THE SMALL GTP-BINDING PROTEIN RHO BINDS TO AND ACTIVATES A 160-KDA SER THR PROTEIN-KINASE HOMOLOGOUS TO MYOTONIC-DYSTROPHY KINASE/

The small GTP-binding protein Rho functions as a molecular switch in t he formation of focal adhesions and stress fibers, cytokinesis and tra nscriptional activation. The biochemical mechanism underlying these ac tions remains unknown. Using a ligand overlay assay, we purified a 160 kDa platelet protein that bound specifically to GTP-bound Rho. This p rotein, p160, underwent autophosphorylation at its serine and threonin e residues and showed the kinase activity to exogenous substrates. Bot h activities were enhanced by the addition of GTP-bound Rho. A cDNA en coding p160 coded for a 1354 amino acid protein. This protein has a Se r/Thr kinase domain in its N-terminus, followed by a coiled-coil struc ture similar to 600 amino acids long, and a cysteine-rich zinc fingerl ike motif and a pleckstrin homology region in the C-terminus. The N-te rminus region including a kinase domain and a part of coiled-coil stru cture showed strong homology to myotonic dystrophy kinase over 500 res idues. When co-expressed with RhoA in COS cells, p160 was co-precipita ted with the expressed Rho and its kinase activity was activated, indi cating that p160 can associate physically and functionally with Rho bo th in vitro and in vivo.