IDENTIFICATION OF IQGAP AS A PUTATIVE TARGET FOR THE SMALL GTPASES, CDC42 AND RAC1

Cdc42 and Rac1 have been implicated in the regulation of various cell functions such as cell morphology, polarity, and cell proliferation. W e have partially purified a Cdc42- and Rac1-associated protein with mo lecular mass of about 170 kDa (p170) from bovine brain cytosol. This p rotein interacted with guanosine 5'-(3-O-thio)triphosphate (GTP gamma S)-glutathione S-transferase (GST)-Cdc42 and GTP gamma-GST-Rac1, or GT P gamma S-GST-RhoA). We identified p170 as an IQGAP, which is original ly identified as putative Ras GTPase-activating protein. Recombinant I QGAP specifically interacted with GTP gamma S-Cdc42 and GTP gamma S-Ra c1. The C-terminal fragment of IQGAP was specifically immunoprecipitat ed with dominant-active Cdc42(Val12) or Rac1(Val12) from the COS7 cell s expressing Cdc42(Val12) or Rac1(Val12), respectively, Immunofluoresc ence analysis revealed that IQGAP was accumulated at insulin- or Rac1- induced membrane ruffling areas. This accumulation of IQGAP was blocke d by the microinjection of the dominant-negative Rac1(Asn17) or Cdc42( Asn17). Moreover, IQGAP was accumulated at the cell-cell junction in M DCK cells, where alpha-catenin and ZO-1 were localized. These results suggest that IQGAP is a novel target molecule for Cdc42 and Rac1.