An IQGAP-related protein controls actin-ring formation and cytokinesis in yeast.
BACKGROUND
Proteins of the IQGAP family have been identified as candidate effectors 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 amino-terminal 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. IQG1 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.
Epp
J A
JA
Department of Molecular and Cellular Biology Harvard University 7 Divinity Avenue, Cambridge, Massachusetts 02138, USA.
Chant
J
J
D008841:Actin Cytoskeleton / Q000378:metabolism; D000199:Actins / Q000378:metabolism*; D000595:Amino Acid Sequence; D002455:Cell Division / Q000502:physiology*; D005656:Fungal Proteins / Q000235:genetics / Q000502:physiology*; D020558:GTP Phosphohydrolases / Q000235:genetics / Q000378:metabolism; D020690:GTPase-Activating Proteins; D015870:Gene Expression; D008969:Molecular Sequence Data; D011506:Proteins / Q000737:chemistry*; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000254:growth & development / Q000502:physiology*; D017386:Sequence Homology, Amino Acid
D000199:Actins; D005656:Fungal Proteins; D020690:GTPase-Activating Proteins; D011506:Proteins; D020558:GTP Phosphohydrolases