Ostip2, a novel oncoprotein that associates with the Rho exchange factor Ost

The ost protooncogene encodes a guanine nucleotide exchange factor for the Rho family of small GTPases, RhoA and Cdc42. The N-terminal domain of Ost ( Ost-N) appears to negatively regulate the oncogenic activity of the protein , as deletion of this domain drastically increases its transforming activit y in NIH 3T3 cells. Using a yeast two-hybrid system, we identified five gen es encoding proteins that can interact with Ost-N. One of them, designated OSTIP2 (Ost interacting protein 2), encoded a previously uncharacterized pr otein. The OSTIP2 product is highly expressed in skeletal muscle as a 1.2-k b transcript. Full-length OSTIP2 cDNA contained an ORF of 193 amino acids. Transcription-coupled translation of OSTIP2 cDNA in reticulocyte lysates re vealed a protein product of 20 kDa, which corresponded to the predicted siz e of the protein. Bacterially expressed glutathione S-transferase (GST)-Ost ip2 fusion protein efficiently associated in vitro with baculovirus-express ed Ost. Interestingly, expression of Ostip2 in NIH 3T3 cells efficiently in duced foci of morphologically transformed cells. Moreover, inoculation of a thymic (nude) mice with OSTIP2 transfectants strongly induced tumor formati on. These results suggest that Ostip2 is a novel oncoprotein that can inter act with the Rho exchange factor Ost.