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

Structure of the class XI myosin globular tail reveals evolutionary hallmarks for cargo recognition in plants

The plant-specific class XI myosins (MyoXIs) play key roles at the molecular, cellular and tissue levels, engaging diverse adaptor proteins to transport cargoes along actin filaments. To recognize their cargoes, MyoXIs have a C-terminal globular tail domain (GTD) that is evolutionarily related to those of class V myosins (MyoVs) from animals and fungi. Despite recent advances in understanding the functional roles played by MyoXI in plants, the structure of its GTD, and therefore the molecular determinants for cargo selectivity and recognition, remain elusive. In this study, the first crystal structure of a MyoXI GTD, that of MyoXI-K from Arabidopsis thaliana, was elucidated at 2.35 Å resolution using a low-identity and fragment-based phasing approach in ARCIMBOLDO_SHREDDER. The results reveal that both the composition and the length of the α5-α6 loop are distinctive features of MyoXI-K, providing evidence for a structural stabilizing role for this loop, which is otherwise carried out by a molecular zipper in MyoV GTDs. The crystal structure also shows that most of the characterized cargo-binding sites in MyoVs are not conserved in plant MyoXIs, pointing to plant-specific cargo-recognition mechanisms. Notably, the main elements involved in the self-regulation mechanism of MyoVs are conserved in plant MyoXIs, indicating this to be an ancient ancestral trait.

Myosin V-mediated transport of Snc1 and Vps10 toward the trans-Golgi network

Retrieval of cargo proteins from the endosome towards the trans-Golgi network (TGN) is a crucial intracellular process for cellular homeostasis. Its dysfunction is associated with pathogenesis of Alzheimer and Parkinson's diseases. Myosin family proteins are cellular motors walking along actin filaments by utilizing the chemical energy from ATP hydrolysis, known to involve in pleiotropic cellular trafficking pathways. However, the question of whether myosins play a role in the trafficking of Snc1 and Vps10 has not been addressed yet. The present study assesses the potential roles of all five yeast myosins in the recycling of two membrane cargo, Snc1 and Vps10. It appears that all myosins except Myo2 are not required for the Snc1 traffic, while it was found that Myo1 and 2 play important roles for Vps10 retrieval from the endosome and the vacuole. Multiple myo2 mutants harboring a point mutation in the actin binding or the cargo binding tail domain were characterized to demonstrate abnormal Vps10-GFP and GFP-Snc1 distribution phenotypes, suggesting a severe defect in their sorting and trafficking at the endosome. Furthermore, Vps10-GFP patches in all tested myo2 mutants were found to be near stationary with quantitative live cell imaging. Finally, we found that actin cables in the myo2 mutant cells were considerably disrupted, which may aggravate the trafficking of Vps10 from the endosome. Together, our results provide novel insights into the function of Myo-family proteins in the recycling traffic of Vps10 and Snc1 destined for the TGN.

Structural mechanism for versatile cargo recognition by the yeast class V myosin Myo2

Class V myosins are actin-dependent motors, which recognize numerous cellular cargos mainly via the C-terminal globular tail domain (GTD). Myo2, a yeast class V myosin, can transport a broad range of organelles. However, little is known about the capacity of Myo2-GTD to recognize such a diverse array of cargos specifically at the molecular level. Here, we solved crystal structures of Myo2-GTD (at 1.9-3.1 Å resolutions) in complex with three cargo adaptor proteins: Smy1 (for polarization of secretory vesicles), Inp2 (for peroxisome transport), and Mmr1 (for mitochondria transport). The structures of Smy1- and Inp2-bound Myo2-GTD, along with site-directed mutagenesis experiments, revealed a binding site in subdomain-I having a hydrophobic groove with high flexibility enabling Myo2-GTD to accommodate different protein sequences. The Myo2-GTD-Mmr1 complex structure confirmed and complemented a previously identified mitochondrion/vacuole-specific binding region. Moreover, differences between the conformations and locations of cargo-binding sites identified here for Myo2 and those reported for mammalian MyoVA (MyoVA) suggest that class V myosins potentially have co-evolved with their specific cargos. Our structural and biochemical analysis not only uncovers a molecular mechanism that explains the diverse cargo recognition by Myo2-GTD, but also provides structural information useful for future functional studies of class V myosins in cargo transport.

Identification of the Isoform-specific Interactions between the Tail and the Head of Class V Myosin

Vertebrates have three isoforms of class V myosin (Myo5), Myo5a, Myo5b, and Myo5c, which are involved in transport of multiple cargoes. It is well established that the motor functions of Myo5a and Myo5b are regulated by a tail inhibition mechanism. Here we found that the motor function of Myo5c was also inhibited by its globular tail domain (GTD), and this inhibition was abolished by high Ca(2+), indicating that the tail inhibition mechanism is conserved in vertebrate Myo5. Interestingly, we found that Myo5a-GTD and Myo5c-GTD were not interchangeable in terms of inhibition of motor function, indicating isoform-specific interactions between the GTD and the head of Myo5. To identify the isoform-specific interactions, we produced a number of Myo5 chimeras by swapping the corresponding regions of Myo5a and Myo5c. We found that Myo5a-GTD, with its H11-H12 loop being substituted with that of Myo5c, was able to inhibit the ATPase activity of Myo5c and that Myo5a-GTD was able to inhibit the ATPase activity of Myo5c-S1 and Myo5c-HMM only when their IQ1 motif was substituted with that of Myo5a. Those results indicate that the H11-H12 loop in the GTD and the IQ1 motif in the head dictate the isoform-specific interactions between the GTD and head of Myo5. Because the IQ1 motif is wrapped by calmodulin, whose conformation is influenced by the sequence of the IQ1 motif, we proposed that the calmodulin bound to the IQ1 motif interacts with the H11-H12 loop of the GTD in the inhibited state of Myo5.

Processing-body movement in Arabidopsis depends on an interaction between myosins and DECAPPING PROTEIN1

Processing (P)-bodies are cytoplasmic RNA protein aggregates responsible for the storage, degradation, and quality control of translationally repressed messenger RNAs in eukaryotic cells. In mammals, P-body-related RNA and protein exchanges are actomyosin dependent, whereas P-body movement requires intact microtubules. In contrast, in plants, P-body motility is actin based. In this study, we show the direct interaction of the P-body core component DECAPPING PROTEIN1 (DCP1) with the tails of different unconventional myosins in Arabidopsis (Arabidopsis thaliana). By performing coexpression studies with AtDCP1, dominant-negative myosin fragments, as well as functional full-length myosin XI-K, the association of P-bodies and myosins was analyzed in detail. Finally, the combination of mutant analyses and characterization of P-body movement patterns showed that myosin XI-K is essential for fast and directed P-body transport. Together, our data indicate that P-body movement in plants is governed by myosin XI members through direct binding to AtDCP1 rather than through an adapter protein, as known for membrane-coated organelles. Interspecies and intraspecies interaction approaches with mammalian and yeast protein homologs suggest that this mechanism is evolutionarily conserved among eukaryotes.

Structural insights into functional overlapping and differentiation among myosin V motors

Myosin V (MyoV) motors have been implicated in the intracellular transport of diverse cargoes including vesicles, organelles, RNA-protein complexes, and regulatory proteins. Here, we have solved the cargo-binding domain (CBD) structures of the three human MyoV paralogs (Va, Vb, and Vc), revealing subtle structural changes that drive functional differentiation and a novel redox mechanism controlling the CBD dimerization process, which is unique for the MyoVc subclass. Moreover, the cargo- and motor-binding sites were structurally assigned, indicating the conservation of residues involved in the recognition of adaptors for peroxisome transport and providing high resolution insights into motor domain inhibition by CBD. These results contribute to understanding the structural requirements for cargo transport, autoinhibition, and regulatory mechanisms in myosin V motors.

Myosin XIK is a major player in cytoplasm dynamics and is regulated by two amino acids in its tail

It has recently been found that among the 17 Arabidopsis myosins, six (XIC, XIE, XIK, XI-I, MYA1, and MYA2) have a major role in the motility of Golgi bodies and mitochondria in Nicotiana benthamiana and Nicotiana tabacum. Here, the same dominant negative tail fragments were also found to arrest the movement of Gogi bodies when transiently expressed in Arabidopsis plants. However, when a Golgi marker was transiently expressed in plants knocked out in these myosins, its movement was dramatically inhibited only in the xik mutant. In addition, a tail fragment of myosin XIK could inhibit the movement of several post-Golgi organelles, such as the trans-Golgi network, pre-vacuolar compartment, and endosomes, as well as total cytoplasmic streaming, suggesting that myosin XIK is a major player in cytoplasm kinetics. However, no co-localization of myosin tails with the arrested organelles was observed. Several deletion truncations of the myosin XIK tail were generated to corroborate function with localization. All deletion mutants possessing an inhibitory effect on organelle movement exhibited a diffuse cytoplasmic distribution. Point mutations in the tail of myosin XIK revealed that Arg1368 and Arg1443 are essential for its activity. These residues correspond to Lys1706 and Lys1779 from mouse myosin Va, which mediate the inhibitory head-tail interaction in this myosin. Therefore, such an interaction might underlie the dominant negative effect of truncated plant myosin tails and explain the mislocalization with target organelles.

The myosin-related motor protein Myo2 is an essential mediator of bud-directed mitochondrial movement in yeast

The myosin-related motor protein Myo2 collaborates with the rab-GTPase Ypt11 to traffic mitochondria to the yeast bud during cell division.

The inheritance of mitochondria in yeast depends on bud-directed transport along actin filaments. It is a matter of debate whether anterograde mitochondrial movement is mediated by the myosin-related motor protein Myo2 or by motor-independent mechanisms. We show that mutations in the Myo2 cargo binding domain impair entry of mitochondria into the bud and are synthetically lethal with deletion of the YPT11 gene encoding a rab-type guanosine triphosphatase. Mitochondrial distribution defects and synthetic lethality were rescued by a mitochondria-specific Myo2 variant that carries a mitochondrial outer membrane anchor. Furthermore, immunoelectron microscopy revealed Myo2 on isolated mitochondria. Thus, Myo2 is an essential and direct mediator of bud-directed mitochondrial movement in yeast. Accumulating genetic evidence suggests that maintenance of mitochondrial morphology, Ypt11, and retention of mitochondria in the bud contribute to Myo2-dependent inheritance of mitochondria.

Whole-genome SNP association in the horse: identification of a deletion in myosin Va responsible for Lavender Foal Syndrome

Lavender Foal Syndrome (LFS) is a lethal inherited disease of horses with a suspected autosomal recessive mode of inheritance. LFS has been primarily diagnosed in a subgroup of the Arabian breed, the Egyptian Arabian horse. The condition is characterized by multiple neurological abnormalities and a dilute coat color. Candidate genes based on comparative phenotypes in mice and humans include the ras-associated protein RAB27a (RAB27A) and myosin Va (MYO5A). Here we report mapping of the locus responsible for LFS using a small set of 36 horses segregating for LFS. These horses were genotyped using a newly available single nucleotide polymorphism (SNP) chip containing 56,402 discriminatory elements. The whole genome scan identified an associated region containing these two functional candidate genes. Exon sequencing of the MYO5A gene from an affected foal revealed a single base deletion in exon 30 that changes the reading frame and introduces a premature stop codon. A PCR–based Restriction Fragment Length Polymorphism (PCR–RFLP) assay was designed and used to investigate the frequency of the mutant gene. All affected horses tested were homozygous for this mutation. Heterozygous carriers were detected in high frequency in families segregating for this trait, and the frequency of carriers in unrelated Egyptian Arabians was 10.3%. The mapping and discovery of the LFS mutation represents the first successful use of whole-genome SNP scanning in the horse for any trait. The RFLP assay can be used to assist breeders in avoiding carrier-to-carrier matings and thus in preventing the birth of affected foals.

Genetic disorders affect many domesticated species, including the horse. In this study we have focused on Lavender Foal Syndrome, a seizure disorder that leads to suffering and death in foals soon after birth. A recessively inherited disorder, its occurrence is often unpredictable and difficult for horse breeders to avoid without a diagnostic test for carrier status. The recent completion of the horse genome sequence has provided new tools for mapping traits with unprecedented resolution and power. We have applied one such tool, the Equine SNP50 genotyping chip, to a small sample set from horses affected with Lavender Foal Syndrome. A single genetic location associated with the disorder was rapidly identified using this approach. Subsequent sequencing of functional candidate genes in this location revealed a single base deletion that likely causes Lavender Foal Syndrome. From a practical standpoint, this discovery and the development of a diagnostic test for the LFS allele provides a valuable new tool for breeders seeking to avoid the disease in their foal crop. However, this work also illustrates the utility of whole-genome association studies in the horse.

A myosin XI tail domain homologous to the yeast myosin vacuole-binding domain interacts with plastids and stromules in Nicotiana benthamiana

The actin cytoskeleton plays a role in mobility of many different organelles in plant cells, including chloroplasts, mitochondria, Golgi, and peroxisomes. While progress has been made in identifying the myosin motors involved in trafficking of various plant organelles, not all of the cargoes mobilized by different members of the myosin XI family have yet been identified. The involvement of myosins in chloroplast positioning and mitochondrial movement was demonstrated by expression of a virus-induced gene silencing (VIGS) construct in tobacco. When VIGS with two different conserved sequences from a myosin XI motor was performed in plants with either GFP-labeled plastids or mitochondria, chloroplast positioning in the dark was abnormal, and mitochondrial movement ceased. Because these and prior observations have implicated a role for myosins and the actin cytoskeleton in plastid and stromule movement, we searched for myosin tail domains that could associate with plastids and stromules. While a yellow fluorescent protein (YFP) fusion with the entire tail region of myosin XI-F was usually found only in the cytoplasm, we observed that an Arabidopsis or Nicotiana benthamiana YFP::myosin XI-F tail domain homologous to the yeast myo2p vacuole-binding domain associated with plastids and stromules after transient expression in N. benthamiana. Taken together, these observations implicate myosin motor proteins in dynamics of plastids and stromules.

Myosin-driven peroxisome partitioning in S. cerevisiae

In Saccharomyces cerevisiae, the class V myosin motor Myo2p propels the movement of most organelles. We recently identified Inp2p as the peroxisome-specific receptor for Myo2p. In this study, we delineate the region of Myo2p devoted to binding peroxisomes. Using mutants of Myo2p specifically impaired in peroxisome binding, we dissect cell cycle–dependent and peroxisome partitioning–dependent mechanisms of Inp2p regulation. We find that although total Inp2p levels oscillate with the cell cycle, Inp2p levels on individual peroxisomes are controlled by peroxisome inheritance, as Inp2p aberrantly accumulates and decorates all peroxisomes in mother cells when peroxisome partitioning is abolished. We also find that Inp2p is a phosphoprotein whose level of phosphorylation is coupled to the cell cycle irrespective of peroxisome positioning in the cell. Our findings demonstrate that both organelle positioning and cell cycle progression control the levels of organelle-specific receptors for molecular motors to ultimately achieve an equidistribution of compartments between mother and daughter cells.

Systematic definition of protein constituents along the major polarization axis reveals an adaptive reuse of the polarization machinery in pheromone-treated budding yeast

Polarizing cells extensively restructure cellular components in a spatially and temporally coupled manner along the major axis of cellular extension. Budding yeast are a useful model of polarized growth, helping to define many molecular components of this conserved process. Besides budding, yeast cells also differentiate upon treatment with pheromone from the opposite mating type, forming a mating projection (the ‘shmoo’) by directional restructuring of the cytoskeleton, localized vesicular transport and overall reorganization of the cytosol. To characterize the proteomic localization changes accompanying polarized growth, we developed and implemented a novel cell microarray-based imaging assay for measuring the spatial redistribution of a large fraction of the yeast proteome, and applied this assay to identify proteins localized along the mating projection following pheromone treatment. We further trained a machine learning algorithm to refine the cell imaging screen, identifying additional shmoo-localized proteins. In all, we identified 74 proteins that specifically localize to the mating projection, including previously uncharacterized proteins (Ycr043c, Ydr348c, Yer071c, Ymr295c, and Yor304c-a) and known polarization complexes such as the exocyst. Functional analysis of these proteins, coupled with quantitative analysis of individual organelle movements during shmoo formation, suggests a model in which the basic machinery for cell polarization is generally conserved between processes forming the bud and the shmoo, with a distinct subset of proteins used only for shmoo formation. The net effect is a defined ordering of major organelles along the polarization axis, with specific proteins implicated at the proximal growth tip.

Upon sensing mating pheromone, budding yeast cells form a mating projection (the ‘shmoo’) that serves as a model for polarized cell growth, involving cytoskeletal/cytosolic restructuring and directed vesicular transport. We developed a cell microarray-based imaging assay for measuring localization of the yeast proteome during polarized growth. We find major organelles ordered along the polarization axis, localize 74 proteins to the growth tip, and observe adaptive reuse of general polarization machinery.

Tropomyosin function in yeast

Tropomyosins were discovered as regulators of actomyosin contractility in muscle cells, making yeasts and other fungi seem unlikely to harbor such proteins. Fungal cells are encased in a rigid cell wall and do not engage in the same sorts of contractile shape changes of animal cells. However, discovery of actin and myosin in yeast raised the possibility for a role for tropomyosin in regulating their interaction. Through a biochemical search, fungal tropomyosins were identified with strong similarities to their animal counterparts in terms ofprotein structure and physical properties. Two particular fungi, the buddingyeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe, have provided powerful genetic systems for studying tropomyosins in nonmetazoans. In these yeasts, tropomyosins associate with subsets ofactin filamentous structures. Mutational studies oftropomyosin genes and biochemical assays of purified proteins point to roles for these proteins as factors that stabilize actin filaments, promote actin-based structures of particular architecture and help maintain distinct biochemical identities among different filament populations. Tropomyosin-enriched filaments are the cytoskeletal structures that promote the major cell shape changes of these organisms: polarized growth and cell division.

Cell and molecular biology of the fastest myosins

Chara myosin is a class XI plant myosin in green algae Chara corallina and responsible for fast cytoplasmic streaming. The Chara myosin exhibits the fastest sliding movement of F-actin at 60 mum/s as observed so far, 10-fold of the shortening speed of muscle. It has some distinct properties differing from those of muscle myosin. Although knowledge about Chara myosin is very limited at present, we have tried to elucidate functional bases of its characteristics by comparing with those of other myosins. In particular, we have built the putative atomic model of Chara myosin by using the homology-based modeling system and databases. Based on the putative structure of Chara myosin obtained, we have analyzed the relationship between structure and function of Chara myosin to understand its distinct properties from various aspects by referring to the accumulated knowledge on mechanochemical and structural properties of other classes of myosin, particularly animal and fungal myosin V. We will also discuss the functional significance of Chara myosin in a living cell.

Myosin V from head to tail

Myosin V (myoV), a processive cargo transporter, has arguably been the most well-studied unconventional myosin of the past decade. Considerable structural information is available for the motor domain, the IQ motifs with bound calmodulin or light chains, and the cargo-binding globular tail, all of which have been crystallized. The repertoire of adapter proteins that link myoV to a particular cargo is becoming better understood, enabling cellular transport processes to be dissected. MyoV is processive, meaning that it takes many steps on actin filaments without dissociating. Its extended lever arm results in long 36-nm steps, making it ideal for single molecule studies of processive movement. In addition, electron microscopy revealed the structure of the inactive, folded conformation of myoV when it is not transporting cargo. This review provides a background on myoV, and highlights recent discoveries that show why myoV will continue to be an active focus of investigation.

The class V myosin motor protein, Myo2, plays a major role in mitochondrial motility in Saccharomyces cerevisiae

The actin cytoskeleton is essential for polarized, bud-directed movement of cellular membranes in Saccharomyces cerevisiae and thus ensures accurate inheritance of organelles during cell division. Also, mitochondrial distribution and inheritance depend on the actin cytoskeleton, though the precise molecular mechanisms are unknown. Here, we establish the class V myosin motor protein, Myo2, as an important mediator of mitochondrial motility in budding yeast. We found that mutants with abnormal expression levels of Myo2 or its associated light chain, Mlc1, exhibit aberrant mitochondrial morphology and loss of mitochondrial DNA. Specific mutations in the globular tail of Myo2 lead to aggregation of mitochondria in the mother cell. Isolated mitochondria lacking functional Myo2 are severely impaired in their capacity to bind to actin filaments in vitro. Time-resolved fluorescence microscopy revealed a block of bud-directed anterograde mitochondrial movement in cargo binding–defective myo2 mutant cells. We conclude that Myo2 plays an important and direct role for mitochondrial motility and inheritance in budding yeast.

Pushing for answers: is myosin V directly involved in moving mitochondria?

In budding yeast, the actin-based class V myosin motors, Myo2 and Myo4, transport virtually all organelles from mother to bud during cell division. Until recently, it appeared that mitochondria may be an exception, with studies showing that the Arp2/3 complex is required for their movement. However, several recent studies have proposed that Myo2 has a direct involvement in mitochondria inheritance. In this issue, Altmann et al. (Altmann, K., M. Frank, D. Neumann, S. Jakobs, and B. Westermann. 2008. J. Cell Biol. 181:119–130) provide the strongest support yet that Myo2 and its associated light chain Mlc1 function directly and significantly in both mitochondria–actin interactions and in the movement of mitochondria from mother to bud. The conflicting functions of Arp 2/3 and Myo2 may be reconciled by the existence of multiple pathways involved in mitochondrial transport.

Ypt32p and Mlc1p bind within the vesicle binding region of the class V myosin Myo2p globular tail domain

Myosin V is an actin-based motor essential for a variety of cellular processes including skin pigmentation, cell separation and synaptic transmission. Myosin V transports organelles, vesicles and mRNA by binding, directly or indirectly, to cargo-bound receptors via its C-terminal globular tail domain (GTD). We have used the budding yeast myosin V Myo2p to shed light on the mechanism of how Myo2p interacts with post-Golgi carriers. We show that the Rab/Ypt protein Ypt32p, which associates with membranes of the trans-Golgi network, secretory vesicles and endosomes and is related to the mammalian Rab11, interacts with the Myo2p GTD within a region previously identified as the 'vesicle binding region'. Furthermore, we show that the essential myosin light chain 1 (Mlc1p), required for vesicle delivery at the mother-bud neck during cytokinesis, binds to the Myo2p GTD in a region overlapping that of Ypt32p. Our data are consistent with a role of Ypt32p and Mlc1p in regulating the interaction of post-Golgi carriers with Myo2p subdomain II.

The tail that wags the dog: the globular tail domain defines the function of myosin V/XI

Actin-based organelle movements are driven by the related multifunctional myosin motors of class V in animals and fungi and class XI in plants. The versatility of these motors depends critically on their C-terminal globular tail domain that allows them to bind to a broad variety of cargo molecules. Regulation of this motor-cargo attachment is frequently employed to modulate organelle movement. While the overall structure of the cargo-binding globular tail appears to be conserved between myosin V and XI, it has become apparent that the motor-cargo interactions differ widely even within a single organism and involve protein complexes with different architecture and completely unrelated protein domains. At the same time, indirect evidence suggests that adaptor or receptor dimerization could facilitate efficient myosin capture. Comparison of myosin V and XI across the large evolutionary distance between animals and plants will likely reveal more fundamental insights into these important motors.

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.

Acto-Myosin motorises the flow of auxin

Auxin has always been a hormone of intensive investigation because of its crucial roles in plant growth. With the recent confirmation of PIN1 and AUX1 as proteins specifically controlling auxin distribution, the ongoing debate about possible mechanisms in polar auxin transport (PAT) made further progress. Another new set of data suggested vesicle-based processes to have meaningful roles in auxin pathways. This view got additional support by the finding that cytoplasmic streaming as well as PAT decreased under inhibition of actin or myosin. Inhibitory effects on both events were also observed when actin-binding proteins (ABPs) were overexpressed for fluorescent actin-labelling. Here, the block of motility was reasoned by myosins and ABPs competing for F-actin binding. In summary, the observations constantly show that acto-myosin controls the motility of vesicles and in parallel, is essential for directional auxin flow. A direct functional link between PAT and vesicles can not be concluded from these experiments, but might be either understood as the shuttling of components of the auxin-efflux machinery by myosin-guided vesicles, or, in the more unusual meaning of PAT by secretion, as the delivery of auxin in membraneous compartments moved by myosin.

Regulating cytoskeleton-based vesicle motility

During vesicular transport, the assembly of the coat complexes and the selection of cargo proteins must be coordinated with the subsequent translocation of vesicles from the donor to an acceptor compartment. Here, we review recent progress toward uncovering the molecular mechanisms that connect transport vesicles to the protein machinery responsible for cytoskeleton-mediated motility. An emerging theme is that vesicle cargo proteins, either directly or through binding interactions with coat proteins, are able to influence cytoskeletal dynamics and motor protein function. Hence, a vesicle's cargo composition may help direct its intracellular motility and targeting.

Organelle targeting of myosin XI is mediated by two globular tail subdomains with separate cargo binding sites

Myosin XI are actin-based molecular motors that are thought to drive organelle movements in plants, analogous to myosin V in animals and fungi. Similar domain structure of these myosins suggests that binding to organelles may occur via the globular tail domain in both types of motors, even though sequence similarity is low. To address this hypothesis, we developed a structure homology model for the globular tail of MYA1, a myosin XI from Arabidopsis, based on the known structure of yeast myosin V (Myo2p) globular tail. This model suggested an interaction between two subdomains of the globular tail which was verified by yeast two-hybrid assay and by in vivo bimolecular fluorescence complementation (BiFC). Interface mapping demonstrated that this subdomain interaction depends critically on the C terminus of helix H6 as well as three specific residues in helices H3 and H15, consistent with the structural prediction. The reconstituted globular tails of several Arabidopsis myosin XIs in BiFC assays targeted to peroxisomes in plant cells, identifying this domain as sufficient for cargo binding. Unlike myosin V, either subdomain of myosin XI alone was targeting-competent and responsible for association with different organelles. In addition, our data suggest that organelle binding is regulated by an allosteric interaction between two tail subdomains. We conclude that the globular tail of myosin XI shares a similar structure with that of myosin V, but has evolved plant-specific cargo binding mechanisms.

Differential organelle movement on the actin cytoskeleton in lily pollen tubes

We have examined the arrangement and movement of three major compartments, the endoplasmic reticulum (ER), mitochondria, and the vacuole during oscillatory, polarized growth in lily pollen tubes. These movements are dependent on the actin cytoskeleton, because they are strongly perturbed by the anti-microfilament drug, latrunculin-B, and unaffected by the anti-microtubule agent, oryzalin. The ER, which has been labeled with mGFP5-HDEL or cytochalasin D tetramethylrhodamine, displays an oscillatory motion in the pollen tube apex. First it moves apically in the cortical region, presumably along the cortical actin fringe, and then periodically folds inward creating a platform that transects the apical domain in a plate-like structure. Finally, the ER reverses its direction and moves basipetally through the central core of the pollen tube. When subjected to cross-correlation analysis, the formation of the platform precedes maximal growth rates by an average of 3 s (35-40 degrees ). Mitochondria, labeled with Mitotracker Green, are enriched in the subapical region, and their movement closely resembles that of the ER. The vacuole, labeled with carboxy-dichlorofluorescein diacetate, consists of thin tubules arranged longitudinally in a reticulate network, which undergoes active motion. In contrast to the mitochondria and ER, the vacuole is located back from the apex, and never extends into the apical clear zone. We have not been able to decipher an oscillatory pattern in vacuole motion. Because this motion is dependent on actin and not tubulin, we think this is due to a different myosin from that which drives the ER and mitochondria.

Preparing the way: fungal motors in microtubule organization

Fungal growth, development and pathogenicity require hyphal tip growth, which is supported by polar exocytosis at the expanding growth region. It is assumed that molecular motors transport growth supplies along the fibrous elements of the cytoskeleton, such as microtubules, to the hyphal apex. Recent advances in live-cell imaging of fungi revealed additional roles for motors in organizing their own tracks. These unexpected roles of the molecular motors are modifying microtubule dynamics directly, targeting stability-determining factors to microtubule plus ends, and transporting and arranging already-assembled microtubules.

Organelles on the move: insights from yeast vacuole inheritance

Organelle inheritance is one of several processes that occur during cell division. Recent studies on yeast vacuole inheritance have indicated rules that probably apply to most organelle-inheritance pathways. They have uncovered a molecular mechanism for membrane-cargo transport that is partially conserved from yeast to humans. They have also shown that the transport complex, which is composed of a molecular motor and its receptor, regulates the destination and timing of vacuole movement and might coordinate organelle movement with several other organelle functions.

A critical role for the type V myosin, Myo52, in septum deposition and cell fission during cytokinesis in Schizosaccharomyces pombe

Cytokinesis in fission yeast involves the coordination of septum deposition with the contraction of a cytokinetic actomyosin ring. We have examined the role of the type V myosin Myo52 in the coupling of these two events by the construction of a series of deletion mutants of the Myo52 tail and a further mutant within the ATP binding domain of the head. Each mutant protein was ectopically expressed in fission yeast cells. Each truncation was assayed for the ability to localize to the cell poles and septum (the normal cellular locations of Myo52) and to rescue the morphology defects and temperature sensitivity of a myo52Delta strain. A region within the Myo52 tail (amino acids 1320-1503), with a high degree of similarity to the vesicle-binding domain of the budding yeast type V myosin Myo2p, was essential for Myo52's role in the maintenance of growth polarity and cell division. A separate region (amino acids 1180-1320) was required for Myo52 foci to move throughout the cytoplasm; however, constructs lacking this region, but which retained the ability to dimerize still associated with actin at sites of cell growth. Not all of the Myo52 truncations which localized rescued the morphological defects of myo52Delta, demonstrating that loss of function was not simply brought about by an inability of mutant proteins to target the correct cellular location. By contrast, Myo52 motor activity was required for both localization and cellular function. myo52Delta cells were unable to efficiently localize the beta-1,3-glucan synthase, Bgs1, either at the cell poles or at the division septum, regions of cell wall deposition. Bgs1 and Myo52 localized to vesicle-like dots at the poles in interphase and these moved together to the septum at division. These data have led to the formulation of a model in which Myo52 is responsible for the delivery of Bgs1 and associated molecules to polar cell growth regions during interphase. On the commencement of septum formation, Myo52 transports Bgs1 to the cell equator, thus ensuring the accurate deposition of beta-1,3-glucan at the leading edge of the primary septum.

Regulated phosphorylation of budding yeast's essential myosin V heavy chain, Myo2p

The tail of the yeast myosin V encoded by Myo2p is known to bind several receptors for cargo delivery along polarized actin cables. However, it is not known how Myo2p activity is regulated or how it selects between cargoes. Here we show that Myo2p is reversibly phosphorylated in vivo. A short peptide at the N-terminal end of the cargo-binding domain contains three residues contributing to single or doubly phosphorylated species. We confirm that the tail consists of two proteolytically resistant subdomains and identify a functionally important region N-terminal to subdomain 1 that includes the phosphorylation sites. Mutagenesis of the phosphorylation sites to alanine abolished a mobility shift diagnostic of phosphorylation, whereas mutagenesis to glutamic acid produced the shift and the formation of an additional phosphorylated species. These substitutions did not affect overall cell growth. However, one of the sites is predicted to be a substrate of cAMP-dependent protein kinase (PKA), and yeast expressing Myo2p with alanine substitutions is resistant to otherwise lethal overexpression of PKA, whereas the glutamic acid mutant is supersensitive to overexpression of PKA. These results suggest that in yeast, Myo2p is subject to phosphoregulation involving a PKA-related signaling pathway.

Structural basis for myosin V discrimination between distinct cargoes

Myosin V molecular motors move cargoes on actin filaments. A myosin V may move multiple cargoes to distinct places at different times. The cargoes attach to the globular tail of myosin V via cargo-specific receptors. Here we report the crystal structure at 2.2 A of the myosin V globular tail. The overall tertiary structure has not been previously observed. There are several patches of highly conserved regions distributed on the surface of the tail. These are candidate attachment sites for cargo-specific receptors. Indeed, we identified a region of five conserved surface residues that are solely required for vacuole inheritance. Likewise, we identified a region of five conserved surface residues that are required for secretory vesicle movement, but not vacuole movement. These two regions are at opposite ends of the oblong-shaped cargo-binding domain, and moreover are offset by 180 degrees. The fact that the cargo-binding areas are distant from each other and simultaneously exposed on the surface of the globular tail suggests that major targets for the regulation of cargo attachment are organelle-specific myosin V receptors.

A point mutation in the cargo-binding domain of myosin V affects its interaction with multiple cargoes

Class V myosins move diverse intracellular cargoes, which attach via interaction of cargo-specific proteins to the myosin V globular tail. The globular tail of the yeast myosin V, Myo2p, contains two structural and functional subdomains. Subdomain I binds to the vacuole-specific protein, Vac17p, while subdomain II likely binds to an as yet unidentified secretory vesicle-specific protein. All functions of Myo2p require the tight association of subdomains I and II, which suggests that binding of a cargo to one subdomain may inhibit cargo-binding to a second subdomain. Thus, two types of mutations are predicted to specifically affect a subset of Myo2p cargoes: first are mutations within a cargo-specific binding region; second are mutations that mimic the inhibited conformation of one of the subdomains. Here we analyze a point mutation in subdomain I, myo2-2(G1248D), which is likely to be this latter type of mutation. myo2-2 has no effect on secretory vesicle movement. The secretory vesicle binding site is in subdomain II. However, myo2-2 is impaired in several Myo2p-related functions. While subdomains I and II of myo2-2p tightly associate, there are measurable differences in the conformation of its globular tail. Based solely on the ability to restore vacuole inheritance, a set of intragenic suppressors of myo2-2 were identified. All suppressor mutations reside in subdomain I. Moreover, subdomain I and II interactions occurred in all suppressors, demonstrating the importance of subdomain I and II association for Myo2p function. Furthermore, 3 of the 10 suppressors globally restored all tested defects in myo2-2. This large proportion of global suppressors strongly suggests that myo2-2(G1248) causes a conformational change in subdomain I that simultaneously affects multiple cargoes.