Subcellular localization and dynamics of MysPDZ (Myo18A) in live mammalian cells

Emerging concepts of myosin 18A isoform mechanobiology in organismal and immune system physiology, development, and function

Alternative and combinatorial splicing of myosin 18A (MYO18A) gene transcripts results in expression of MYO18A protein isoforms and isoform variants with different membrane and subcellular localizations, and functional properties. MYO18A proteins are members of the myosin superfamily consisting of a myosin-like motor domain, an IQ motif, and a coiled-coil domain. MYO18A isoforms, however, lack the ability to hydrolyze ATP and do not perform ATP-dependent motor activity. MYO18A isoforms are distinguished by different amino- and carboxy-terminal extensions and domains. The domain organization and functions of MYO18Aα, MYO18Aβ, and MYO18Aγ have been studied experimentally. MYO18Aα and MYO18Aβ have a common carboxy-terminal extension but differ by the presence or absence of an amino-terminal KE repeat and PDZ domain, respectively. The amino- and carboxy-terminal extensions of MYO18Aγ contain unique proline and serine-rich domains. Computationally predicted MYO18Aε and MYO18Aδ isoforms contain the carboxy-terminal serine-rich extension but differ by the presence or absence of the amino-terminal KE/PDZ extension. Additional isoform variants within each category arise by alternative utilization or inclusion/exclusion of small exons. MYO18Aα variants are expressed in somatic cells and mature immune cells, whereas MYO18Aβ variants occur mainly in myeloid and natural killer cells. MYO18Aγ expression is selective to cardiac and skeletal muscle. In the present review perspective, we discuss current and emerging concepts of the functional specialization of MYO18A proteins in membrane and cytoskeletal dynamics, cellular communication and signaling, endocytic and exocytic organelle movement, viral infection, and as the SP-R210 receptor for surfactant protein A.

Are the class 18 myosins Myo18A and Myo18B specialist sarcomeric proteins?

Initially, the two members of class 18 myosins, Myo18A and Myo18B, appeared to exhibit highly divergent functions, complicating the assignment of class-specific functions. However, the identification of a striated muscle-specific isoform of Myo18A, Myo18Aγ, suggests that class 18 myosins may have evolved to complement the functions of conventional class 2 myosins in sarcomeres. Indeed, both genes, Myo18a and Myo18b, are predominantly expressed in the heart and somites, precursors of skeletal muscle, of developing mouse embryos. Genetic deletion of either gene in mice is embryonic lethal and is associated with the disorganization of cardiac sarcomeres. Moreover, Myo18Aγ and Myo18B localize to sarcomeric A-bands, albeit the motor (head) domains of these unconventional myosins have been both deduced and biochemically demonstrated to exhibit negligible ATPase activity, a hallmark of motor proteins. Instead, Myo18Aγ and Myo18B presumably coassemble with thick filaments and provide structural integrity and/or internal resistance through interactions with F-actin and/or other proteins. In addition, Myo18Aγ and Myo18B may play distinct roles in the assembly of myofibrils, which may arise from actin stress fibers containing the α-isoform of Myo18A, Myo18Aα. The β-isoform of Myo18A, Myo18Aβ, is similar to Myo18Aα, except that it lacks the N-terminal extension, and may serve as a negative regulator through heterodimerization with either Myo18Aα or Myo18Aγ. In this review, we contend that Myo18Aγ and Myo18B are essential for myofibril structure and function in striated muscle cells, while α- and β-isoforms of Myo18A play diverse roles in nonmuscle cells.

Myosin-18B Regulates Higher-Order Organization of the Cardiac Sarcomere through Thin Filament Cross-Linking and Thick Filament Dynamics

MYO18B loss-of-function mutations and depletion significantly compromise the structural integrity of striated muscle sarcomeres. The molecular function of the encoded protein, myosin-18B (M18B), within the developing muscle is unknown. Here, we demonstrate that recombinant M18B lacks motor ATPase activity and harbors previously uncharacterized N-terminal actin-binding domains, properties that make M18B an efficient actin cross-linker and molecular brake capable of regulating muscle myosin-2 contractile forces. Spatiotemporal analysis of M18B throughout cardiomyogenesis and myofibrillogenesis reveals that this structural myosin undergoes nuclear-cytoplasmic redistribution during myogenic differentiation, where its incorporation within muscle stress fibers coincides with actin striation onset. Furthermore, this analysis shows that M18B is directly integrated within the muscle myosin thick filament during myofibril maturation. Altogether, our data suggest that M18B has evolved specific biochemical properties that allow it to define and maintain sarcomeric organization from within the thick filament via its dual actin cross-linking and motor modulating capabilities.

Multifaceted Function of Myosin-18, an Unconventional Class of the Myosin Superfamily

Myosin is a diverse superfamily of motor proteins responsible for actin-based motility and contractility in eukaryotic cells. Myosin-18 family, including myosin-18A and myosin-18B, belongs to an unconventional class of myosin, which lacks ATPase motor activity, and the investigations on their functions and molecular mechanisms in vertebrate development and diseases have just been initiated in recent years. Myosin-18A is ubiquitously expressed in mammalian cells, whereas myosin-18B shows strong enrichment in striated muscles. Myosin-18 family is important for cell motility, sarcomere formation, and mechanosensing, mostly by interacting with other cytoskeletal proteins and cellular apparatus. Myosin-18A participates in several intracellular transport processes, such as Golgi trafficking, and has multiple roles in focal adhesions, stress fibers, and lamellipodia formation. Myosin-18B, on the other hand, participates in actomyosin alignment and sarcomere assembly, thus relating to cell migration and muscle contractility. Mutations of either Myo18a or Myo18b cause cardiac developmental defects in mouse, emphasizing their crucial role in muscle development and cardiac diseases. In this review, we revisit the discovery history of myosin-18s and summarize the evolving understanding of the molecular functions of myosin-18A and myosin-18B, with an emphasis on their separate yet closely related functions in cell motility and contraction. Moreover, we discuss the diseases tightly associated with myosin-18s, especially cardiovascular defects and cancer, as well as highlight the unanswered questions and potential future research perspectives on myosin-18s.

Myosin XVIII

Class XVIII myosins represent a branch of the myosin family tree characterized by the presence of large N- and C-terminal extensions flanking a generic myosin core. These myosins display the highest sequence similarity to conventional class II muscle myosins and are compatible with but not restricted to myosin-2 contractile structures. Instead, they fulfill their functions at diverse localities, such as lamella, actomyosin bundles, the Golgi apparatus, focal adhesions, the cell membrane, and within sarcomeres. Sequence comparison of active-site residues and biochemical data available thus far indicate that this myosin class lacks active ATPase-driven motor activity, suggesting that its members function as structural myosins. An emerging body of evidence indicates that this structural capability is essential for the organization, maturation, and regulation of the contractile machinery in both muscle and nonmuscle cells. This is supported by the clear association of myosin-18A (Myo18A) and myosin-18B (Myo18B) dysregulation with diseases such as cancer and various myopathies.

A novel isoform of myosin 18A (Myo18Aγ) is an essential sarcomeric protein in mouse heart

Whereas myosin 18B (Myo18B) is known to be a critical sarcomeric protein, the function of myosin 18A (Myo18A) is unclear, although it has been implicated in cell motility and Golgi shape. Here, we show that homozygous deletion (homozygous tm1a, tm1b, or tm1d alleles) of Myo18a in mouse is embryonic lethal. Reminiscent of Myo18b, Myo18a was highly expressed in the embryo heart, and cardiac-restricted Myo18a deletion in mice was embryonic lethal. Surprisingly, using Western blot analysis, we were unable to detect the known isoforms of Myo18A, Myo18Aα and Myo18Aβ, in mouse heart using a custom C-terminal antibody. However, alternative anti-Myo18A antibodies detected a larger than expected protein, and RNA-Seq analysis indicated that a novel Myo18A transcript is expressed in mouse ventricular myocytes (and human heart). Cloning and sequencing revealed that this cardiac isoform, denoted Myo18Aγ, lacks the PDZ-containing N terminus of Myo18Aα but includes an alternative N-terminal extension and a long serine-rich C terminus. EGFP-tagged Myo18Aγ expressed in ventricular myocytes localized to the level of A-bands in sarcomeres, and Myo18a knockout embryos at day 10.5 exhibited disorganized sarcomeres with wavy thick filaments. We additionally generated myeloid-restricted Myo18a knockout mice to investigate the role of Myo18A in nonmuscle cells, exemplified by macrophages, which express more Myo18Aβ than Myo18Aα, but no defects in cell shape, motility, or Golgi shape were detected. In summary, we have identified a previously unrecognized sarcomere component, a large novel isoform (denoted Myo18Aγ) of Myo18A. Thus, both members of class XVIII myosins are critical components of cardiac sarcomeres.

MYO18A: An unusual myosin

MYO18A is a divergent member of the myosin family characterized by the presence of an amino-terminal PDZ domain. MYO18A has been found in a few different complexes involved in intracellular transport processes. MYO18A is found in a complex with LURAP1 and MRCK that functions in retrograde treadmilling of actin. It also has been found in a complex with PAK2, βPIX, and GIT1, functioning to transport that protein complex from focal adhesions to the leading edge. Finally, a high proportion of MYO18A is found in complex with GOLPH3 at the trans Golgi, where it functions to promote vesicle budding for Golgi-to-plasma membrane trafficking. Interestingly, MYO18A has been implicated as a cancer driver, as have other components of the GOLPH3 pathway. It remains uncertain as to whether or not MYO18A has intrinsic motor activity. While many questions remain, MYO18A is a fascinatingly unique myosin that is essential in higher organisms.

Re-evaluating the roles of myosin 18Aα and F-actin in determining Golgi morphology

The peri-centrosomal localization and morphology of the Golgi apparatus depends largely on the microtubule cytoskeleton and the microtubule motor protein dynein. Recent studies proposed that myosin 18Aα (M18Aα) also contributes to Golgi morphology by binding the Golgi protein GOLPH3 and walking along adjacent actin filaments to stretch the Golgi into its classic ribbon structure. Biochemical analyses have shown, however, that M18A is not an actin-activated ATPase and lacks motor activity. Our goal, therefore, was to define the precise molecular mechanism by which M18Aα determines Golgi morphology. We show that purified M18Aα remains inactive in the presence of GOLPH3, arguing against the Golgi-specific activation of the myosin. Using M18A-specific antibodies and expression of GFP-tagged M18Aα, we find no evidence that it localizes to the Golgi. Moreover, several cell lines with reduced or eliminated M18Aα expression exhibited normal Golgi morphology. Interestingly, actin filament disassembly resulted in a marked reduction in lateral stretching of the Golgi in both control and M18Aα-deficient cells. Importantly, this reduction was accompanied by an expansion of the Golgi in the vertical direction, vertical movement of the centrosome, and increases in the height of both the nucleus and the cell. Collectively, our data indicate that M18Aα does not localize to the Golgi or play a significant role in determining its morphology, and suggest that global F-actin disassembly alters Golgi morphology indirectly by altering cell shape.

Identification of novel MYO18A interaction partners required for myoblast adhesion and muscle integrity

The unconventional myosin MYO18A that contains a PDZ domain is required for muscle integrity during zebrafish development. However, the mechanism by which it functions in myofibers is not clear. The presence of a PDZ domain suggests that MYO18A may interact with other partners to perform muscle-specific functions. Here we performed double-hybrid screening and co-immunoprecipitation to identify MYO18A-interacting proteins, and have identified p190RhoGEF and Golgin45 as novel partners for the MYO18A PDZ domain. We have also identified Lurap1, which was previously shown to bind MYO18A. Functional analyses indicate that, similarly as myo18a, knockdown of lurap1, p190RhoGEF and Golgin45 by morpholino oligonucleotides disrupts dystrophin localization at the sarcolemma and produces muscle lesions. Simultaneous knockdown of myo18a with either of these genes severely disrupts myofiber integrity and dystrophin localization, suggesting that they may function similarly to maintain myofiber integrity. We further show that MYO18A and its interaction partners are required for adhesion of myoblasts to extracellular matrix, and for the formation of the Golgi apparatus and organization of F-actin bundles in myoblast cells. These findings suggest that MYO18A has the potential to form a multiprotein complex that links the Golgi apparatus to F-actin, which regulates muscle integrity and function during early development.

SP-R210 (Myo18A) Isoforms as Intrinsic Modulators of Macrophage Priming and Activation

The surfactant protein (SP-A) receptor SP-R210 has been shown to increase phagocytosis of SP-A-bound pathogens and to modulate cytokine secretion by immune cells. SP-A plays an important role in pulmonary immunity by enhancing opsonization and clearance of pathogens and by modulating macrophage inflammatory responses. Alternative splicing of the Myo18A gene results in two isoforms: SP-R210S and SP-R210L, with the latter predominantly expressed in alveolar macrophages. In this study we show that SP-A is required for optimal expression of SP-R210L on alveolar macrophages. Interestingly, pre-treatment with SP-A prepared by different methods either enhances or suppresses responsiveness to LPS, possibly due to differential co-isolation of SP-B or other proteins. We also report that dominant negative disruption of SP-R210L augments expression of receptors including SR-A, CD14, and CD36, and enhances macrophages' inflammatory response to TLR stimulation. Finally, because SP-A is known to modulate CD14, we used a variety of techniques to investigate how SP-R210 mediates the effect of SP-A on CD14. These studies revealed a novel physical association between SP-R210S, CD14, and SR-A leading to an enhanced response to LPS, and found that SP-R210L and SP-R210S regulate internalization of CD14 via distinct macropinocytosis-like mechanisms. Together, our findings support a model in which SP-R210 isoforms differentially regulate trafficking, expression, and activation of innate immune receptors on macrophages.

The PDZ-containing unconventional myosin XVIIIA regulates embryonic muscle integrity in zebrafish

Myosin XVIIIA, or MYO18A, is a unique PDZ domain-containing unconventional myosin and is evolutionarily conserved from Drosophila to vertebrates. Although there is evidence indicating its expression in the somites, whether it regulates muscle function remains unclear. We show that the two zebrafish myo18a genes (myo18aa and myo18ab) are predominantly expressed at somite borders during early developmental stages. Knockdown of these genes or overexpression of the MYO18A PDZ domain disrupts myofiber integrity, induces myofiber lesions, and compromises the localization of dystrophin, α-dystroglycan (α-DG) and laminin at the myotome boundaries. Cell transplantation experiments indicate that myo18a morphant myoblasts fail to form elongated myofibers in the myotomes of wild-type embryos, which can be rescued by the full-length MYO18A protein. These results suggest that MYO18A likely functions in the adhesion process that maintains the stable attachment of myofibers to ECM (extracellular matrix) and muscle integrity during early development.

Binding of the extreme carboxyl-terminus of PAK-interacting exchange factor β (βPIX) to myosin 18A (MYO18A) is required for epithelial cell migration

The PAK2/βPIX/GIT1 (p21-activated kinase 2/PAK-interacting exchange factor-β/G protein-coupled receptor kinase-interactor 1) complex has been shown to distribute to both membrane ruffles and focal adhesions of cells, where it plays an important role in regulating focal adhesion turnover. However, the detailed mechanism underlying this regulation is largely unknown. We previously reported that MYO18Aα interacts via its carboxyl terminus with the PAK2/βPIX/GIT1 complex through direct binding to βPIX, and that knockdown of MYO18Aα in epithelial cells causes accumulation of the complex in focal adhesions and decreased cell migration ability (Hsu et al., 2010). The current study characterized the detailed MYO18Aα-βPIX interaction mechanism and the biological significance of this interaction. We found that deletion of the carboxyl-terminal globular domain of MYO18Aα profoundly altered the cellular localization of βPIX and inhibited cell migration. βPIX interacts through its most carboxyl-terminus, PAWDETNL (639-646), with MYO18Aα and partially colocalized with MYO18Aα in membrane ruffles of cells, whereas βPIX(1-638), a mutant with deletion of PAWDETNL, accumulated in focal adhesions. Both focal adhesion numbers and area in βPIX(1-638)-expressing cells were greater than those in cells expressing wild-type βPIX(FL). Further experiments using deletion mutants of MYO18A and βPIX showed that disruption of MYO18A-βPIX interaction not only impaired cell motility but also decreased Rac1 activity. Collectively, our data unravel the interaction regions between MYO18A and βPIX and provide evidence for the critical role of this interaction in regulating cellular localization of βPIX, Rac1 activity, and adhesion and migration in epithelial cells.

Functional characterization of human myosin-18A and its interaction with F-actin and GOLPH3

Molecular motors of the myosin superfamily share a generic motor domain region. They commonly bind actin in an ATP-sensitive manner, exhibit actin-activated ATPase activity, and generate force and movement in this interaction. Class-18 myosins form heavy chain dimers and contain protein interaction domains located at their unique N-terminal extension. Here, we characterized human myosin-18A molecular function in the interaction with nucleotides, F-actin, and its putative binding partner, the Golgi-associated phosphoprotein GOLPH3. We show that myosin-18A comprises two actin binding sites. One is located in the KE-rich region at the start of the N-terminal extension and appears to mediate ATP-independent binding to F-actin. The second actin-binding site resides in the generic motor domain and is regulated by nucleotide binding in the absence of intrinsic ATP hydrolysis competence. This core motor domain displays its highest actin affinity in the ADP state. Electron micrographs of myosin-18A motor domain-decorated F-actin filaments show a periodic binding pattern independent of the nucleotide state. We show that the PDZ module mediates direct binding of myosin-18A to GOLPH3, and this interaction in turn modulates the actin binding properties of the N-terminal extension. Thus, myosin-18A can act as an actin cross-linker with multiple regulatory modulators that targets interacting proteins or complexes to the actin-based cytoskeleton.

Mammalian myosin-18A, a highly divergent myosin

The Mus musculus myosin-18A gene is expressed as two alternatively spliced isoforms, α and β, with reported roles in Golgi localization, in maintenance of cytoskeleton, and as receptors for immunological surfactant proteins. Both myosin-18A isoforms feature a myosin motor domain, a single predicted IQ motif, and a long coiled-coil reminiscent of myosin-2. The myosin-18Aα isoform, additionally, has an N-terminal PDZ domain. Recombinant heavy meromyosin- and subfragment-1 (S1)-like constructs for both myosin-18Aα and -18β species were purified from the baculovirus/Sf9 cell expression system. These constructs bound both essential and regulatory light chains, indicating an additional noncanonical light chain binding site in the neck. Myosin-18Aα-S1 and -18Aβ-S1 molecules bound actin weakly with Kd values of 4.9 and 54 μm, respectively. The actin binding data could be modeled by assuming an equilibrium between two myosin conformations, a competent and an incompetent form to bind actin. Actin binding was unchanged by presence of nucleotide. Both myosin-18A isoforms bound N-methylanthraniloyl-nucleotides, but the rate of ATP hydrolysis was very slow (<0.002 s(-1)) and not significantly enhanced by actin. Phosphorylation of the regulatory light chain had no effect on ATP hydrolysis, and neither did the addition of tropomyosin or of GOLPH3, a myosin-18A binding partner. Electron microscopy of myosin-18A-S1 showed that the lever is strongly angled with respect to the long axis of the motor domain, suggesting a pre-power stroke conformation regardless of the presence of ATP. These data lead us to conclude that myosin-18A does not operate as a traditional molecular motor in cells.

A three-way translocation of MLL, MLLT11, and the novel reciprocal partner gene MYO18A in a child with acute myeloid leukemia

Translocations of the MLL gene are common among neonates and infants with acute lymphoblastic and acute myeloid leukemias. We characterized a new three-way translocation involving MLL in an infant with acute myeloid leukemia who subsequently relapsed and underwent a hematopoietic stem cell transplant from an unrelated stem cell donor. The translocation was characterized using karyotyping and fluorescence in situ hybridization. In this patient, a complex rearrangement fused the distal part of 11q23 with 17q11.2, the distal part of 17q11.2 with 1q21, and the distal part of 1q21 with 11q23, resulting in a three-way translocation; t(1;11;17)(q21;q23;q11.2). The two reciprocal MLL fusion sites were cloned by long-distance inverse polymerase chain reaction, which led to the identification of MLL-MLLT11 and the reciprocal MYO18A-MLL fusion alleles. Both fusion genes are in-frame and can be translated into functional fusion proteins. Although the MLL-MLLT11 fusion gene has been described in the literature, the reciprocal MYO18A fusion partner is a novel candidate gene in the growing list of reciprocal MLL fusions.

Proteomics analysis of the ezrin interactome in B cells reveals a novel association with Myo18aα

The molecular regulation of recruitment and assembly of signalosomes near the B cell receptor (BCR) is poorly understood. We have previously demonstrated a role for the ERM family protein ezrin in regulating antigen-dependent lipid raft coalescence in B cells. In this study, we addressed the possibility that ezrin may collaborate with other adaptor proteins to regulate signalosome dynamics at the membrane. Using mass spectrometry-based proteomics analysis, we identified Myo18aα as a novel binding partner of ezrin. Myo18aα is an attractive candidate as it has several protein-protein interaction domains and an intrinsic motor activity. The expression of Myo18aα varied during B cell development in the bone marrow and in mature B cell subsets suggesting functional differences. Interestingly, BCR stimulation increased the association between ezrin and Myo18aα, and induced co-segregation of Myo18aα with the BCR and phosphotyrosine-containing proteins. Our data raise an intriguing possibility that the Myo18aα/ezrin complex may facilitate BCR-mediated signaling by recruiting signaling proteins that are in close proximity of the antigen receptor. Our study is not only significant with respect to understanding the molecular regulation of BCR signaling but also provides a broader basis for understanding the mechanism of action of ezrin in other cellular systems.

Drosophila melanogaster myosin-18 represents a highly divergent motor with actin tethering properties

The gene encoding Drosophila myosin-18 is complex and can potentially yield six alternatively spliced mRNAs. One of the major features of this myosin is an N-terminal PDZ domain that is included in some of the predicted alternatively spliced products. To explore the biochemical properties of this protein, we engineered two minimal motor domain (MMD)-like constructs, one that contains the N-terminal PDZ (myosin-18 M-PDZ) domain and one that does not (myosin-18 M-ΔPDZ). These two constructs were expressed in the baculovirus/Sf9 system. The results suggest that Drosophila myosin-18 is highly divergent from most other myosins in the superfamily. Neither of the MMD constructs had an actin-activated MgATPase activity, nor did they even bind ATP. Both myosin-18 M-PDZ and M-ΔPDZ proteins bound to actin with K_d values of 2.61 and 1.04 μm, respectively, but only about 50–75% of the protein bound to actin even at high actin concentrations. Unbound proteins from these actin binding assays reiterated the 60% saturation maximum, suggesting an equilibrium between actin-binding and non-actin-binding conformations of Drosophila myosin-18 in vitro. Neither the binding affinity nor the substoichiometric binding was significantly affected by ATP. Optical trapping of single molecules in three-bead assays showed short lived interactions of the myosin-18 motors with actin filaments. Combined, these data suggest that this highly divergent motor may function as an actin tethering protein.

Identification of MYO18A as a novel interacting partner of the PAK2/betaPIX/GIT1 complex and its potential function in modulating epithelial cell migration

MYO18A is found as a novel PAK2 binding partner via βPIX/GIT1. MYO18A-depleted cells showed dramatic changes in shape, actin stress fiber and membrane ruffle formation, and displayed increases in the number and size of focal adhesions and a decrease in cell migration, suggesting an important role of MYO18A in regulating epithelial cell migration.

The p21-activated kinase (PAK) 2 is known to be involved in numerous biological functions, including the regulation of actin reorganization and cell motility. To better understand the mechanisms underlying this regulation, we herein used a proteomic approach to identify PAK2-interacting proteins in human epidermoid carcinoma A431 cells. We found that MYO18A, an emerging member of the myosin superfamily, is a novel PAK2 binding partner. Using a siRNA knockdown strategy and in vitro binding assay, we discovered that MYO18A binds to PAK2 through the βPIX/GIT1 complex. Under normal conditions, MYO18A and PAK2 colocalized in lamellipodia and membrane ruffles. Interestingly, knockdown of MYO18A in cells did not prevent formation of the PAK2/βPIX/GIT1 complex, but rather apparently changed its localization to focal adhesions. Moreover, MYO18A-depleted cells showed dramatic changes in morphology and actin stress fiber and membrane ruffle formation and displayed increases in the number and size of focal adhesions. Migration assays revealed that MYO18A-depleted cells had decreased cell motility, and reexpression of MYO18A restored their migration ability. Collectively, our findings indicate that MYO18A is a novel binding partner of the PAK2/βPIX/GIT1 complex and suggest that MYO18A may play an important role in regulating epithelial cell migration via affecting multiple cell machineries.

GOLPH3 bridges phosphatidylinositol-4- phosphate and actomyosin to stretch and shape the Golgi to promote budding

Golgi membranes, from yeast to humans, are uniquely enriched in phosphatidylinositol-4-phosphate (PtdIns(4)P), although the role of this lipid remains poorly understood. Using a proteomic lipid-binding screen, we identify the Golgi protein GOLPH3 (also called GPP34, GMx33, MIDAS, or yeast Vps74p) as a PtdIns(4)P-binding protein that depends on PtdIns(4)P for its Golgi localization. We further show that GOLPH3 binds the unconventional myosin MYO18A, thus connecting the Golgi to F-actin. We demonstrate that this linkage is necessary for normal Golgi trafficking and morphology. The evidence suggests that GOLPH3 binds to PtdIns(4)P-rich trans-Golgi membranes and MYO18A conveying a tensile force required for efficient tubule and vesicle formation. Consequently, this tensile force stretches the Golgi into the extended ribbon observed by fluorescence microscopy and the familiar flattened form observed by electron microscopy.

A tripartite complex containing MRCK modulates lamellar actomyosin retrograde flow

Actomyosin retrograde flow underlies the contraction essential for cell motility. Retrograde flow in both lamellipodia and lamella is required for membrane protrusion and for force generation by coupling to cell adhesion. We report that the Rac/Cdc42-binding kinase MRCK and myosin II-related MYO18A linked by the adaptor protein LRAP35a form a functional tripartite complex, which is responsible for the assembly of lamellar actomyosin bundles and of a subnuclear actomyosin network. LRAP35a binds independently to MYO18A and MRCK. This binding leads to MRCK activation and its phosphorylation of MYO18A, independently of ROK and MLCK. The MRCK complex moves in concert with the retrograde flow of actomyosin bundles, with MRCK being able to influence other flow components such as MYO2A. The promotion of persistent protrusive activity and inhibition of cell motility by the respective expression of wild-type and dominant-negative mutant components of the MRCK complex show it to be crucial to cell protrusion and migration.

The immortalized cell lines with differentiation potentials: their establishment and possible application

Approximately 200 types of the cells are qualified as differentiated cells in the human body. If these different types of cells can be separated from each other (or cloned) and obtained in sufficient quantity, it will be beneficial for studying development, morphogenesis, tissue maintenance, cancer and aging, and for reconstructing functional tissues in vitro for regenerative medicine. We produced the transgenic mouse and rat harboring SV40 T-antigen gene to make the immortalized cell lines in the primary tissue culture and succeeded in establishing many functionally active cell lines from various tissues. Many immortalized cell lines from various tissues are shown to exhibit the unique characteristics of tissue functions and they should be useful as an in vitro model of various tissues for physiological and pharmacological investigations. Future application of these cells to drug screening is discussed.

MYO18B interacts with the proteasomal subunit Sug1 and is degraded by the ubiquitin-proteasome pathway

MYO18B is a class XVIIIB unconventional myosin encoded by a candidate tumor suppressor gene. To gain insights into the cellular function of this protein, we searched for MYO18B-interacting proteins by a yeast two-hybrid screen. Sug1, a 19S regulator subunit of the 26S proteasome, was identified as a binding partner of the C-terminal tail region of MYO18B. The association of MYO18B with Sug1 was further confirmed by GST pull-down, co-immunoprecipitation, and immunocytochemistry. Furthermore, proteasome dysfunction by a proteasome inhibitor or siRNA-mediated knock-down of Sug1 caused the up-regulation of MYO18B protein and MYO18B was polyubiquitinated in vivo. Collectively, these results suggested that MYO18B is a substrate for proteasomal degradation.

Identification of the surfactant protein A receptor 210 as the unconventional myosin 18A

Mass spectrometric characterization of the surfactant protein A (SP-A) receptor 210 (SP-R210) led to the identification of myosin (Myo) XVIIIA and nonmuscle myosin IIA. Antibodies generated against the unique C-terminal tail of MyoXVIIIA revealed that MyoXVIIIA, MyoIIA, and SP-R210 have overlapping tissue distribution, all being highly expressed in myeloid cells, bone marrow, spleen, lymph nodes, and lung. Western blot analysis of COS-1 cells stably transfected with either MyoXVIIIA or MyoIIA indicated that SP-R210 antibodies recognize MyoXVIIIA. Furthermore, MyoXVIIIA but not MyoIIA localized to the surface of COS-1 cells, and most importantly, expression of MyoXVIIIA in COS-1 cells conferred SP-A binding. Western analysis of recombinant MyoXVIIIA domains expressed in bacteria mapped the epitopes of previously derived SP-R210 antibodies to the neck region of MyoXVIIIA. Antibodies raised against the neck domain of MyoXVIIIA blocked the binding of SP-A to macrophages. Together, these findings indicate that MyoXVIIIA constitutes a novel receptor for SP-A.