Cyk3 acts in actomyosin ring independent cytokinesis by recruiting Inn1 to the yeast bud neck

Abstract

Cytokinesis in yeast can be achieved by plasma membrane ingression, which is dependent on actomyosin ring constriction. Inn1 presumably couples these processes by interaction with both the plasma membrane and the temporary actomyosin ring component Hof1. In addition, an actomyosin ring independent cytokinesis pathway exists in yeast. We here identified Cyk3, a key component of the alternative pathway, as a novel interaction partner of Inn1. The carboxy-terminal proline rich part of Inn1 binds the SH3 domains of either Cyk3 or Hof1. Strains with truncated proteins lacking either of these SH3 domains do not display any severe phenotypes, but are synthetically lethal, demonstrating their crucial role in cytokinesis. Overexpression of CYK3 leads to an actomyosin ring independent recruitment of Inn1 to the bud neck, further supporting the significance of this interaction in vivo. Moreover, overexpression of CYK3 in a myo1 or an iqg1 deletion not only restores viability, but also the recruitment of Inn1 to the bud neck. We propose that Cyk3 is part of an actomyosin ring independent cytokinesis pathway, which acts as a rescue mechanism to recruit Inn1 to the bud neck.

Cdk1-dependent phosphorylation of Iqg1 governs actomyosin ring assembly prior to cytokinesis

“…Cyk3 provides an appealing candidate. Overexpression of Cyk3 can rescue defects in both AMR and primary septum pathways, and induces premature Inn1 localization (Jendretzki et al, 2009). Cyk3 is also a Cdk1 substrate in vivo (Holt et al, 2009).…”

“…The two processes are at once physically distinct, mutually interdependent and partially redundant, a relationship that invites and facilitates more detailed molecular investigation. Genetic studies have illuminated the dependencies underlying sequential protein recruitment to the bud neck (Jendretzki et al, 2009;Meitinger et al, 2010;Roh et al, 2002), but information on the temporal regulation of these components remains limited.…”

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.…”

“…The machinery that orchestrates AMR contraction, plasma membrane ingression, and primary septum formation is only partially understood. Recently, Inn1 has been suggested to couple ring contraction to plasma membrane ingression (Sanchez -Diaz et al, 2008 ;Jendretzki et al , 2009 ). The proline-rich region of Inn1 binds to the SH3 domain of the transient actomyosin component Hof1, an F-BAR protein, which appears at the septin ring already early during cell cycle (Nishihama et al , 2009 ).…”

“…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 ).…”