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. We have partially purified a Cdc42- and Rac1-associated protein with molecular mass of about 170 kDa (p170) from bovine brain cytosol. This protein interacted with guanosine 5'-(3-O-thio)triphosphate (GTPγS).glutathione S-transferase (GST)-Cdc42 and GTPγS-GST-Rac1 but not with the GDP-GST-Cdc42, GDP-GST- Rac1, or GTPγS-GST-RhoA). We identified p170 as an IQGAP, which is originally identified as a putative Ras GTPase-activating protein. Recombinant IQGAP specifically interacted with GTPγS-Cdc42 and GTPγS-Rac1. The C-terminal fragment of IQGAP was responsible for their interactions. IQGAP was specifically immunoprecipitated with dominant-active Cdc42(val)/¹² or Rac1(val)/¹² from the COS7 cells expressing Cdc42(val)/¹² or Rac1(val)/¹, respectively. Immunofluorescence analysis revealed that IQGAP was accumulated at insulin- or Rac1-induced membrane ruffling areas. This accumulation of IQGAP was blocked by the microinjection of the dominant-negative Rac1(Asn)/¹⁷ or Cdc42(Asn)/¹⁷. Moreover, IQGAP was accumulated at the cell-cell junction in MDCK cells, where α-catenin and ZO-1 were localized. These results suggest that IQGAP is a novel target molecule for Cdc42 and Rac1.