Identification of Yeast IQGAP (Iqg1p) as an Anaphase-Promoting-Complex Substrate and Its Role in Actomyosin-Ring-Independent Cytokinesis

Abstract

In the yeast Saccharomyces cerevisiae, a ring of myosin II forms in a septin-dependent manner at the budding site in late G1. This ring remains at the bud neck until the onset of cytokinesis, when actin is recruited to it. The actomyosin ring then contracts, septum formation occurs concurrently, and cytokinesis is soon completed. Deletion of MYO1 (the only myosin II gene) is lethal on rich medium in the W303 strain background and causes slow-growth and delayed-cell-separation phenotypes in the S288C strain background. These phenotypes can be suppressed by deletions of genes encoding nonessential components of the anaphase-promoting complex (APC/C). This suppression does not seem to result simply from a delay in mitotic exit, because overexpression of a nondegradable mitotic cyclin does not suppress the same phenotypes. Overexpression of either IQG1 or CYK3 also suppresses the myo1Delta phenotypes, and Iqg1p (an IQGAP protein) is increased in abundance and abnormally persistent after cytokinesis in APC/C mutants. In vitro assays showed that Iqg1p is ubiquitinated directly by APC/C(Cdh1) via a novel recognition sequence. A nondegradable Iqg1p (lacking this recognition sequence) can suppress the myo1Delta phenotypes even when expressed at relatively low levels. Together, the data suggest that compromise of APC/C function allows the accumulation of Iqg1p, which then promotes actomyosin-ring-independent cytokinesis at least in part by activation of Cyk3p.

Mechanisms of cytokinesis in budding yeast

“…Finally, AMR disassembly is regulated by the anaphase promoting complex/cyclosome (APC/C)-mediated degradation of Iqg1 [Ko et al, 2007;Tully et al, 2009]. APC/ C is an E3 ligase that ubiquitinates Iqg1 and targets it for degradation by the 26S proteasome [Ko et al, 2007;Tully et al, 2009].…”

“…APC/ C is an E3 ligase that ubiquitinates Iqg1 and targets it for degradation by the 26S proteasome [Ko et al, 2007;Tully et al, 2009]. In the absence of Cdh1 (an activator of the APC/C), Iqg1, Myo1, Mlc1, and Mlc2 all linger at the division site and also colocalize in ''ectopic patches'' for more than 10 min after AMR constriction [Tully et al, 2009].…”

“…Also required for AMR-independent cytokinesis [Epp and Chant, 1997;Shannon and Li, 1999;Boyne et al, 2000;Ko et al, 2007;Tully et al, 2009;Fang et al, 2010] Bni1 Formin Plays a major role in actin ring assembly [Vallen et al, 2000;Pruyne et al, 2004;Buttery et al, 2007] …”

Microcompartments within the yeast plasma membrane

“…They orchestrate cell division and act as a signaling platform for cytokinesis Vrabioiu and Mitchison , 2006 ;McMurray and Thorner , 2009a ;Roncero and Sanchez , 2010 AMR (Myo1, Mlc1, Act1) AMR assembles during early mitosis Myo1; type II myosin of the AMR; Mcl1, type V myosin, light chain of Myo1; Act1, forming filamentous actin Vallen et al , 2000 Iqg1 ( = Cyk1) Ring-like structure at the bud neck Essential for actomyosin formation but has also a cytokinetic function that is independent from the AMR; interacts with Myo1 independently of septins Epp and Chant , 1997 ;Ko et al , 2007 Cyk3 Ring-like structure at the bud neck Recruits Inn1 to the bud neck; when overexpressed, Inn1 recruitment is AMR independent Jendretzki et al , 2009 ;Meitinger et al , 2010 Inn1 Localizes to the bud neck during late mitosis Required for plasma membrane ingression concomitant with AMR constriction; binds to Hof1 and Cyk3 Jendretzki et al , 2009 ;Meitinger et al , 2010 Hof1 Bud neck at anaphase; initially forms a double ring that fuses after AMR constriction Binds to the formin Bnr1 and triggers AMR constriction; then, it relocates to the septin filaments Vallen et al , 2000 ;Meitinger et al , 2011 Formins (Bni1, Bnr1) Bni1 localizes at the bud tip during bud growth and later at the bud neck; Bnr1 stably associates with septins at the bud neck Promote actin polymerization in a Rho1 dependent manner.…”

“…In the late anaphase, Hof1 is phosphorylated by the components of the mitotic exit network (MEN; specifically by the Dbf2-Mob1 heterotrimer), which triggers its release from the septin filaments and a shift to the AMR, where it promotes ring contraction and membrane ingression (Meitinger et al , 2011 ). Besides, with Hof1, Inn1 also interacts with Cyk3, which might have a role in a cytokinesis pathway that is triggered by Iqg1 independently from the AMR (Ko et al , 2007 ;Jendretzki et al , 2009 ). Inn1-Cyk3 complex formation depends on the phosphorylation status of Inn1, which is controlled by Cdk1 and Cdc14 (Palani et al , 2012 ).…”