THE MULTIDOMAIN PROTEIN TRIO BINDS THE LAR TRANSMEMBRANE TYROSINE PHOSPHATASE, CONTAINS A PROTEIN-KINASE DOMAIN, AND HAS SEPARATE RAC-SPECIFIC AND RHO-SPECIFIC GUANINE-NUCLEOTIDE EXCHANGE FACTOR DOMAINS

rho-like GTP binding proteins play an essential role in regulating cel l growth and actin polymerization. These molecular switch are positive ly regulated by guanine nucleotide exchange factors (GEFs) that promot e the exchange of GDP for GTP. Using the interaction-trap assay to ide ntify candidate proteins that bind the cytoplasmic region of the LAR t ransmembrane protein tyrosine phosphatase (PTPase), we isolated a cDNA encoding a 2861-amino acid protein termed Trio that contains three en zyme domains: two functional GEF domains and a protein serine/threonin e kinase (PSK) domain. One of the Trio GEF domains (Trio GEF-D1) has r ac-specific GEF activity, while the other Trio GEF domain (Trio GEF-D2 ) has rho-specific activity. The C-terminal PSK domain is adjacent to an Ig-like domain and is most similar to calcium/calmodulin-dependent kinases, such as smooth muscle myosin light chain kinase which similar ly contains associated Ig-like domains. Near the N terminus, Trio has four spectrin-like repeats that may play a role in intracellular targe ting. Northern blot analysis indicates that Trio has a broad tissue di stribution. Trio appears to be phosphorylated only on serine residues, suggesting that Trio is not a LAR substrate, but rather that it forms a complex with LAR. As the LAR PTPase localizes to the ends of focal adhesions, we propose that LAR and the Trio GEF/PSK may orchestrate ce ll-matrix and cytoskeletal rearrangements necessary for cell migration .