-5B malfunction causes the congenital enteropathy microvillus inclusion disease, underlining its importance in cellular homeostasis. Here we describe the interaction of myosin-5B with F-actin, nucleotides, and the pyrazolopyrimidine compound myoVin-1. We show that single-headed myosin-5B is an intermediate duty ratio motor with a kinetic ATPase cycle that is rate-limited by the release of phosphate. The presence of a second head generates strain and gating in the myosin-5B dimer that alters the kinetic signature by reducing the actin-activated ADP release rate to become rate-limiting. This kinetic transition into a high-duty ratio motor is a prerequisite for the proposed transport function of myosin-5B in cellular recycling pathways. Moreover, we show that the small molecule compound myoVin-1 inhibits the enzymatic and functional activity of myosin-5B in vitro Partial inhibition of the actin-activated steady-state ATPase activity and sliding velocity suggests that caution should be used when probing the effect of myoVin-1 on myosin-5-dependent transport processes in cells.

Diverse and pervasive subcellular distributions for both coding and long noncoding RNAs

Here, Wilk et al. examined ∼8000 mRNA transcripts throughout Drosophila embryogenesis. They found that almost all RNAs, both coding and noncoding RNAs, are subcellularly localized at some stage of development, thus providing an important resource for functional gene analysis.

In a previous analysis of 2300 mRNAs via whole-mount fluorescent in situ hybridization in cellularizing Drosophila embryos, we found that 70% of the transcripts exhibited some form of subcellular localization. To see whether this prevalence is unique to early Drosophila embryos, we examined ∼8000 transcripts over the full course of embryogenesis and ∼800 transcripts in late third instar larval tissues. The numbers and varieties of new subcellular localization patterns are both striking and revealing. In the much larger cells of the third instar larva, virtually all transcripts observed showed subcellular localization in at least one tissue. We also examined the prevalence and variety of localization mechanisms for >100 long noncoding RNAs. All of these were also found to be expressed and subcellularly localized. Thus, subcellular RNA localization appears to be the norm rather than the exception for both coding and noncoding RNAs. These results, which have been annotated and made available on a recompiled database, provide a rich and unique resource for functional gene analyses, some examples of which are provided.

Myosin light chains: Teaching old dogs new tricks

The myosin holoenzyme is a multimeric protein complex consisting of heavy chains and light chains. Myosin light chains are calmodulin family members which are crucially involved in the mechanoenzymatic function of the myosin holoenzyme. This review examines the diversity of light chains within the myosin superfamily, discusses interactions between the light chain and the myosin heavy chain as well as regulatory and structural functions of the light chain as a subunit of the myosin holoenzyme. It covers aspects of the myosin light chain in the localization of the myosin holoenzyme, protein-protein interactions and light chain binding to non-myosin binding partners. Finally, this review challenges the dogma that myosin regulatory and essential light chain exclusively associate with conventional myosin heavy chains while unconventional myosin heavy chains usually associate with calmodulin.

The motor function of Drosophila melanogaster myosin-5 is activated by calcium and cargo-binding protein dRab11

In the Drosophila melanogaster compound eye, myosin-5 (DmM5) plays two distinct roles in response to light stimulation: transport of pigment granules to the rhabdomere base to decrease light exposure and transport of rhodopsin-bearing vesicles to the rhabdomere base to compensate for the rhodopsin loss during light exposure. However, little is known of how the motor function of DmM5 is regulated at the molecular level. In the present study, we overexpressed DmM5 in Sf9 insect cells and investigated its regulation using purified proteins. We found that the actin-activated ATPase activity of DmM5 is significantly lower than that of the truncated DmM5 having the C-terminal globular tail domain (GTD) deleted, indicating that the GTD is the inhibitory domain. The actin-activated ATPase activity of DmM5 is significantly activated by micromolar levels of calcium. DmM5 associates with pigment granules and rhodopsin-bearing vesicles through cargo-binding proteins Lightoid (Ltd) and dRab11 respectively. We found that GTP-bound dRab11, but not Ltd, significantly activates DmM5 actin-activated ATPase activity. Moreover, we identified Gln(1689) in the GTD as the critical residue for the interaction with dRab11 and activation of DmM5 motor function by dRab11. Based on those results, we propose that DmM5-dependent transport of pigment granules is directly activated by light-induced calcium influx and the DmM5-dependent transport of rhodopsin-bearing vesicle is activated by active GTP-bound dRab11, whose formation is stimulated by light-induced calcium influx.

Myosin motor isoforms direct specification of actomyosin function by tropomyosins

Myosins and tropomyosins represent two cytoskeletal proteins that often work together with actin filaments in contractile and motile cellular processes. While the specialized role of tropomyosin in striated muscle myosin-II regulation is well characterized, its role in nonmuscle myosin regulation is poorly understood. We previously showed that fission yeast tropomyosin (Cdc8p) positively regulates myosin-II (Myo2p) and myosin-V (Myo52p) motors. To understand the broader implications of this regulation we examined the role of two mammalian tropomyosins (Tpm3.1cy/Tm5NM1 and Tpm4.2cy/Tm4) recently implicated in cancer cell proliferation and metastasis. Like Cdc8p, the Tpm3.1cy and Tpm4.2cy isoforms significantly enhance Myo2p and Myo52p motor activity, converting nonprocessive Myo52p molecules into processive motors that can walk along actin tracks as single molecules. In contrast to the positive regulation of Myo2p and Myo52p, Cdc8p and the mammalian tropomyosins potently inhibited skeletal muscle myosin-II, while having negligible effects on the highly processive mammalian myosin-Va. In support of a conserved role for certain tropomyosins in regulating nonmuscle actomyosin structures, Tpm3.1cy supported normal contractile ring function in fission yeast. Our work reveals that actomyosin regulation by tropomyosin is dependent on the myosin isoform, highlighting a general role for specific isoforms of tropomyosin in sorting myosin motor outputs.

The novel proteins Rng8 and Rng9 regulate the myosin-V Myo51 during fission yeast cytokinesis

The myosin-V family of molecular motors is known to be under sophisticated regulation, but our knowledge of the roles and regulation of myosin-Vs in cytokinesis is limited. Here, we report that the myosin-V Myo51 affects contractile ring assembly and stability during fission yeast cytokinesis, and is regulated by two novel coiled-coil proteins, Rng8 and Rng9. Both rng8Δ and rng9Δ cells display similar defects as myo51Δ in cytokinesis. Rng8 and Rng9 are required for Myo51's localizations to cytoplasmic puncta, actin cables, and the contractile ring. Myo51 puncta contain multiple Myo51 molecules and walk continuously on actin filaments in rng8(+) cells, whereas Myo51 forms speckles containing only one dimer and does not move efficiently on actin tracks in rng8Δ. Consistently, Myo51 transports artificial cargos efficiently in vivo, and this activity is regulated by Rng8. Purified Rng8 and Rng9 form stable higher-order complexes. Collectively, we propose that Rng8 and Rng9 form oligomers and cluster multiple Myo51 dimers to regulate Myo51 localization and functions.

Getting myosin-V on the right track: tropomyosin sorts transport in yeast

Recent studies have revealed a novel mechanism of myosin regulation in which the actin-binding protein tropomyosin converts atypical type-V myosins into processive cargo transporters. To achieve this, tropomyosin's primary role appears to lie in its ability to influence myosin's enzyme kinetics, prolonging the strong actin-bound ADP/apo state to enable hand-over-hand walking of myosin-V dimers along actin tracks. Activation of myosin-V mediated transport by tropomyosin underscores its function in helping to direct cargos to specific actin tracks and subcellular destinations. This type of regulation supports the broader notion that tropomyosin plays a key role in actomyosin sorting.

Fission yeast tropomyosin specifies directed transport of myosin-V along actin cables

Fission yeast tropomyosin targets myosin-V to actin cables by favoring processivity of the motor. Live-cell imaging is used to estimate the number of myosin-V molecules per motile particle in vivo. In vitro reconstitution demonstrates the physiological relevance of tropomyosin-based targeting of this motor.

A hallmark of class-V myosins is their processivity—the ability to take multiple steps along actin filaments without dissociating. Our previous work suggested, however, that the fission yeast myosin-V (Myo52p) is a nonprocessive motor whose activity is enhanced by tropomyosin (Cdc8p). Here we investigate the molecular mechanism and physiological relevance of tropomyosin-mediated regulation of Myo52p transport, using a combination of in vitro and in vivo approaches. Single molecules of Myo52p, visualized by total internal reflection fluorescence microscopy, moved processively only when Cdc8p was present on actin filaments. Small ensembles of Myo52p bound to a quantum dot, mimicking the number of motors bound to physiological cargo, also required Cdc8p for continuous motion. Although a truncated form of Myo52p that lacked a cargo-binding domain failed to support function in vivo, it still underwent actin-dependent movement to polarized growth sites. This result suggests that truncated Myo52p lacking cargo, or single molecules of wild-type Myo52p with small cargoes, can undergo processive movement along actin-Cdc8p cables in vivo. Our findings outline a mechanism by which tropomyosin facilitates sorting of transport to specific actin tracks within the cell by switching on myosin processivity.

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.

Tropomyosin is essential for processive movement of a class V myosin from budding yeast

Myosin V is an actin-based motor protein involved in intracellular cargo transport [1]. Given this physiological role, it was widely assumed that all class V myosins are processive, able to take multiple steps along actin filaments without dissociating. This notion was challenged when several class V myosins were characterized as nonprocessive in vitro, including Myo2p, the essential class V myosin from S. cerevisiae [2-6]. Myo2p moves cargo including secretory vesicles and other organelles for several microns along actin cables in vivo. This demonstrated cargo transporter must therefore either operate in small ensembles or behave processively in the cellular context. Here we show that Myo2p moves processively in vitro as a single motor when it walks on an actin track that more closely resembles the actin cables found in vivo. The key to processivity is tropomyosin: Myo2p is not processive on bare actin but highly processive on actin-tropomyosin. The major yeast tropomyosin isoform, Tpm1p, supports the most robust processivity. Tropomyosin slows the rate of MgADP release, thus increasing the time the motor spends strongly attached to actin. This is the first example of tropomyosin switching a motor from nonprocessive to processive motion on actin.

Two single-headed myosin V motors bound to a tetrameric adapter protein form a processive complex

Myo4p, one of two class V myosins in budding yeast, continuously transports messenger RNA (mRNA) cargo in the cell but is nonprocessive when characterized in vitro. The adapter protein She3p tightly binds to the Myo4p rod, forming a single-headed motor complex. In this paper, we show that two Myo4p-She3p motors are recruited by the tetrameric mRNA-binding protein She2p to form a processive double-headed complex. The binding site for She3p was mapped to a single α helix that protrudes at right angles from She2p. Processive runs of several micrometers on yeast actin-tropomyosin filaments were observed only in the presence of She2p, and, thus, motor activity is regulated by cargo binding. While moving processively, each head steps ~72 nm in a hand-over-hand motion. Coupling two high-duty cycle monomeric motors via a common cargo-binding adapter protein creates a complex with transport properties comparable with a single dimeric processive motor such as vertebrate myosin Va.

Walking to work: roles for class V myosins as cargo transporters

Cells use molecular motors, such as myosins, to move, position and segregate their organelles. Class V myosins possess biochemical and structural properties that should make them ideal actin-based cargo transporters. Indeed, studies show that class V myosins function as cargo transporters in yeast, moving a range of organelles, such as the vacuole, peroxisomes and secretory vesicles. There is also increasing evidence in vertebrate cells that class V myosins not only tether organelles to actin but also can serve as short-range, point-to-point organelle transporters, usually following long-range, microtubule-dependent organelle transport.

Shaking the myosin family tree: biochemical kinetics defines four types of myosin motor

Although all myosin motors follow the same basic cross-bridge cycle, they display a large variety in the rates of transition between different states in the cycle, allowing each myosin to be finely tuned for a specific task. Traditionally, myosins have been classified by sequence analysis into a large number of sub-families (∼35). Here we use a different method to classify the myosin family members which is based on biochemical and mechanical properties. The key properties that define the type of mechanical activity of the motor are duty ratio (defined as the fraction of the time myosin remains attached to actin during each cycle), thermodynamic coupling of actin and nucleotide binding to myosin and the degree of strain-sensitivity of the ADP release step. Based on these properties we propose to classify myosins into four different groups: (I) fast movers, (II) slow/efficient force holders, (III) strain sensors and (IV) gates.

Regulation and function of the fission yeast myosins

It is now quarter of a century since the actin cytoskeleton was first described in the fission yeast, Schizosaccharomyces pombe. Since then, a substantial body of research has been undertaken on this tractable model organism, extending our knowledge of the organisation and function of the actomyosin cytoskeleton in fission yeast and eukaryotes in general. Yeast represents one of the simplest eukaryotic model systems that has been characterised to date, and its genome encodes genes for homologues of the majority of actin regulators and actin-binding proteins found in metazoan cells. The ease with which diverse methodologies can be used, together with the small number of myosins, makes fission yeast an attractive model system for actomyosin research and provides the opportunity to fully understand the biochemical and functional characteristics of all myosins within a single cell type. In this Commentary, we examine the differences between the five S. pombe myosins, and focus on how these reflect the diversity of their functions. We go on to examine the role that the actin cytoskeleton plays in regulating the myosin motor activity and function, and finally explore how research in this simple unicellular organism is providing insights into the substantial impacts these motors can have on development and viability in multicellular higher-order eukaryotes.

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.

Functional adaptation of the switch-2 nucleotide sensor enables rapid processive translocation by myosin-5

Active site loops that are conserved across superfamilies of myosins, kinesins, and G proteins play key roles in allosteric coupling of NTP hydrolysis to interaction with track filaments or effector proteins. In this study, we investigated how the class-specific natural variation in the switch-2 active site loop contributes to the motor function of the intracellular transporter myosin-5. We used single-molecule, rapid kinetic and spectroscopic experiments and semiempirical quantum chemical simulations to show that the class-specific switch-2 structure including a tyrosine (Y439) in myosin-5 enables rapid processive translocation along actin filaments by facilitating Mg(2+)-dependent ADP release. Using wild-type control and Y439 point mutant myosin-5 proteins, we demonstrate that the translocation speed precisely correlates with the kinetics of nucleotide exchange. Switch-2 variants can thus be used to fine-tune translocation speed while maintaining high processivity. The class-specific variation of switch-2 in various NTPase superfamilies indicates its general role in the kinetic tuning of Mg(2+)-dependent nucleotide exchange.

Extensibility of the extended tail domain of processive and nonprocessive myosin V molecules

Myosin V is a single-molecule motor that moves organelles along actin. When myosin V pulls loads inside the cell in a highly viscous environment, the force on the motor is unlikely to be constant. We propose that the tether between the single-molecule motor and the cargo (i.e., the extended tail domain of the molecule) must be able to absorb the sudden mechanical motions of the motor and allow smooth relaxation of the motion of the cargo to a new position. To test this hypothesis, we compared the elastic properties of the extended tail domains of processive (mouse myosin Va) and nonprocessive (Drosophila myosin V) molecular motors. The extended tail domain of these myosins consists of mechanically strong coiled-coil regions interspersed with flexible loops. In this work we explored the mechanical properties of coiled-coil regions using atomic force microscopy. We found that the processive and nonprocessive coiled-coil fragments display different unfolding patterns. The unfolding of coiled-coil structures occurs much later during the atomic force microscopy stretch cycle for processive myosin Va than for nonprocessive Drosophila myosin V, suggesting that this elastic tether between the cargo and motor may play an important role in sustaining the processive motions of this single-molecule motor.

A nonprocessive class V myosin drives cargo processively when a kinesin- related protein is a passenger

During secretory events, kinesin transports cargo along microtubules and then shifts control to myosin V for delivery on actin filaments to the cell membrane [1]. When kinesin and myosin V are present on the same cargo, kinesin interacts electrostatically with actin to enhance myosin V-based transport in vitro [2]. The relevance of this observation within the cell was questioned. In budding yeast, overexpression of a kinesin-family protein (Smy1p) suppressed a transport defect in a strain with a mutant class V myosin (Myo2p) [3]. We postulate that this is a cellular manifestation of the in vitro observation. We demonstrate that Smy1p binds electrostatically to actin bundles. Although a single Myo2p cannot transport cargo along actin bundles, addition of Smy1p causes the complex to undergo long-range, continuous movement. We propose that the kinesin-family protein acts as a tether that prevents cargo dissociation from actin, allowing the myosin to take many steps before dissociating. We demonstrate that both the tether and the motor reside on moving secretory vesicles in yeast cells, a necessary feature for this mechanism to apply in vivo. The presence of both kinesin and myosin on the same cargo may be a general mechanism to enhance cellular transport in yeast and higher organisms.

Fission yeast Myo51 is a meiotic spindle pole body component with discrete roles during cell fusion and spore formation

Class V myosins are dimeric actin-associated motor proteins that deliver cellular cargoes to discrete cellular locations. Fission yeast possess two class V myosins, Myo51 and Myo52. Although Myo52 has been shown to have roles in vacuole distribution, cytokinesis and cell growth, Myo51 has no as yet discernible function in the vegetative life cycle. Here, we uncover distinct functions for this motor protein during mating and meiosis. Not only does Myo51 transiently localise to a foci at the site of cell fusion upon conjugation, but overexpression of the Myo51 globular tail also leads to disruption of cell fusion. Upon completion of meiotic prophase Myo51 localises to the outside of the spindle pole bodies (SPBs), where it remains until completion of meiosis II. Association of Myo51 with SPBs is not dependent upon actin or the septation initiation network (SIN); however, it is dependent on a stable microtubule cytoskeleton and the presence of the Cdc2-CyclinB complex. We observe a rapid and dynamic exchange of Myo51 at the SPB during meiosis I but not meiosis II. Finally, we show that Myo51 has an important role in regulating spore formation upon completion of meiosis.

Myosin Vc is a molecular motor that functions in secretory granule trafficking

Class V myosins are actin-based motor proteins that have critical functions in organelle trafficking. Of the three class V myosins expressed in mammals, relatively little is known about Myo5c except that it is abundant in exocrine tissues. Here we use MCF-7 cells to identify the organelles that Myo5c associates with, image the dynamics of Myo5c in living cells, and test the functions of Myo5c. Endogenous Myo5c localizes to two distinct compartments: small puncta and slender tubules. Myo5c often exhibits a highly polarized distribution toward the leading edge in migrating cells and is clearly distinct from the Myo5a or Myo5b compartments. Imaging with GFP-Myo5c reveals that Myo5c puncta move slowly (approximately 30 nm/s) and microtubule independently, whereas tubules move rapidly (approximately 440 nm/s) and microtubule dependently. Myo5c puncta colocalize with secretory granule markers such as chromogranin A and Rab27b, whereas Myo5c tubules are labeled by Rab8a. TIRF imaging indicates that the granules can be triggered to undergo secretion. To test if Myo5c functions in granule trafficking, we used the Myo5c tail as a dominant negative and found that it dramatically perturbs the distribution of granule markers. These results provide the first live-cell imaging of Myo5c and indicate that Myo5c functions in secretory granule trafficking.

Essential features of the class V myosin from budding yeast for ASH1 mRNA transport

Myo4p, a single-headed and nonprocessive class V myosin in budding yeast, transports >20 different mRNAs asymmetrically to the bud. Here, we determine the features of the Myo4p motor that are necessary for correct localization of ASH1 mRNA to the daughter cell, a process that also requires the adapter protein She3p and the dimeric mRNA-binding protein She2p. The rod region of Myo4p, but not the globular tail, is essential for correct localization of ASH1 mRNA, confirming that the rod contains the primary binding site for She3p. The requirement for both the rod region and She3p can be bypassed by directly coupling the mRNA-binding protein She2p to Myo4p. ASH1 mRNA was also correctly localized when one motor was bound per dimeric She2p, or when two motors were joined together by a leucine zipper. Because multiple mRNAs are cotransported to the bud, it is likely that this process involves multiple motor transport regardless of the number of motors per zip code. Our results show that the most important feature for correct localization is the retention of coupling between all the members of the complex (Myo4p-She3p-She2p-ASH1 mRNA), which is aided by She3p being a tightly bound subunit of Myo4p.

Dictyostelium myosin-5b is a conditional processive motor

Dictyostelium myosin-5b is the gene product of myoJ and one of two closely related myosin-5 isoenzymes produced in Dictyostelium discoideum. Here we report a detailed investigation of the kinetic and functional properties of the protein. In standard assay buffer conditions, Dictyostelium myosin-5b displays high actin affinity in the presence of ADP, fast ATP hydrolysis, and a high steady-state ATPase activity in the presence of actin that is rate limited by ADP release. These properties are typical for a processive motor that can move over long distances along actin filaments without dissociating. Our results show that a physiological decrease in the concentration of free Mg(2+)-ions leads to an increased rate of ADP release and shortening of the fraction of time the motor spends in the strong actin binding states. Consistently, the ability of the motor to efficiently translocate actin filaments at very low surface densities decreases with decreasing concentrations of free Mg(2+)-ions. In addition, we provide evidence that the observed changes in Dd myosin-5b motor activity are of physiological relevance and propose a mechanism by which this molecular motor can switch between processive and non-processive movement.

Calcium-activated Myosin V closes the Drosophila pupil

Approximately 40 years ago, an elegant automatic-gain control was revealed in compound eye photoreceptors: In bright light, an assembly of small pigment granules migrates to the cytoplasmic face of the photosensitive membrane organelle, the rhabdomere, where they attenuate waveguide propagation along the rhabdomere. This migration results in a "longitudinal pupil" that reduces rhodopsin exposure by a factor of 0.8 log units. Light-induced elevation of cytosolic free Ca(2+) triggers the migration of pigment granules, and pigment granules fail to migrate in a mutant deficient in photoactivated TRP calcium channels. However, the mechanism that moves photoreceptor pigment granules remains elusive. Are the granules actively pulled toward the rhabdomere upon light, or are they instead actively pulled into the cytoplasm in the absence of light? Here we show that Ca(2+)-activated Myosin V (MyoV) pulls pigment granules to the rhabdomere. Thus, one of MyoV's several functions is also as a sensory-adaptation motor. In vitro, Ca(2+) both activates and inhibits MyoV motility; in vivo, its role is undetermined. This first demonstration of an in vivo role for Ca(2+) in MyoV activity shows that in Drosophila photoreceptors, Ca(2+) stimulates MyoV motility.

Human myosin Vc is a low duty ratio, nonprocessive molecular motor

Myosin Vc is the product of one of the three genes of the class V myosin found in vertebrates. It is widely found in secretory and glandular tissues, with a possible involvement in transferrin trafficking. Transient and steady-state kinetic studies of human myosin Vc were performed using a truncated, single-headed construct. Steady-state actin-activated ATPase measurements revealed a V(max) of 1.8 +/- 0.3 s(-1) and a K(ATPase) of 43 +/- 11 microm. Unlike previously studied vertebrate myosin Vs, the rate-limiting step in the actomyosin Vc ATPase pathway is the release of inorganic phosphate (~1.5 s(-1)), rather than the ADP release step (~12.0-16.0 s(-1)). Nevertheless, the ADP affinity of actomyosin Vc (K(d) = 0.25 +/- 0.02 microm) reflects a higher ADP affinity than seen in other myosin V isoforms. Using the measured kinetic rates, the calculated duty ratio of myosin Vc was approximately 10%, indicating that myosin Vc spends the majority of the actomyosin ATPase cycle in weak actin-binding states, unlike the other vertebrate myosin V isoforms. Consistent with this, a fluorescently labeled double-headed heavy meromyosin form showed no processive movements along actin filaments in a single molecule assay, but it did move actin filaments at a velocity of approximately 24 nm/s in ensemble assays. Kinetic simulations reveal that the high ADP affinity of actomyosin Vc may lead to elevations of the duty ratio of myosin Vc to as high as 64% under possible physiological ADP concentrations. This, in turn, may possibly imply a regulatory mechanism that may be sensitive to moderate changes in ADP concentration.

She3p binds to the rod of yeast myosin V and prevents it from dimerizing, forming a single-headed motor complex

Vertebrate myosin Va is a dimeric processive motor that walks on actin filaments to deliver cargo. In contrast, the two class V myosins in budding yeast, Myo2p and Myo4p, are non-processive (Reck-Peterson, S. L., Tyska, M. J., Novick, P. J., and Mooseker, M. S. (2001) J. Cell Biol. 153, 1121-1126). We previously showed that a chimera with the motor domain of Myo4p on the backbone of vertebrate myosin Va was processive, demonstrating that the Myo4p motor domain has a high duty ratio. Here we examine the properties of a chimera containing the rod and globular tail of Myo4p joined to the motor domain and neck of mouse myosin Va. Surprisingly, the adaptor protein She3p binds to the rod region of Myo4p and forms a homogeneous single-headed myosin-She3p complex, based on sedimentation equilibrium and velocity data. We propose that She3p forms a heterocoiled-coil with Myo4p and is a subunit of the motor. She3p does not affect the maximal actin-activated ATPase in solution or the velocity of movement in an ensemble in vitro motility assay. At the single molecule level, the monomeric myosin-She3p complex showed no processivity. When this construct was dimerized with a leucine zipper, short processive runs were obtained. Robust continuous movement was observed when multiple monomeric myosin-She3p motors were bound to a quantum dot "cargo." We propose that continuous transport of mRNA by Myo4p-She3p in yeast is accomplished either by multiple high duty cycle monomers or by molecules that may be dimerized by She2p, the homodimeric downstream binding partner of She3p.

Human myosin Vc is a low duty ratio nonprocessive motor

There are three distinct members of the myosin V family in vertebrates, and each isoform is involved in different membrane trafficking pathways. Both myosin Va and Vb have demonstrated that they are high duty ratio motors that are consistent with the processive nature of these motors. Here we report that the ATPase cycle mechanism of the single-headed construct of myosin Vc is quite different from those of other vertebrate myosin V isoforms. K(ATPase) of the actin-activated ATPase was 62 microm, which is much higher than that of myosin Va ( approximately 1 mum). The rate of ADP release from actomyosin Vc was 12.7 s(-1), which was 2 times greater than the entire ATPase cycle rate, 6.5 s(-1). P(i) burst size was 0.31, indicating that the equilibrium of the ATP hydrolysis step is shifted to the prehydrolysis form. Our kinetic model, based on all kinetic data we determined in this study, suggests that myosin Vc spends the majority of the ATPase cycle time in the weak actin binding state in contrast to myosin Va and Vb. Consistently, the two-headed myosin Vc construct did not show processive movement in total internal reflection fluorescence microscope analysis, demonstrating that myosin Vc is a nonprocessive motor. Our findings suggest that myosin Vc fulfills its function as a cargo transporter by different mechanisms from other myosin V isoforms.

In vivo movement of the type V myosin Myo52 requires dimerisation but is independent of the neck domain

Intracellular movement is a fundamental property of all cell types. Many organelles and molecules are actively transported throughout the cytoplasm by molecular motors, such as the dimeric type V myosins. These possess a long neck, which contains an IQ motif, that allow it to make 36-nm steps along the actin polymer. Live cell imaging of the fission yeast type V myosin Myo52 reveals that the protein moves rapidly throughout the cytoplasm. Here, we describe analysis of this movement and have established that Myo52 moves long distances on actin filaments in an ATP-dependent manner at approximately 0.5 mum/second. Myo51 and the microtubule cytoskeleton have no discernable role in modulating Myo52 movements, whereas rigour mutations in Myo52 abrogated its movement. We go on to show that, although dimerisation is required for Myo52 movement, deleting its neck has no discernable affect on Myo52 function or velocity in vivo.

Myo4p is a monomeric myosin with motility uniquely adapted to transport mRNA

The yeast Saccharomyces cerevisiae uses two class V myosins to transport cellular material into the bud: Myo2p moves secretory vesicles and organelles, whereas Myo4p transports mRNA. To understand how Myo2p and Myo4p are adapted to transport physically distinct cargos, we characterize Myo2p and Myo4p in yeast extracts, purify active Myo2p and Myo4p from yeast lysates, and analyze their motility. We find several striking differences between Myo2p and Myo4p. First, Myo2p forms a dimer, whereas Myo4p is a monomer. Second, Myo4p generates higher actin filament velocity at lower motor density. Third, single molecules of Myo2p are weakly processive, whereas individual Myo4p motors are nonprocessive. Finally, Myo4p self-assembles into multi-motor complexes capable of processive motility. We show that the unique motility of Myo4p is not due to its motor domain and that the motor domain of Myo2p can transport ASH1 mRNA in vivo. Our results suggest that the oligomeric state of Myo4p is important for its motility and ability to transport mRNA.

Native nonmuscle myosin II stability and light chain binding inDrosophila melanogaster

Native nonmuscle myosin IIs play essential roles in cellular and developmental processes throughout phylogeny. Individual motor molecules consist of a heterohexameric complex of three polypeptides which, when properly assembled, are capable of force generation. Here, we more completely characterize the properties, relationships and associations that each subunit has with one another in Drosophila melanogaster. All three native nonmuscle myosin II polypeptide subunits are expressed in close to constant stoichiometry to each other throughout development. We find that the stability of two subunits, the heavy chain and the regulatory light chain, depend on one another whereas the stability of the third subunit, the essential light chain, does not depend on either the heavy chain or regulatory light chain. We demonstrate that heavy chain aggregates, which form when regulatory light chain is lacking, associate with the essential light chain in vivo-thus showing that regulatory light chain association is required for heavy chain solubility. By immunodepletion we find that the majority of both light chains are associated with the nonmuscle myosin II heavy chain but pools of free light chain and/or light chain bound to other proteins are present. We identify four myosins (myosin II, myosin V, myosin VI and myosin VIIA) and a microtubule-associated protein (asp/Abnormal spindle) as binding partners for the essential light chain (but not the regulatory light chain) through mass spectrometry and co-precipitation. Using an in silico approach we identify six previously uncharacterized genes that contain IQ-motifs and may be essential light chain binding partners.

Myosin at work: motor adaptations for a variety of cellular functions

Cells have evolved multiple mechanisms to overcome the effects of entropy and diffusion to create a highly ordered environment. For cells to function properly, some components must be anchored to provide a framework or structure. Others must be rapidly transported over long distances to generate asymmetries in cell morphology and composition. To accomplish long-range transport, cells cannot rely on diffusion alone as many large organelles and macromolecular complexes are essentially immobilized by the dense meshwork of the cytosol. One strategy used by cells to overcome diffusion is to harness the free energy liberated by ATP hydrolysis through molecular motors. Myosins are a family of actin based molecular motors that have evolved a variety of ways to contribute to cellular organization through numerous modifications to the manner they convert that free energy into mechanical work.

Dimerized Drosophila myosin VIIa: a processive motor

The molecular mechanism of processive movement of single myosin molecules from classes V and VI along their actin tracks has recently attracted extraordinary attention. Another member of the myosin superfamily, myosin VII, plays vital roles in the sensory function of Drosophila and mammals. We studied the molecular mechanism of Drosophila myosin VIIa, using transient kinetics and single-molecule motility assays. Myosin VIIa moves along actin filaments as a processive, double-headed single molecule when dimerized by the inclusion of a leucine zipper at the C terminus of the coiled-coil domain. Its motility is approximately 8-10 times slower than that of myosin V, and its step size is 30 nm, which is consistent with the presence of five IQ motifs in its neck region. The kinetic basis for the processive motility of myosin VIIa is the relative magnitude of the release rate constants of phosphate (fast) and ADP (slow) as in myosins V and VI. The ATPase pathway is rate-limited by a reversible interconversion between two distinct ADP-bound actomyosin states, which results in high steady-state occupancy of a strongly actin-bound myosin species. The distinctive features of myosin VIIa (long run lengths, slow motility) will be very useful in video-based single-molecule applications. In cells, this kinetic behavior would allow myosin VIIa to exert and hold tension on actin filaments and, if dimerized, to function as a processive cargo transporter.

Walking with myosin V

The cytoplasm of cells is teaming with vesicles and other cargo that are moving along tracks of microtubules or actin filaments, powered by myosins, kinesins and dyneins. Myosin V has been implicated in several types of intracellular transport. The mechanism by which myosin V moves processively along actin filaments has been the subject of many biophysical and biochemical studies and a consensus is starting to emerge about how this minute molecular motor operates.

Processivity of chimeric class V myosins

Unconventional myosin V takes many 36-nm steps along an actin filament before it dissociates, thus ensuring its ability to move cargo intracellularly over long distances. In the present study we assessed the structural features that affect processive run length by analyzing the properties of chimeras of mouse myosin V and a non-processive class V myosin from yeast (Myo4p) (Reck-Peterson, S. L., Tyska, M. J., Novick, P. J., and Mooseker, M. S. (2001) J. Cell Biol. 153, 1121-1126). Surprisingly a chimera containing the yeast motor domain on the neck and rod of mouse myosin V (Y-MD) showed longer run lengths than mouse wild type at low salt. Run lengths of mouse myosin V showed little salt dependence, whereas those of Y-MD decreased steeply with ionic strength, similar to a chimera containing yeast loop 2 in the mouse myosin V backbone. Loop 2 binds to acidic patches on actin in the weak binding states of the cycle (Volkmann, N., Liu, H., Hazelwood, L., Krementsova, E. B., Lowey, S., Trybus, K. M., and Hanein, D. (2005) Mol. Cell 19, 595-605). Constructs containing yeast loop 2, which has no net charge compared with +6 for wild type, showed a higher K(m) for actin in steady-state ATPase assays. The results imply that a positively charged loop 2 and a high affinity for actin are important to maintain processivity near physiologic ionic strength.

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.