Phosphorylation-Dependent Regulation of Septin Dynamics during the Cell Cycle

Abstract

Septins are GTPases involved in cytokinesis. In yeast, they form a ring at the cleavage site. Using FRAP, we show that septins are mobile within the ring at bud emergence and telophase and are immobile during S, G2, and M phases. Immobilization of the septins is dependent on both Cla4, a PAK-like kinase, and Gin4, a septin-dependent kinase that can phosphorylate the septin Shs1/Sep7. Induction of septin ring dynamics in telophase is triggered by the translocation of Rts1, a kinetochore-associated regulatory subunit of PP2A phosphatase, to the bud neck and correlates with Rts1-dependent dephosphorylation of Shs1. In rts1-Delta cells, the actomyosin ring contracts properly but cytokinesis fails. Together our results implicate septins in a late step of cytokinesis and indicate that proper regulation of septin dynamics, possibly through the control of their phosphorylation state, is required for the completion of cytokinesis.

Stable and Dynamic Axes of Polarity Use Distinct Formin Isoforms in Budding Yeast

“…The septin dependence of Bnr1p provides a likely explanation for the synthetic lethality between defects in Bni1p function and septins (Longtine et al, 1996), which recapitulate the lethality of the simultaneous loss of Bni1p-and Bnr1p-dependent cable assembly ( Figure 5A). Septindependent recruitment of Bnr1p also might explain the delay in Bnr1p recruitment to the bud neck until after bud emergence, because during this transition the septins undergo significant changes in their organization (reviewed in Gladfelter et al, 2001; recent results in Caviston et al, 2003;Dobbelaere et al, 2003).…”

Some assembly required: yeast septins provide the instruction manual

“…In S. cerevisiae in G1, an apparent cap, or patch, of septins in which the subunits are highly mobile as judged by fluorescence recovery after photobleaching (FRAP) experiments [47,48] accumulates in the juxta-membrane region immediately subtending the incipient bud site. Septins seem to be more concentrated at the edge of this patch than at its center.…”

“…Phosphorylation by two protein kinases, Cla4 [28,[66][67][68] and Gin4 [39,48,69], plays a prominent role in initiating and/or stabilizing filament assembly during collar formation during emergence of the bud. Cla4 is a clear-cut ortholog of mammalian p21-activated protein kinases (PAKs), and the closest mammalian relative of Gin4 is the neuron-specific, AMPK-related, kinase-family member, BRSK1.…”

“…At the end of the cell cycle in budding yeast, during cytokinesis, septin-collar scission into separate rings correlates with dephosphorylation of Shs1 by phosphoprotein phosphatase 2A (PP2A), specifically an isoform that contains Rts1, a homolog of the mammalian B′-type regulatory subunit [48]. Also, direct phosphorylation by Cdc28, an ortholog of mammalian cyclin-dependent kinase 1 (Cdk1/Cdc2), of the extreme C-terminus of Cdc3 has been implicated in septin disassembly [72].…”

Do septins have a role in cancer?

“…They all have a central P loop-based GTP-binding domain and there is evidence that septins form hetero-and homo-oligomeric structures and can form filaments, although their significance remains uncertain (Hall and Russell, 2004). However, post translational modifications such as phosphorylation (Dobbelaere et al, 2003) and protein -protein interactions (Casamayor and Snyder, 2003) are crucial to septin function. They have been suggested to form scaffold-like structures upon which other proteins bind, thus allowing proper spatial and temporal control of processes such as polarity determination and cytokinesis.…”