An IQGAP-related protein controls actin-ring formation and cytokinesis in yeast

Background: Proteins of the IQGAP family have been identified as candidate effecters for the Rho family of GTPases; however, little is known about their cellular functions. The domain structures of IQGAP family members make them excellent candidates as regulators of the cytoskeleton: their sequences include an actin-binding domain homologous to that found in calponin, IQ motifs for interaction with calmodulin, and a GTPase-binding domain. Results: The genomic sequence of Saccharomyces cerevisiae revealed a single gene encoding an IQGAP family member (denoted IQGAP-related protein: Iqg1). Iqg1 and IQGAPs share similarity along their entire length, with an aminoterminal calponin-homology (CH) domain, IQ repeats, and a conserved carboxyl terminus. In contrast to IQGAPs, Iqg1 lacks an identifiable GAP motif, a WW domain, and IR repeats, although the functions of these domains in IQGAPs are not well defined. Deletion of the IQG1 gene resulted in lethality. Cellular defects included a deficiency in cytokinesis, altered actin organization, aberrant nuclear segregation, and cell lysis. The primary defect appeared to be a cytokinesis defect, and the other problems possibly arose as a consequence of this initial defect. Consistent with a role in cytokinesis, Iqg1 co-localizes with an actin ring encircling the mother-bud neck late in the cell cycle-a putative cytokinetic ring. lQG1 overexpression resulted in premature actin-ring formation, suggesting that Iqg1 activity temporally controls formation of this structure during the cell cycle. Conclusions: Yeast IQGAP-related protein, Iqg1, is an important regulator of cellular morphogenesis, inducing actin-ring formation in association with cytokinesis.