The ras-like GTP binding proteins cdc42, rac, and rho regulate diverse cellular processes including cell growth and actin remodeling associated with changes in cell morphology, growth, adhesion, and motility (1-4). In fibroblasts, cdc42 regulates actin polymerization and focal complexes necessary for filopodia formation, rac mediates actin polymerization and focal complex assembly within lamellipodia and membrane ruffles, and rho induces actin stress fiber and focal adhesion (FA) complex formation (5). A hierarchical relationship exists among cdc42, rac, and rho, whereby cdc42 regulates rac activity and rac regulates rho activity, suggesting that these proteins may orchestrate the spatial and temporal changes in the actin cytoskeleton necessary for cell movement (5, 6). cdc42 and rac also regulate activation of the c-Jun N-terminal kinase/stressactivated kinase via a mitogen-activated protein (MAP) kinase pathway (7-9), and rac and rho are essential for ras transformation (10, 11). cdc42, rho, and rac all appear to stimulate c-fos transcription (12), as well as cell cycle progression through GI and subsequent DNA synthesis (9). The activation state of ras-like GTP binding proteins is positively regulated by guanine nucleotide exchange factors (GEFs) that promote the exchange of GDP for GTP, and negatively by GTPase activating proteins (2). A number of putative GEFs for rho-like GTPases have been identified by sequence comparison (2), and several of these demonstrate GEF activity in vitro. The dbl and ost oncogene products have cdc42 and rho GEF activity (13-15); the lbc oncogene product has rho GEF activity (16); the invasion-inducing Tiaml gene product has cdc42, rho, and rac GEF activity (17); and the yeast CDC24 gene product has cdc42 GEF activity (18). LAR is a broadly expressed transmembrane protein tyrosine phosphatase (PTPase) comprised of a cell adhesion-like extracellular region and two intracellular PTPase domains (19)(20)(21)(22). A role for LAR in regulating cell-matrix interactions was proposed, as LAR colocalizes with a coiled-coil protein, termed LAR interacting protein 1 (LIP.1) at the ends of FAs (23), and LAR expression was observed at regions of association between cells and basement membrane in various tissues (19). To identify putative substrates and other proteins involved in LAR-mediated signal transduction, we screened for proteins that bind the LAR PTPase domains using the interaction-trap assay and coimmunoprecipitation studies. A protein thus isolated is a novel multidomain GEF we have named Trio because it contains three enzyme domains: two GEF domains, one of which has racl GEF activity and the other has rhoA GEF activity, and a serine/threonine kinase (PSK) domain. In addition, Trio contains four N-terminal spectrinlike domains, two pleckstrin-like domains, and an Ig-like domain. Because proteins with cdc42, rac, or rho GEF activity are generally involved in regulating cytoskeletal organization (1), it is likely that Trio in conjunction with LAR plays a key role in coordinating the ...

No abstract

Focal adhesions are sites of cell‐extracellular matrix interactions that function in anchoring stress fibers to the plasma membrane and in adhesion‐mediated signal transduction. Both focal adhesion structure and signaling ability involve protein tyrosine phosphorylation. LAR is a broadly expressed transmembrane protein tyrosine phosphatase comprised of a cell adhesion‐like ectodomain and two intracellular protein tyrosine phosphatase domains. We have identified a novel cytoplasmic 160 kDa phosphoserine protein termed LAR‐interacting protein 1 (LIP.1), which binds to the LAR membrane‐distal D2 protein tyrosine phosphatase domain and appears to localize LAR to focal adhesions. Both LAR and LIP.1 decorate the ends of focal adhesions most proximal to the cell nucleus and are excluded from the distal ends of focal adhesions, thus localizing to regions of focal adhesions presumably undergoing disassembly. We propose that LAR and LIP.1 may regulate the disassembly of focal adhesions and thus help orchestrate cell‐matrix interactions.

Interaction with the multi-PDZ protein GRIP is required for the synaptic targeting of AMPA receptors, but the underlying mechanism is unknown. We show that GRIP binds to the liprin-alpha/SYD2 family of proteins that interact with LAR receptor protein tyrosine phosphatases (LAR-RPTPs) and that are implicated in presynaptic development. In neurons, liprin-alpha and LAR-RPTP are enriched at synapses and coimmunoprecipitate with GRIP and AMPA receptors. Dominant-negative constructs that interfere with the GRIP-liprin interaction disrupt the surface expression and dendritic clustering of AMPA receptors in cultured neurons. Thus, by mediating the targeting of liprin/GRIP-associated proteins, liprin-alpha is important for postsynaptic as well as presynaptic maturation.