Cultured adherent cells divide on the substratum, leading to formation of the cell monolayer. However, how the orientation of this anchorage-dependent cell division is regulated remains unknown. We have previously shown that integrin-dependent adhesion orients the spindle parallel to the substratum, which ensures this anchorage-dependent cell division. Here, we show that phosphatidylinositol-3,4,5-triphosphate (PtdIns(3,4,5)P3) is essential for this spindle orientation control. In metaphase, PtdIns(3,4,5)P3 is accumulated in the midcortex in an integrin-dependent manner. Inhibition of phosphatidylinositol-3-OH kinase (PI(3)K) reduces the accumulation of PtdIns(3,4,5)P3 and induces spindle misorientation. Introduction of PtdIns(3,4,5)P3 to these cells restores the midcortical accumulation of PtdIns(3,4,5)P3 and proper spindle orientation. PI(3)K inhibition causes dynein-dependent spindle rotations along the z-axis, resulting in spindle misorientation. Moreover, dynactin, a dynein-binding partner, is accumulated in the midcortex in a PtdIns(3,4,5)P3-dependent manner. We propose that PtdIns(3,4,5)P3 directs dynein/dynactin-dependent pulling forces on spindles to the midcortex, and thereby orients the spindle parallel to the substratum.

Arp2/3 actin filament nucleating complex drives circumnavigation of cortical actin clusters during mitosis.

The spindle orientation is regulated by the interaction of astral microtubules with the cell cortex. We have previously shown that spindles in nonpolarized adherent cells are oriented parallel to the substratum by an actin cytoskeleton-and phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3]-dependent mechanism. Here, we show that Cdc42, a Rho family of small GTPases, has an essential role in this mechanism of spindle orientation by regulating both the actin cytoskeleton and PtdIns(3,4,5)P3. Knockdown of Cdc42 suppresses PI(3)K activity in M phase and induces spindle misorientation. Moreover, knockdown of Cdc42 disrupts the cortical actin structures in metaphase cells. Our results show that p21-activated kinase 2 (PAK2), a target of Cdc42 and/or Rac1, plays a key role in regulating actin reorganization and spindle orientation downstream from Cdc42. Surprisingly, PAK2 regulates spindle orientation in a kinase activity-independent manner. ␤Pix, a guanine nucleotide exchange factor for Rac1 and Cdc42, is shown to mediate this kinase-independent function of PAK2. This study thus demonstrates that spindle orientation in adherent cells is regulated by two distinct pathways downstream from Cdc42 and uncovers a novel role of the Cdc42-PAK2-␤Pix-actin pathway for this mechanism.Alignment of the mitotic spindles with a predetermined axis, which confines the plane of cell division, occurs in many types of cells and is crucial for morphogenesis and embryogenesis. Cell geometry (30,32,47), cell polarity (6,24,35), and cell-cell adhesions (20,22,48) are proposed to be the determinants for the axis of the spindles. In most cases, spindle alignment along the predetermined axis requires both astral microtubules and the actin cytoskeleton and is believed to involve dynein-dependent microtubule pulling forces functioning at the cell cortex (4, 12, 31).We have previously shown that in nonpolarized adherent cells, such as HeLa cells, integrin-mediated cell-substrate adhesion orients the spindles parallel to the substratum, which ensures that both daughter cells remain attached to the substrate after cell division (42). This mechanism requires the actin cytoskeleton, astral microtubules, the microtubule plusend-tracking protein EB1, and myosin X. Furthermore, our recent study has shown that the lipid second messenger phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3] is also essential to this mechanism. PtdIns(3,4,5)P3 is accumulated in the midcortex of metaphase cells, which is important for the localized accumulation of dynactin, a dynein-binding partner, at the midcortex. We have proposed that PtdIns(3,4,5)P3 directs dynein/dynactin-dependent pulling forces on the spindle to the midcortex and orients the spindle parallel to the substratum (43). However, the molecular mechanisms that regulate the actin cytoskeleton and PtdIns(3,4,5)P3 in the spindle orientation control remain unknown.The Rho family of GTPases, including Rho, Rac, and Cdc42, plays central roles in the regulation of not only the actin cytoskeleton but als...

Vinexin is a recently identified cytoskeletal protein and plays a key role in the regulation of cytoskeletal organization and signal transduction. Vinexin localizes at sites of cell-extracellular matrix adhesion in NIH3T3 fibroblasts and at sites of cell-cell contact in epithelial LLC-PK1 cells. Expression of vinexin promotes the formation of actin stress fiber, but the role of vinexin at sites of cell-cell contact is unclear. Here we identified lp-dlg/KIAA0583 as a novel binding partner for vinexin by using yeast two-hybrid screening. lp-dlg/KIAA0583 has a NH 2 -terminal coiled-coil-like domain, in addition to four PDZ domains, an Src homology (SH) 3 domain, and a guanylate kinase domain, which are conserved structures in membrane-associated guanylate kinase family proteins. The third SH3 domain of vinexin bound to the region between the second and third PDZ domain of lp-dlg, which contains a proline-rich sequence. lp-dlg colocalized with vinexin at sites of cell-cell contact in LLC-PK1 cells. Furthermore, lp-dlg colocalized with ␤-catenin, a major adherens junction protein, in LLC-PK1 cells. Co-immunoprecipitation experiments revealed that both endogenous and epitope-tagged deletion mutants of lp-dlg/KIAA0583 associated with ␤-catenin. We also showed that these three proteins could form a ternary complex. Together these findings suggest that lp-dlg/KIAA0583 is a novel scaffolding protein that can link the vinexin-vinculin complex and ␤-catenin at sites of cell-cell contact.Cell-cell adhesion is important for cell polarity, tissue morphogenesis development, and homeostasis (1-3). To this end, epithelial cells exhibit specialized structures involved in cell-cell contacts such as tight junctions and adherens junctions. Adherens junctions contain the transmembrane cell adhesion molecules, cadherins and nectins, which mediate the calcium-dependent and -independent cell-cell adhesion (1, 3, 4), respectively. The cytoplasmic domain of cadherin binds to ␤-catenin, which then binds to ␣-catenin. ␣-Catenin binds to actin and actin-binding proteins such as vinculin, ␣-actinin, and ZO-1, resulting in the link of cadherin to the actin cytoskeleton (3, 5, 6). The cytoplasmic domain of nectin binds to l-afadin, which then binds to actin and a vinculin-binding protein ponsin (4,7,8). Multiple protein complexes of these cytoplasmic proteins play important roles in communicating between cell adhesion systems, regulating cellcell adhesion, and transducing signals into cells.Vinexin is a protein localizing at cell-cell and cell-extracellular matrix junctions (9). There are at least two types of vinexin, vinexin ␣ and vinexin ␤, which share a common carboxyl-terminal sequence containing three SH (Src homology) 3 1 domains. The larger vinexin ␣ has an additional amino-terminal sequence containing a sorbin homology domain. Vinexin is a member of a novel adaptor protein family, including ArgBP2 and ponsin, all of which have a sorbin homology domain in the NH 2 -terminal half and three SH3 domains in the COOHterminal half (8 -12). Vin...

Extracellular signal-regulated kinase 1/2 (ERK1/2) is activated by various extracellular stimuli including growth factors and cytokines and plays a pivotal role in regulating cell proliferation and differentiation by phosphorylating nuclear transcription factors. Recently, it was reported that activated ERK1/2 also concentrates at adhesion sites and regulates cell spreading and migration. Vinexin is a focal adhesion protein regulating both cell spreading and growth factor signaling. We show here that vinexin was directly phosphorylated by ERK1/2 upon stimulation with growth factors. ERK1/2 phosphorylated the linker region of vinexin between the second and third SH3 domains. Site-directed mutagenesis revealed that ERK2 mainly phosphorylated the serine 189 residue of vinexin ␤. Furthermore, vinexin ␤ interacted with ERK1/2 both in vitro and in vivo. Vinexin interacted with the active but not inactive form of ERK1/2. A putative DEF (docking for ERK FXFP) domain located in the linker region of vinexin was required for the interaction with ERK1/2 and efficient phosphorylation of vinexin ␤ by ERK2. Finally, we showed that cell adhesion to fibronectin also induced the association of vinexin ␤ with ERK2 and the phosphorylation of vinexin ␤. Furthermore, vinexin and ERK were co-localized to the periphery of cells during cell spreading on fibronectin. Together, these results suggest that vinexin is a novel substrate of ERK2 and may play roles in ERK-dependent cell regulation during cell spreading as well as in growth factor-induced responses.Mitogen-activated protein kinase (MAPK) 1 consists of four subfamilies, extracellular signal-regulated kinase 1/2 (ERK1/ 2), c-Jun N-terminal kinase/stress-activated kinase, p38MAPK, and ERK5. ERK1/2 is mainly activated by various growth factors and regulates a diverse array of cellular events including cell proliferation, survival, and differentiation (1, 2). Many extracellular signals activate membrane receptors, including receptor tyrosine kinases, G protein-coupled receptors, or integrins, leading to the activation of Raf. Activated Raf, in turn, activates MEK (MAPK/ERK kinase) by the direct phosphorylation of dual serine residues. Activated MEK also directly phosphorylates and activates ERK1/2. Once activated, ERK1/2 enters the nucleus and phosphorylates nuclear transcription factors, including Elk-1, Sap1, c-Myc, and c-Fos (3).Recently, ERK has been shown to play crucial roles in regulating cell adhesion and cell migration independent of its nuclear functions (4 -7). ERK is involved in the migration of breast cancer cells stimulated by urokinase-type tissue plasminogen activator. Neither de novo gene transcription nor protein synthesis is required for this process (5). Activation of ERK is also suggested to be involved in the chemotaxis or the extension of pseudopodia, probably through the phosphorylation of cytoplasmic proteins (8, 9). Furthermore, ERK is well known to be activated by integrin engagement and to be localized to adhesion sites (7, 10 -14). Although some cytoplasm...