GAPA is an IQGAP-related protein and is involved in Dictyostelium cytokinesis. Since mammalian IQ-GAPs are effectors for Rac/Cdc42, GAPA is also predicted to bind to small GTPases, which are to be identified. In this study, mutant GAPAs were examined for functions in cytokinesis by genetic complementation of gapA- cells. Positively charged side chains of Arg442 and Lys474 of GAPA, predicted to be present on the surface of interaction with small GTPases, were found to be essential, suggesting an interaction between GAPA and putative small GTPase in cytokinesis. Also, results from truncated GAPAs indicated that almost the entire region of GAPA homologous to IQGAP is required for cytokinesis in Dictyostelium.