Localization and specificity of the phospholipid and actin binding sites on the tail of Acanthamoeba myosin IC

Myosin-I synergizes with Arp2/3 complex to enhance the pushing forces of branched actin networks

Class I myosins (myosin-Is) colocalize with Arp2/3 complex-nucleated actin networks at sites of membrane protrusion and invagination, but the mechanisms by which myosin-I motor activity coordinates with branched actin assembly to generate force are unknown. We mimicked the interplay of these proteins using the "comet tail" bead motility assay, where branched actin networks are nucleated by the Arp2/3 complex on the surface of beads coated with myosin-I and nucleation-promoting factor. We observed that myosin-I increased bead movement efficiency by thinning actin networks without affecting growth rates. Myosin-I triggered symmetry breaking and comet tail formation in dense networks resistant to spontaneous fracturing. Even with arrested actin assembly, myosin-I alone could break the network. Computational modeling recapitulated these observations, suggesting myosin-I acts as a repulsive force shaping the network's architecture and boosting its force-generating capacity. We propose that myosin-I leverages its power stroke to amplify the forces generated by Arp2/3 complex-nucleated actin networks.

Discovery of the first unconventional myosin: Acanthamoeba myosin-I

Having characterized actin from Acanthamoeba castellanii (Weihing and Korn, Biochemistry, 1971, 10, 590-600) and knowing that myosin had been isolated from the slime mold Physarum (Hatano and Tazawa, Biochim. Biophys. Acta, 1968, 154, 507-519; Adelman and Taylor, Biochemistry, 1969, 8, 4976-4988), we set out in 1969 to find myosin in Acanthamoeba. We used K-EDTA-ATPase activity to assay myosin, because it is a unique feature of muscle myosins. After slightly less than 3 years, we purified a K-EDTA ATPase that interacted with actin. Actin filaments stimulated the Mg-ATPase activity of the crude enzyme, but this was lost with further purification. Recombining fractions from the column where this activity was lost revealed a "cofactor" that allowed actin filaments to stimulate the Mg-ATPase of the purified enzyme. The small size of the heavy chain and physical properties of the purified myosin were unprecedented, so many were skeptical, assuming that our myosin was a proteolytic fragment of a larger myosin similar to muscle or Physarum myosin. Subsequently our laboratories confirmed that Acanthamoeba myosin-I is a novel unconventional myosin that interacts with membrane lipids (Adams and Pollard, Nature, 1989, 340 (6234), 565-568) and that the cofactor is a myosin heavy chain kinase (Maruta and Korn, J. Biol. Chem., 1977, 252, 8329-8332). Phylogenetic analysis (Odronitz and Kollmar, Genome Biology, 2007, 8, R196) later established that class I myosin was the first myosin to appear during the evolution of eukaryotes.

Myosins, an Underestimated Player in the Infectious Cycle of Pathogenic Bacteria

Myosins play a key role in many cellular processes such as cell migration, adhesion, intracellular trafficking and internalization processes, making them ideal targets for bacteria. Through selected examples, such as enteropathogenic E. coli (EPEC), Neisseria, Salmonella, Shigella, Listeria or Chlamydia, this review aims to illustrate how bacteria target and hijack host cell myosins in order to adhere to the cell, to enter the cell by triggering their internalization, to evade from the cytosolic autonomous cell defense, to promote the biogenesis of intracellular replicative niche, to disseminate in tissues by cell-to-cell spreading, to exit out the host cell, and also to evade from macrophage phagocytosis. It highlights the diversity and sophistication of the strategy evolved by bacteria to manipulate one of their privileged targets, the actin cytoskeleton.

Roles of Myosin-Mediated Membrane Trafficking in TGF-β Signaling

Recent findings have revealed the role of membrane traffic in the signaling of transforming growth factor-β (TGF-β). These findings originate from the pivotal function of TGF-β in development, cell proliferation, tumor metastasis, and many other processes essential in malignancy. Actin and unconventional myosin have crucial roles in subcellular trafficking of receptors; research has also revealed a growing number of unconventional myosins that have crucial roles in TGF-β signaling. Unconventional myosins modulate the spatial organization of endocytic trafficking and tether membranes or transport them along the actin cytoskeletons. Current models do not fully explain how membrane traffic forms a bridge between TGF-β and the downstream effectors that produce its functional responsiveness, such as cell migration. In this review, we present a brief overview of the current knowledge of the TGF-β signaling pathway and the molecular components that comprise the core pathway as follows: ligands, receptors, and Smad mediators. Second, we highlight key role(s) of myosin motor-mediated protein trafficking and membrane domain segregation in the modulation of the TGF-β signaling pathway. Finally, we review future challenges and provide future prospects in this field.

Cell Motility and Cytokinesis: From Mysteries to Molecular Mechanisms in Five Decades

This is the story of someone who has been fortunate to work in a field of research where essentially nothing was known at the outset but that blossomed with the discovery of profound insights about two basic biological processes: cell motility and cytokinesis. The field started with no molecules, just a few people, and primitive methods. Over time, technological advances in biophysics, biochemistry, and microscopy allowed the combined efforts of scientists in hundreds of laboratories to explain mysterious processes with molecular mechanisms that can be embodied in mathematical equations and simulated by computers. The success of this field is a tribute to the power of the reductionist strategy for understanding biology.

ERK signaling promotes cell motility by inducing the localization of myosin 1E to lamellipodial tips

Tanimura et al. demonstrate that SH3P2 binds to and functions as a cytosolic anchor for myosin 1E (Myo1E). ERK signaling–dependent phosphorylation of SH3P2 induces the dissociation of bound Myo1E and its consequent localization to the tips of lamellipodia, where it promotes cell motility.

Signaling by extracellular signal–regulated kinase (ERK) plays an essential role in the induction of cell motility, but the precise mechanism underlying such regulation has remained elusive. We recently identified SH3P2 as a negative regulator of cell motility whose function is inhibited by p90 ribosomal S6 kinase (RSK)–mediated phosphorylation downstream of ERK. We here show that myosin 1E (Myo1E) is a binding partner of SH3P2 and that the interaction of the two proteins in the cytosol prevents the localization of Myo1E to the plasma membrane. Serum-induced phosphorylation of SH3P2 at Ser²⁰² by RSK results in dissociation of Myo1E from SH3P2 in the cytosol and the subsequent localization of Myo1E to the tips of lamellipodia mediated by binding of its TH2 domain to F-actin. This translocation of Myo1E is essential for lamellipodium extension and consequent cell migration. The ERK signaling pathway thus promotes cell motility through regulation of the subcellular localization of Myo1E.

Structure of the Single-lobe Myosin Light Chain C in Complex with the Light Chain-binding Domains of Myosin-1C Provides Insights into Divergent IQ Motif Recognition

Myosin light chains are key regulators of class 1 myosins and typically comprise two domains, with calmodulin being the archetypal example. They bind IQ motifs within the myosin neck region and amplify conformational changes in the motor domain. A single lobe light chain, myosin light chain C (MlcC), was recently identified and shown to specifically bind to two sequentially divergent IQ motifs of the Dictyostelium myosin-1C. To provide a molecular basis of this interaction, the structures of apo-MlcC and a 2:1 MlcC·myosin-1C neck complex were determined. The two non-functional EF-hand motifs of MlcC pack together to form a globular four-helix bundle that opens up to expose a central hydrophobic groove, which interacts with the N-terminal portion of the divergent IQ1 and IQ2 motifs. The N- and C-terminal regions of MlcC make critical contacts that contribute to its specific interactions with the myosin-1C divergent IQ motifs, which are contacts that deviate from the traditional mode of calmodulin-IQ recognition.

Myosin-I molecular motors at a glance

Myosin-I molecular motors are proposed to play various cellular roles related to membrane dynamics and trafficking. In this Cell Science at a Glance article and the accompanying poster, we review and illustrate the proposed cellular functions of metazoan myosin-I molecular motors by examining the structural, biochemical, mechanical and cell biological evidence for their proposed molecular roles. We highlight evidence for the roles of myosin-I isoforms in regulating membrane tension and actin architecture, powering plasma membrane and organelle deformation, participating in membrane trafficking, and functioning as a tension-sensitive dock or tether. Collectively, myosin-I motors have been implicated in increasingly complex cellular phenomena, yet how a single isoform accomplishes multiple types of molecular functions is still an active area of investigation. To fully understand the underlying physiology, it is now essential to piece together different approaches of biological investigation. This article will appeal to investigators who study immunology, metabolic diseases, endosomal trafficking, cell motility, cancer and kidney disease, and to those who are interested in how cellular membranes are coupled to the underlying actin cytoskeleton in a variety of different applications.

In situ quantification of protein binding to the plasma membrane

This study presents a fluorescence-based assay that allows for direct measurement of protein binding to the plasma membrane inside living cells. An axial scan through the cell generates a fluorescence intensity profile that is analyzed to determine the membrane-bound and cytoplasmic concentrations of a peripheral membrane protein labeled by the enhanced green fluorescent protein (EGFP). The membrane binding curve is constructed by mapping those concentrations for a population of cells with a wide range of protein expression levels, and a fit of the binding curve determines the number of binding sites and the dissociation coefficient. We experimentally verified the technique, using myosin-1C-EGFP as a model system and fit its binding curve. Furthermore, we studied the protein-lipid interactions of the membrane binding domains from lactadherin and phospholipase C-δ1 to evaluate the feasibility of using competition binding experiments to identify specific lipid-protein interactions in living cells. Finally, we applied the technique to determine the lipid specificity, the number of binding sites, and the dissociation coefficient of membrane binding for the Gag matrix domain of human T-lymphotropic virus type 1, which provides insight into early assembly steps of the retrovirus.

Class I myosins in B-cell physiology: functions in spreading, immune synapses, motility, and vesicular traffic

Myosins comprise a family of motor proteins whose role in muscle contraction and motility in a large range of eukaryotic cells has been widely studied. Although these proteins have been characterized extensively and much is known about their function in different cellular compartments, little is known about these molecules in hematopoietic cells. Myosins expressed by cells from the immune response are involved in maintaining plasma membrane tension, moving and secreting vesicles, endo- and exocytotic processes, and promoting the adhesion and motility of cells. Herein, we summarize our current understanding of class I myosins in B cells, with an emphasis on the emerging roles of these molecular motors in immune functions.

Myosin 1g regulates cytoskeleton plasticity, cell migration, exocytosis, and endocytosis in B lymphocytes

Myosin 1g (Myo1g) is a hematopoietic-specific myosin expressed mainly by lymphocytes. Here, we report the localization of Myo1g in B-cell membrane compartments such as lipid rafts, microvilli, and membrane extensions formed during spreading. By using Myo1g-deficient mouse B cells, we detected abnormalities in the adhesion ability and chemokine-induced directed migration of these lymphocytes. We also assessed a role for Myo1g in phagocytosis and exocytosis processes, as these were also irregular in Myo1g-deficient B cells. Taken together, our results show that Myo1g acts as a main regulator of different membrane/cytoskeleton-dependent processes in B lymphocytes.

Regulation and control of myosin-I by the motor and light chain-binding domains

Members of the myosin-I family of molecular motors are expressed in many eukaryotes, where they are involved in a multitude of critical processes. Humans express eight distinct members of the myosin-I family, making it the second largest family of myosins expressed in humans. Despite the high degree of sequence conservation in the motor and light chain-binding domains (LCBDs) of these myosins, recent studies have revealed surprising diversity of function and regulation arising from isoform-specific differences in these domains. Here we review the regulation of myosin-I function and localization by the motor and LCBDs.

Molecular basis of dynamic relocalization of Dictyostelium myosin IB

Class I myosins have a single heavy chain comprising an N-terminal motor domain with actin-activated ATPase activity and a C-terminal globular tail with a basic region that binds to acidic phospholipids. These myosins contribute to the formation of actin-rich protrusions such as pseudopodia, but regulation of the dynamic localization to these structures is not understood. Previously, we found that Acanthamoeba myosin IC binds to acidic phospholipids in vitro through a short sequence of basic and hydrophobic amino acids, BH site, based on the charge density of the phospholipids. The tail of Dictyostelium myosin IB (DMIB) also contains a BH site. We now report that the BH site is essential for DMIB binding to the plasma membrane and describe the molecular basis of the dynamic relocalization of DMIB in live cells. Endogenous DMIB is localized uniformly on the plasma membrane of resting cells, at active protrusions and cell-cell contacts of randomly moving cells, and at the front of motile polarized cells. The BH site is required for association of DMIB with the plasma membrane at all stages where it colocalizes with phosphoinositide bisphosphate/phosphoinositide trisphosphate (PIP(2)/PIP(3)). The charge-based specificity of the BH site allows for in vivo specificity of DMIB for PIP(2)/PIP(3) similar to the PH domain-based specificity of other class I myosins. However, DMIB-head is required for relocalization of DMIB to the front of migrating cells. Motor activity is not essential, but the actin binding site in the head is important. Thus, dynamic relocalization of DMIB is determined principally by the local PIP(2)/PIP(3) concentration in the plasma membrane and cytoplasmic F-actin.

Myosin-1C associates with microtubules and stabilizes the mitotic spindle during cell division

The mitotic spindle in eukaryotic cells is composed of a bipolar array of microtubules (MTs) and associated proteins that are required during mitosis for the correct partitioning of the two sets of chromosomes to the daughter cells. In addition to the well-established functions of MT-associated proteins (MAPs) and MT-based motors in cell division, there is increasing evidence that the F-actin-based myosin motors are important mediators of F-actin-MT interactions during mitosis. Here, we report the functional characterization of the long-tailed class-1 myosin myosin-1C from Dictyostelium discoideum during mitosis. Our data reveal that myosin-1C binds to MTs and has a role in maintenance of spindle stability for accurate chromosome separation. Both myosin-1C motor function and tail-domain-mediated MT-F-actin interactions are required for the cell-cycle-dependent relocalization of the protein from the cell periphery to the spindle. We show that the association of myosin-1C with MTs is mediated through the tail domain. The myosin-1C tail can inhibit kinesin motor activity, increase the stability of MTs, and form crosslinks between MTs and F-actin. These data illustrate that myosin-1C is involved in the regulation of MT function during mitosis in D. discoideum.

Identification and characterization of an unusual class I myosin involved in vesicle traffic in Trypanosoma brucei

Myosins are a multimember family of motor proteins with diverse functions in eukaryotic cells. African trypanosomes possess only two candidate myosins and thus represent a useful system for functional analysis of these motors. One of these candidates is an unusual class I myosin (TbMyo1) that is expressed at similar levels but organized differently during the life cycle of Trypanosoma brucei. This myosin localizes to the polarized endocytic pathway in bloodstream forms of the parasite. This organization is actin dependent. Knock down of TbMyo1 results in a significant reduction in endocytic activity, a cessation in cell division and eventually cell death. A striking morphological feature in these cells is an enlargement of the flagellar pocket, which is consistent with an imbalance in traffic to and from the surface. In contrast TbMyo1 is distributed throughout procyclic forms of the tsetse vector and a loss of ∼90% of the protein has no obvious effects on growth or morphology. These results reveal a life cycle stage specific requirement for this myosin in essential endocytic traffic and represent the first description of the involvement of a motor protein in vesicle traffic in these parasites.

Calmodulin dissociation regulates Myo5 recruitment and function at endocytic sites

Myosins-I are conserved proteins that bear an N-terminal motor head followed by a Tail Homology 1 (TH1) lipid-binding domain. Some myosins-I have an additional C-terminal extension (C(ext)) that promotes Arp2/3 complex-dependent actin polymerization. The head and the tail are separated by a neck that binds calmodulin or calmodulin-related light chains. Myosins-I are known to participate in actin-dependent membrane remodelling. However, the molecular mechanisms controlling their recruitment and their biochemical activities in vivo are far from being understood. In this study, we provided evidence suggesting the existence of an inhibitory interaction between the TH1 domain of the yeast myosin-I Myo5 and its C(ext). The TH1 domain prevented binding of the Myo5 C(ext) to the yeast WIP homologue Vrp1, Myo5 C(ext)-induced actin polymerization and recruitment of the Myo5 C(ext) to endocytic sites. Our data also indicated that calmodulin dissociation from Myo5 weakened the interaction between the neck and TH1 domains and the C(ext). Concomitantly, calmodulin dissociation triggered Myo5 binding to Vrp1, extended the myosin-I lifespan at endocytic sites and activated Myo5-induced actin polymerization.

Leveraging the membrane - cytoskeleton interface with myosin-1

Class 1 myosins are small motor proteins with the ability to simultaneously bind to actin filaments and cellular membranes. Given their ability to generate mechanical force, and their high prevalence in many cell types, these molecules are well positioned to carry out several important biological functions at the interface of membrane and the actin cytoskeleton. Indeed, recent studies implicate these motors in endocytosis, exocytosis, release of extracellular vesicles, and the regulation of tension between membrane and the cytoskeleton. Many class 1 myosins also exhibit a load-dependent mechano-chemical cycle that enables them to maintain tension for long periods of time without hydrolyzing ATP. These properties put myosins-1 in a unique position to regulate dynamic membrane-cytoskeleton interactions and respond to physical forces during these events.

An experimentally based computer search identifies unstructured membrane-binding sites in proteins: application to class I myosins, PAKS, and CARMIL

Programs exist for searching protein sequences for potential membrane-penetrating segments (hydrophobic regions) and for lipid-binding sites with highly defined tertiary structures, such as PH, FERM, C2, ENTH, and other domains. However, a rapidly growing number of membrane-associated proteins (including cytoskeletal proteins, kinases, GTP-binding proteins, and their effectors) bind lipids through less structured regions. Here, we describe the development and testing of a simple computer search program that identifies unstructured potential membrane-binding sites. Initially, we found that both basic and hydrophobic amino acids, irrespective of sequence, contribute to the binding to acidic phospholipid vesicles of synthetic peptides that correspond to the putative membrane-binding domains of Acanthamoeba class I myosins. Based on these results, we modified a hydrophobicity scale giving Arg- and Lys-positive, rather than negative, values. Using this basic and hydrophobic scale with a standard search algorithm, we successfully identified previously determined unstructured membrane-binding sites in all 16 proteins tested. Importantly, basic and hydrophobic searches identified previously unknown potential membrane-binding sites in class I myosins, PAKs and CARMIL (capping protein, Arp2/3, myosin I linker; a membrane-associated cytoskeletal scaffold protein), and synthetic peptides and protein domains containing these newly identified sites bound to acidic phospholipids in vitro.

Myosin 1G is an abundant class I myosin in lymphocytes whose localization at the plasma membrane depends on its ancient divergent pleckstrin homology (PH) domain (Myo1PH)

Class I myosins, which link F-actin to membrane, are largely undefined in lymphocytes. Mass spectrometric analysis of lymphocytes identified two short tail forms: (Myo1G and Myo1C) and one long tail (Myo1F). We investigated Myo1G, the most abundant in T-lymphocytes, and compared key findings with Myo1C and Myo1F. Myo1G localizes to the plasma membrane and associates in an ATP-releasable manner to the actin-containing insoluble pellet. The IQ+tail region of Myo1G (Myo1C and Myo1F) is sufficient for membrane localization, but membrane localization is augmented by the motor domain. The minimal region lacks IQ motifs but includes: 1) a PH-like domain; 2) a "Pre-PH" region; and 3) a "Post-PH" region. The Pre-PH predicted alpha helices may contribute electrostatically, because two conserved basic residues on one face are required for optimal membrane localization. Our sequence analysis characterizes the divergent PH domain family, Myo1PH, present also in long tail myosins, in eukaryotic proteins unrelated to myosins, and in a probable ancestral protein in prokaryotes. The Myo1G Myo1PH domain utilizes the classic lipid binding site for membrane association, because mutating either of two basic residues in the "signature motif" destroys membrane localization. Mutation of each basic residue of the Myo1G Myo1PH domain reveals another critical basic residue in the beta3 strand, which is shared only by Myo1D. Myo1G differs from Myo1C in its phosphatidylinositol 4,5-bisphosphate dependence for membrane association, because membrane localization of phosphoinositide 5-phosphatase releases Myo1C from the membrane but not Myo1G. Thus Myo1PH domains likely play universal roles in myosin I membrane association, but different isoforms have diverged in their binding specificity.

Head region of unconventional myosin I family members is responsible for the organ-specificity of their roles in left-right polarity in Drosophila

In Drosophila, Myosin31DF (Myo31DF), encoding a Myosin ID protein, has crucial roles in left-right (LR) asymmetric development. Loss of Myo31DF function leads to laterality inversion for many organs, including the embryonic gut. Here, we found that Myo31DF was required before LR asymmetric morphogenesis in the hindgut, suggesting it functions in LR patterning instead of directly in hindgut morphological changes. Myosin61F (Myo61F) encodes another Myosin I, and Myo31DF or Myo61F overexpression reverses the laterality of different organs. Myo31DF and Myo61F have domains conserved in Myosin proteins, particularly in the proteins' head regions. We studied the roles of these domains in LR patterning using overexpression analysis. The Actin-binding and ATP-binding domains were essential for both proteins, but the IQ domains, binding sites for Myosin light chains, were required only by Myo31DF. Our results also suggest that the organ specificities of the Myo31DF and Myo61F activities depended on their head regions.

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.

Acanthamoeba myosin IC colocalizes with phosphatidylinositol 4,5-bisphosphate at the plasma membrane due to the high concentration of negative charge

The tail of Acanthamoeba myosin IC (AMIC) has a basic region (BR), which contains a putative pleckstrin homology (PH) domain, followed by two Gly/Pro/Ala (GPA)-rich regions separated by a Src homology 3 (SH3) domain. Cryoelectron microscopy had shown that the tail is folded back on itself at the junction of BR and GPA1, and nuclear magnetic resonance spectroscopy indicated that the SH3 domain may interact with the putative PH domain. The BR binds to acidic phospholipids, and the GPA region binds to F-actin. We now show that the folded tail does not affect the affinity of AMIC for acidic phospholipids. AMIC binds phosphatidylinositol 4,5-bisphosphate (PIP2) with high affinity (approximately 1 microm), but binding is not stereospecific. When normalized to net negative charge, AMIC binds with equal affinity to phosphatidylserine (PS) and PIP2. This and other data show that the putative PH domain of AMIC is not a typical PIP2-specific PH domain. We have identified a 13-residue sequence of basic-hydrophobic-basic amino acids within the putative PH domain that may be a major determinant of binding of AMIC to acidic phospholipids. Despite the lack of stereospecificity, AMIC binds 10 times more strongly to vesicles containing 5% PIP2 plus 25% PS than to vesicles containing only 25% PS, suggesting that AMIC may be targeted to PIP2-enriched regions of the plasma membrane. In agreement with this, AMIC colocalizes with PIP2 at dynamic, protrusive regions of the plasma membrane. We discuss the possibility that AMIC binding to PIP2 may initiate the formation of a multiprotein complex at the plasma membrane.

Head-neck domain of Arabidopsis myosin XI, MYA2, fused with GFP produces F-actin patterns that coincide with fast organelle streaming in different plant cells

BACKGROUND

The cytoskeletal mechanisms that underlie organelle transport in plants are intimately linked to acto-myosin function. This function is mediated by the attachment of myosin heads to F-actin and the binding of cargo to the tails. Acto-myosin also powers vigorous cytoplasmic streaming in plant cells. Class XI myosins exhibit strikingly fast velocities and may have extraordinary roles in cellular motility. Studies of the structural basis of organelle transport have focused on the cargo-binding tails of myosin XI, revealing a close relationship with the transport of peroxisomes, mitochondria, and Golgi-vesicles. Links between myosin heads and F-actin-based motility have been less investigated. To address this function, we performed localization studies using the head-neck domain of AtMYA2, a myosin XI from Arabidopsis.

RESULTS

We expressed the GFP-fused head-neck domain of MYA2 in epidermal cells of various plant species and found that it associated with F-actin. By comparison to other markers such as fimbrin and talin, we revealed that the myosin-labeled F-actin was of a lower quality and absent from the fine microfilament arrays at the cell cortex. However, it colocalized with cytoplasmic (transvacuolar) F-actin in areas coinciding with the tracks of fast organelles. This observation correlates well with the proposed function of myosin XI in organelle trafficking. The fact that organelle streaming was reduced in cells expressing the GFP-MYA2-head6IQ indicated that the functionless motor protein inhibits endogenous myosins. Furthermore, co-expression of the GFP-MYA2-head6IQ with other F-actin markers disrupted its attachment to F-actin. In nuclei, the GFP-myosin associated with short bundles of F-actin.

CONCLUSION

The localization of the head of MYA2 in living plant cells, as investigated here for the first time, suggests a close linkage between this myosin XI and cytoplasmic microfilaments that support the rapid streaming of organelles such as peroxisomes. Potential roles of MYA2 may also exist in the cell nucleus. Whether the low quality of the F-actin-labeling by MYA2-head6IQ compared to other F-actin-binding proteins (ABPs) signifies a weak association of the myosin with actin filaments remains to be proven by other means than in vivo. Clues for the mode of contact between the myosin molecules and F-actin so far cannot be drawn from sequence-related data.

Binding of chara Myosin globular tail domain to phospholipid vesicles

Binding of Chara myosin globular tail domain to phospholipid vesicles was investigated quantitatively. It was found that the globular tail domain binds to vesicles made from acidic phospholipids but not to those made from neutral phospholipids. This binding was weakened at high KCl concentration, suggesting that the binding is electrostatic by nature. The dissociation constant for the binding of the globular tail domain to 20% phosphatidylserine vesicles (similar to endoplasmic reticulum in acidic phospholipid contents) at 150 mM KCl was 273 nM. The free energy change due to this binding calculated from the dissociation constant was -37.3 kJ mol(-1). Thus the bond between the globular tail domain and membrane phospholipids would not be broken when the motor domain of Chara myosin moves along the actin filament using the energy of ATP hydrolysis (DeltaG degrees ' = -30.5 kJ mol(-1)). Our results suggested that direct binding of Chara myosin to the endoplasmic reticulum membrane through the globular tail domain could work satisfactorily in Chara cytoplasmic streaming. We also suggest a possible regulatory mechanism of cytoplasmic streaming including phosphorylation-dependent dissociation of the globular tail domain from the endoplasmic reticulum membrane.

Multiple myosins are required to coordinate actin assembly with coat compression during compensatory endocytosis

Actin is involved in endocytosis in organisms ranging from yeast to mammals. In activated Xenopus eggs, exocytosing cortical granules (CGs) are surrounded by actin "coats," which compress the exocytosing compartments, resulting in compensatory endocytosis. Here, we examined the roles of two myosins in actin coat compression. Myosin-2 is recruited to exocytosing CGs late in coat compression. Inhibition of myosin-2 slows coat compression without affecting actin assembly. This differs from phenotype induced by inhibition of actin assembly, where exocytosing CGs are trapped at the plasma membrane (PM) completely. Thus, coat compression is likely driven in part by actin assembly itself, but it requires myosin-2 for efficient completion. In contrast to myosin-2, the long-tailed myosin-1e is recruited to exocytosing CGs immediately after egg activation. Perturbation of myosin-1e results in partial actin coat assembly and induces CG collapse into the PM. Intriguingly, simultaneous inhibition of actin assembly and myosin-1e prevents CG collapse. Together, the results show that myosin-1e and myosin-2 are part of an intricate machinery that coordinates coat compression at exocytosing CGs.

The nucleolar channel system of human endometrium is related to endoplasmic reticulum and R-rings

The nucleolar channel system (NCS) is a well-established ultrastructural hallmark of the postovulation endometrium. Its transient presence has been associated with human fertility. Nevertheless, the biogenesis, composition, and function of these intranuclear membrane cisternae are unknown. Membrane systems with a striking ultrastructural resemblance to the NCS, termed R-rings, are induced in nuclei of tissue culture cells by overexpression of the central repeat domain of the nucleolar protein Nopp140. Here we provide a first molecular characterization of the NCS and compare the biogenesis of these two enigmatic organelles. Like the R-rings, the NCS consists of endoplasmic reticulum harboring the marker glucose-6-phosphatase. R-ring formation initiates at the nuclear envelope, apparently by a calcium-mediated Nopp140-membrane interaction, as supported by the calcium-binding ability of Nopp140, the inhibition of R-ring formation by calcium chelators, and the concentration of Nopp140 and complexed calcium in R-rings. Although biogenesis of the NCS may initiate similarly, the reduced presence of complexed calcium and Nopp140 suggests the involvement of additional factors.

Intramolecular interaction in the tail of Acanthamoeba myosin IC between the SH3 domain and a putative pleckstrin homology domain

The 466-aa tail of the heavy chain of Acanthamoeba myosin IC (AMIC) comprises an N-terminal 220-residue basic region (BR) followed by a 56-residue Gly/Pro/Ala-rich region (GPA1), a 55-residue Src homology 3 (SH3) domain, and a C-terminal 135-residue Gly/Pro/Ala-rich region (GPA2). Cryo-electron microscopy of AMIC had shown previously that the AMIC tail is folded back on itself, suggesting the possibility of interactions between its N- and C-terminal regions. We now show specific differences between the NMR spectrum of bacterially expressed full-length tail and the sum of the spectra of individually expressed BR and GPA1-SH3-GPA2 (GSG) regions. These results are indicative of interactions between the two subdomains in the full-length tail. From the NMR data, we could assign many of the residues in BR and GSG that are involved in these interactions. By combining homology modeling with the NMR data, we identify a putative pleckstrin homology (PH) domain within BR, and show that the PH domain interacts with the SH3 domain.

Differential localization of unconventional myosin I and nonmuscle myosin II during B cell spreading

Cross-linking of CD44 in vitro promotes chemokinesis and actin-based dendrite formation in T and B cells. However, the mechanisms by which the adhesion molecule CD44 induces cytoskeleton activation in lymphocytes are still poorly understood. In this study, we have investigated whether myosin isoforms are involved in CD44-dependent dendrite formation in activated B cells. Pharmacological inhibition of myosin with 2,3-butanedione monoxime strongly affected spreading and dendrite formation, suggesting that these cellular motors may participate in these phenomena. Furthermore, immunofluorescence analysis showed differences in subcellular localization of class I and class II myosin during B cell spreading. In response to CD44 cross-linking, myosin-1c was polarized to lamellipodia, where F-actin was high. In contrast, the distribution of cytosplasmic nonmuscle class II myosin was not altered. Expressions of myosin-1c and II were also demonstrated in B cells by Western blot. Although the inhibition of PLCgamma, PI3K and MEK-1 activation affected the spreading and dendrite formation in activated B cells, only PLCgamma and MEK-1 inhibition correlated with absence of myosin-1c polarization. Additionally, myosin-1c polarization was observed upon cross-linking of other surface molecules, suggesting a common mechanism for B cell spreading. This work shows that class I and class II myosin are expressed in B cells, are differentially distributed, and may participate in the morphological changes of these cells.

From transcription to transport: emerging roles for nuclear myosin IThis paper is one of a selection of papers published in this Special Issue, entitled 27th International West Coast Chromatin and Chromosome Conference, and has undergone the Journal's usual peer review process

Myosins are a superfamily of actin-activated ATPases that, in the cytoplasm, work together with actin as molecular motors. The presence of actin in the nucleus has been known for many years. The demonstration of a nuclear isoform of a myosin, nuclear myosin I (NMI), stimulated a great deal of interest in possible intranuclear motor functions of an acto–NMI complex. NMI has been shown to be involved in transcription by RNA polymerases I and II. In both cases, NMI interacts with the respective polymerase and is critically involved in the basic process of transcription. A recent study on intranuclear long-range chromosome movement has now demonstrated a role for NMI in the translocation of chromosome regions as well. Moreover, this movement is based on an active and directed process that is facilitated by an acto–NMI complex, establishing for the first time a functional role for a motor complex consisting of actin and a myosin in the nucleus.

Myo1c binds tightly and specifically to phosphatidylinositol 4,5-bisphosphate and inositol 1,4,5-trisphosphate

Myosin-I is the single-headed member of the myosin superfamily that associates with acidic phospholipids through its basic tail domain. Membrane association is essential for proper myosin-I localization and function. However, little is known about the physiological relevance of the direct association of myosin-I with phospholipids or about phospholipid headgroup-binding specificity. To better understand the mechanism of myosin-I-membrane association, we measured effective dissociation constants for the binding of a recombinant myo1c tail construct (which includes three IQ domains and bound calmodulins) to large unilamellar vesicles (LUVs) composed of phosphatidylcholine and various concentrations of phosphatidylserine (PS) or phosphatidylinositol 4,5-bisphosphate (PIP(2)). We found that the myo1c-tail binds tightly to LUVs containing >60% PS but very weakly to LUVs containing physiological PS concentrations (<40%). The myo1c tail and not the IQ motifs bind tightly to LUVs containing 2% PIP(2). Additionally, we found that the myo1c tail binds to soluble inositol-1,4,5-trisphosphate with nearly the same affinity as to PIP(2) in LUVs, suggesting that myo1c binds specifically to the headgroup of PIP(2). We also show that a GFP-myosin-I-tail chimera expressed in epithelial cells is transiently localized to regions known to be enriched in PIP(2). Our results suggest that myo1c does not bind to physiological concentrations of PS but rather binds tightly to PIP(2).

Role of fission yeast myosin I in organization of sterol-rich membrane domains

Specialized membrane domains containing lipid rafts are thought to be important for membrane processes such as signaling and trafficking. An unconventional type I myosin has been shown to reside in lipid rafts and function to target a disaccharidase to rafts in brush borders of intestinal mammalian cells. In the fission yeast Schizosaccharomyces pombe, distinct sterol-rich membrane domains are formed at the cell division site and sites of polarized cell growth at cell tips. Here, we show that the sole S. pombe myosin I, myo1p, is required for proper organization of these membrane domains. myo1 mutants lacking the TH1 domain exhibit a uniform distribution of sterol-rich membranes all over the plasma membrane throughout the cell cycle. These effects are independent of endocytosis because myo1 mutants exhibit no endocytic defects. Conversely, overexpression of myo1p induces ectopic sterol-rich membrane domains. Myo1p localizes to nonmotile foci that cluster in sterol-rich plasma membrane domains and fractionates with detergent-resistant membranes. Because the myo1p TH1 domain may bind directly to acidic phospholipids, these findings suggest a model for how type I myosin contributes to the organization of specialized membrane domains.

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

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

Functional characterization of myosin I tail regions in Candida albicans

The molecular motor myosin I is required for hyphal growth in the pathogenic yeast Candida albicans. Specific myosin I functions were investigated by a deletion analysis of five neck and tail regions. Hyphal formation requires both the TH1 region and the IQ motifs. The TH2 region is important for optimal hyphal growth. All of the regions, except for the SH3 and acidic (A) regions that were examined individually, were required for the localization of myosin I at the hyphal tip. Similarly, all of the domains were required for the association of myosin I with pelletable actin-bound complexes. Moreover, the hyphal tip localization of cortical actin patches, identified by both rhodamine-phalloidin staining and Arp3-green fluorescent protein signals, was dependent on myosin I. Double deletion of the A and SH3 domains depolarized the distribution of the cortical actin patches without affecting the ability of the mutant to form hyphae, suggesting that myosin I has distinct functions in these processes. Among the six myosin I tail domain mutants, the ability to form hyphae was strictly correlated with endocytosis. We propose that the uptake of cell wall remodeling enzymes and excess plasma membrane is critical for hyphal formation.

Changes in Mg2+ ion concentration and heavy chain phosphorylation regulate the motor activity of a class I myosin

Class I myosins are single-headed motor proteins implicated in various motile processes including organelle translocation, ion channel gating, and cytoskeleton reorganization. Dictyostelium discoideum myosin-ID belongs to subclass 1alpha, whose members are thought to be tuned for rapid sliding. The direct analysis of myosin-ID motor activity is made possible by the production of single polypeptide constructs carrying an artificial lever arm. Using these constructs, we show that the motor activity of myosin-ID is activated 80-fold by phosphorylation at the TEDS site. TEDS site phosphorylation acts by stabilizing the actomyosin complex and increasing the coupling between actin binding and the release of hydrolysis products. A surprising effect of Mg(2+) ions on in vitro motility was discovered. Changes in the level of free Mg(2+) ions within the physiological range are shown to modulate motor activity by inhibiting ADP release. Our results indicate that higher concentrations of free Mg(2+) ions stabilize the tension-bearing actin myosin ADP state and shift the system from the production of rapid movement toward the generation of tension.

Subdomain organization of the Acanthamoeba myosin IC tail from cryo-electron microscopy

Acanthamoeba myosin IC (AMIC) is a single-headed myosin comprised of one heavy chain (129 kDa) and one light chain (17 kDa). The heavy chain has head, neck (light chain-binding), and tail domains. The tail consists of four subdomains: a basic region (BR) (23 kDa) and two Gly/Pro/Ala-rich (GPA) regions, GPA1 (6 kDa) and GPA2 (15 kDa), flanking an Src homology 3 region (6 kDa). Although the AMIC head is similar in sequence, structure, and function (ATPase motor) to other myosin heads, the organization of the tail has been less clear as has its function beyond an assumed role in binding interaction partners, e.g., the BR has a membrane affinity and the GPA components bind F-actin in an ATP-independent manner. To investigate the spatial arrangement of subdomains in the tail, we have used cryo-electron microscopy and image reconstruction to compare actin filaments decorated with WT AMIC and tail-truncated mutants of various lengths. The BR forms an oval-shaped feature, approximately 40 A long, that diverges obliquely from the head, extending azimuthally around the actin filament and toward its barbed end. GPA2 and GPA1 are located together on the inner (actin-proximal) side of the tail, close enough to act in concert in binding the same or another actin filament. The outer face of the BR is strategically exposed for membrane or vesicle binding.

Identification of the membrane receptor of a class XIV myosin in Toxoplasma gondii

Apicomplexan parasites exhibit a unique form of substrate-dependent motility, gliding motility, which is essential during their invasion of host cells and during their spread between host cells. This process is dependent on actin filaments and myosin that are both located between the plasma membrane and two underlying membranes of the inner membrane complex. We have identified a protein complex in the apicomplexan parasite Toxoplasma gondii that contains the class XIV myosin required for gliding motility, TgMyoA, its associated light chain, TgMLC1, and two novel proteins, TgGAP45 and TgGAP50. We have localized this complex to the inner membrane complex of Toxoplasma, where it is anchored in the membrane by TgGAP50, an integral membrane glycoprotein. Assembly of the protein complex is spatially controlled and occurs in two stages. These results provide the first molecular description of an integral membrane protein as a specific receptor for a myosin motor, and further our understanding of the motile apparatus underlying gliding motility in apicomplexan parasites.

Localization of a class III myosin to filopodia tips in transfected HeLa cells requires an actin-binding site in its tail domain

Bass Myo3A, a class III myosin, was expressed in HeLa cells as a GFP fusion in order to study its cellular localization. GFP-Myo3A localized to the cytoplasm and to the tips of F-actin bundles in filopodia, a localization that is consistent with the observed concentration toward the distal ends of F-actin bundles in photoreceptor cells. A mutation in the motor active site resulted in a loss of filopodia localization, suggesting that Myo3A motor activity is required for filopodial tip localization. Deletion analyses showed that the NH2-terminal kinase domain is not required but the CO2H-terminal 22 amino acids of the Myo3A tail are required for filopodial localization. Expression of this tail fragment alone produced fluorescence associated with F-actin throughout the cytoplasm and filopodia and a recombinant tail fragment bound to F-actin in vitro. An actin-binding motif was identified within this tail fragment, and a mutation within this motif abolished both filopodia localization by Myo3A and F-actin binding by the tail fragment alone. Calmodulin localized to filopodial tips when coexpressed with Myo3A but not in the absence of Myo3A, an observation consistent with the previous proposal that class III myosins bind calmodulin and thereby localize it in certain cell types.

Intracellular localization and dynamics of myosin-II and myosin-IC in live Acanthamoeba by transient transfection of EGFP fusion proteins

We developed a reliable method for transient transfection of Acanthamoeba using Superfect (Qiagen) and a vector with the Acanthamoeba ubiquitin promoter and enhanced green fluorescent protein (EGFP) as the reporter gene. The transfection efficiency was 3% for profilin-I-EGFP and EGFP-myosin-II tail, and less than 0.5% for larger constructs such as full length myosin-II or myosin-IC. Profilin-I-EGFP was distributed throughout the cytoplasm as observed previously with rhodamine-labeled profilin, while EGFP alone accumulated in the nucleus. EGFP fused to full length myosin-II or to the C-terminal 256 residues of the myosin-II tail concentrated in fluorescent spots similar to thick filaments and minifilaments identified previously in fixed cells with fluorescent antibodies. Thick filaments were located in the dorsal cytoplasm and along the lateral margins of the back half of the cell. Thick filaments formed behind the leading edge and moved continuously towards the rear of the cell, where they disassembled. If phosphorylation of the myosin-II heavy chain was prevented by mutation of all three phosphorylated serines to alanine, thick filaments of unphosphorylated myosin-II accumulated around vesicles of various sizes. EGFP-myosin-IC was spread throughout the cytoplasm but concentrated transiently around contractile vacuoles and macropinocytosis cups providing that the construct included both the head and a tail with the SH3 domain.

Dynamics of myo1c (myosin-ibeta ) lipid binding and dissociation

Myosin-I is the single-headed member of the myosin superfamily that associates with lipid membranes. Biochemical experiments have shown that myosin-I membrane binding is the result of electrostatic interactions between the basic tail domain and acidic phospholipids. To better understand the dynamics of myosin-I membrane association, we measured the rates of association and dissociation of a recombinant myo1c tail domain (which includes three IQ domains and bound calmodulins) to and from large unilamellar vesicles using fluorescence resonance energy transfer. The apparent second-order rate constant for lipid-tail association in the absence of calcium is fast with nearly every lipid-tail collision resulting in binding. The rate of binding is decreased in the presence of calcium. Time courses of myo1c-tail dissociation are best fit by two exponential rates: a fast component that has a rate that depends on the ratio of acidic phospholipid to myo1c-tail (phosphatidylserine (PS)/tail) and a slow component that predominates at high PS/tail ratios. The dissociation rate of the slow component is slower than the myo1c ATPase rate, suggesting that myo1c is able to stay associated with the lipid membrane during multiple catalytic cycles of the motor. Calcium significantly increases the lifetimes of the membrane-bound state, resulting in dissociation rates 0.001 s(-1).

The novel adaptor protein, Mti1p, and Vrp1p, a homolog of Wiskott-Aldrich syndrome protein-interacting protein (WIP), may antagonistically regulate type I myosins in Saccharomyces cerevisiae

Type I myosins in yeast, Myo3p and Myo5p (Myo3/5p), are involved in the reorganization of the actin cytoskeleton. The SH3 domain of Myo5p regulates the polymerization of actin through interactions with both Las17p, a homolog of mammalian Wiskott-Aldrich syndrome protein (WASP), and Vrp1p, a homolog of WASP-interacting protein (WIP). Vrp1p is required for both the localization of Myo5p to cortical patch-like structures and the ATP-independent interaction between the Myo5p tail region and actin filaments. We have identified and characterized a new adaptor protein, Mti1p (Myosin tail region-interacting protein), which interacts with the SH3 domains of Myo3/5p. Mti1p co-immunoprecipitated with Myo5p and Mti1p-GFP co-localized with cortical actin patches. A null mutation of MTI1 exhibited synthetic lethal phenotypes with mutations in SAC6 and SLA2, which encode actin-bundling and cortical actin-binding proteins, respectively. Although the mti1 null mutation alone did not display any obvious phenotype, it suppressed vrp1 mutation phenotypes, including temperature-sensitive growth, abnormally large cell morphology, defects in endocytosis and salt-sensitive growth. These results suggest that Mti1p and Vrp1p antagonistically regulate type I myosin functions.

Myosins in protists

This review focuses on selected papers that illustrate an historical perspective and the current knowledge of myosin structure and function in protists. The review contains a general description of myosin structure, a phylogenetic tree of the myosin classes, and descriptions of myosin isoforms identified in protists. Each myosin is discussed within the context of the taxonomic group of the organism in which the myosin has been identified. Domain structure, cellular location, function, and regulation are described for each myosin.

Intranuclear endoplasmic reticulum induced by Nopp140 mimics the nucleolar channel system of human endometrium

Exogenous expression of the characteristic repeat domain of the nucleolar chaperone Nopp140 induces the formation of intranuclear structures, termed R-rings. Here, the R-rings are identified as extensive stacks of membrane cisternae in the otherwise membrane-free nucleus. They consist of bona fide endoplasmic reticulum (ER) containing integral membrane proteins of the smooth and rough ER. Although lacking nuclear pore complexes and lamina, the R-rings derive specifically from the inner nuclear membrane. These findings are consistent with the idea that all transmembrane proteins synthesized in the ER and the outer nuclear membrane can freely diffuse through the pore membrane domain into the inner membrane. Uniquely, the soluble transfected Nopp140 is directly involved in the generation of these membrane stacks as it localizes to the electron dense matrix in which they are embedded. The only well-documented example of intranuclear membrane proliferation is the nucleolar channel system of the postovulation human endometrium. The transient emergence of the nucleolar channel system correlates precisely with the readiness of the endometrium for the implantation of the fertilized egg. The nucleolar channel system exhibits an ultrastructure that is indistinguishable from R-rings, and nuclei of human endometrium harbor Nopp140 and ER marker containing structures. Therefore, the nucleolar channel system appears to be identical to the R-rings, suggesting a role for Nopp140 in human reproduction.

Motor domain-dependent localization of myo1b (myr-1)

Myosin-I is the single-headed, membrane binding member of the myosin superfamily that plays a role in membrane dynamics and transport [1-6]. Its molecular functions and its mechanism of regulation are not known. In mammalian cells, myosin-I is excluded from specific microfilament populations, indicating that its localization is tightly regulated. Identifying the mechanism of this localization, and the specific actin populations with which myosin-I interacts, is crucial to understanding the molecular functions of this motor. eGFP chimeras of myo1b [7] were imaged in live and fixed NRK cells. Ratio-imaging microscopy shows that myo1b-eGFP concentrates within dynamic areas of the actin cytoskeleton, most notably in membrane ruffles. Myo1b-eGFP does not associate with stable actin bundles or stress fibers. Truncation mutants consisting of the motor or tail domains show a partially overlapping cytoplasmic localization with full-length myo1b, but do not concentrate in membrane ruffles. A chimera consisting of the light chain and tail domains of myo1b and the motor domain from nonmuscle myosin-IIb (nmMIIb) concentrates on actin filaments in ruffles as well as to stress fibers. In vitro motility assays show that the exclusion of myo1b from certain actin filament populations is due to the regulation of the actomyosin interaction by tropomyosin. Therefore, we conclude that tropomyosin and spatially regulated actin polymerization play important roles in regulating the function and localization of myo1b.

Regulation of molecular motor proteins

Motor proteins in the kinesin, dynein, and myosin superfamilies are tightly regulated to perform multiple functions in the cell requiring force generation. Although motor proteins within families are diverse in sequence and structure, there are general mechanisms by which they are regulated. We first discuss the regulation of the subset of kinesin family members for which such information exists, and then address general mechanisms of kinesin family regulation. We review what is known about the regulation of axonemal and cytoplasmic dyneins. Recent work on cytoplasmic dynein has revealed the existence of multiple isoforms for each dynein chain, making the study of dynein regulation more complicated than previously realized. Finally, we discuss the regulation of myosins known to be involved in membrane trafficking. Myosins and kinesins may be evolutionarily related, and there are common themes of regulation between these two classes of motors.

Regulation of Dictyostelium myosin I and II

Dictyostelium expresses 12 different myosins, including seven single-headed myosins I and one conventional two-headed myosin II. In this review we focus on the signaling pathways that regulate Dictyostelium myosin I and myosin II. Activation of myosin I is catalyzed by a Cdc42/Rac-stimulated myosin I heavy chain kinase that is a member of the p21-activated kinase (PAK) family. Evidence that myosin I is linked to the Arp2/3 complex suggests that pathways that regulate myosin I may also influence actin filament assembly. Myosin II activity is stimulated by a cGMP-activated myosin light chain kinase and inhibited by myosin heavy chain kinases (MHCKs) that block bipolar filament assembly. Known MHCKs include MHCK A and MHCK B, which have a novel type of kinase catalytic domain joined to a WD repeat domain, and MHC-protein kinase C (PKC), which contains both diacylglycerol kinase and PKC-related protein kinase catalytic domains. A Dictyostelium PAK (PAKa) acts indirectly to promote myosin II filament formation, suggesting that the MHCKs may be indirectly regulated by Rac GTPases.

Regulation and expression of metazoan unconventional myosins

Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3-16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplification. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

Recruitment of a specific amoeboid myosin I isoform to the plasma membrane in chemotactic Dictyostelium cells

The Dictyostelium class I myosins, MyoA, -B, -C, and -D, participate in plasma membrane-based cellular processes such as pseudopod extension and macropinocytosis. Given the existence of a high affinity membrane-binding site in the C-terminal tail domain of these motor proteins and their localized site of action at the cortical membrane-cytoskeleton, it was of interest to determine whether each myosin I was directly associated with the plasma membrane. The membrane association of a myosin I heavy chain kinase that regulates the activity of one of the class I myosins, MyoD was also examined. Cellular fractionation experiments revealed that the majority of the Dicyostelium MyoA, -B, -C and -D heavy chains and the kinase are cytosolic. However, a small, but significant, fraction (appr. 7. -15%) of each myosin I and the kinase was associated with the plasma membrane. The level of plasma membrane-associated MyoB, but neither that of MyoC nor MyoD, increases up to 2-fold in highly motile, streaming cells. These results indicate that Dictyostelium specifically recruits myoB to the plasma membrane during directed cell migration, consistent with its known role in pseudopod formation.