Evidence that a septin diffusion barrier is dispensable for cytokinesis in budding yeast

Abstract

Septins are essential for cytokinesis in Saccharomyces cerevisiae, but their precise roles remain elusive. Currently, it is thought that before cytokinesis, the hourglass-shaped septin structure at the mother-bud neck acts as a scaffold for assembly of the actomyosin ring (AMR) and other cytokinesis factors. At the onset of cytokinesis, the septin hourglass splits to form a double ring that sandwiches the AMR and may function as diffusion barriers to restrict diffusible cytokinesis factors to the division site. Here, we show that in cells lacking the septin Cdc10 or the septin-associated protein Bud4, the septins form a ring-like structure at the mother-bud neck that fails to re-arrange into a double ring early in cytokinesis. Strikingly, AMR assembly and constriction, the localization of membrane-trafficking and extracellular-matrix-remodeling factors, cytokinesis, and cell-wall-septum formation all occur efficiently in cdc10Δ and bud4Δ mutants. Thus, diffusion barriers formed by the septin double ring do not appear to be critical for S. cerevisiae cytokinesis. However, an AMR mutation and a septin mutation have synergistic effects on cytokinesis and the localization of cytokinesis proteins, suggesting that tethering to the AMR and a septin diffusion barrier may function redundantly to localize proteins to the division site.

Mechanisms of cytokinesis in budding yeast

“…Thus, septins undergo cell cycle-triggered organizational changes but the underlying mechanisms remain largely unknown. Interestingly, in cdc10D cells, a septin ring/hourglass is assembled at the division site [Frazier et al, 1998;Versele et al, 2004;Wloka et al, 2011] but it disappears precisely at the onset of cytokinesis [Wloka et al, 2011], suggesting that Cdc10 is required for the assembly and/or maintenance of the septin double ring during cytokinesis [Wloka et al, 2011]. Bud4, an anillin-related protein involved in bud-site selection [Sanders and Herskowitz, 1996], is selectively required for the maintenance of the septin ring at the mother side during cytokinesis [Wloka et al, 2011].…”

“…Interestingly, in cdc10D cells, a septin ring/hourglass is assembled at the division site [Frazier et al, 1998;Versele et al, 2004;Wloka et al, 2011] but it disappears precisely at the onset of cytokinesis [Wloka et al, 2011], suggesting that Cdc10 is required for the assembly and/or maintenance of the septin double ring during cytokinesis [Wloka et al, 2011]. Bud4, an anillin-related protein involved in bud-site selection [Sanders and Herskowitz, 1996], is selectively required for the maintenance of the septin ring at the mother side during cytokinesis [Wloka et al, 2011]. Similarly, anillin in animal cells [Field et al, 2005;Maddox et al, 2005;Goldbach et al, 2010] and anillin-like proteins such as Mid2 in fission yeast [Berlin et al, 2003;Tasto et al, 2003] are also involved in septin localization and/or organization.…”

“…After the onset of cytokinesis, the septin double ring is thought to function as a diffusion barrier that cages diffusible factors at the division site to promote the completion of cytokinesis [Dobbelaere and Barral, 2004]. However, in cdc10D and bud4D cells where the septin double ring is defective, AMR assembly and constriction occurs efficiently [Wloka et al, 2011]. Strikingly, membrane trafficking components such as Myo2 (myosin-V) and Exo84 (an exocyst subunit) are delivered to the division site and constrict with the AMR in the mutant cells.…”

Vhs2 is a novel regulator of septin dynamics in budding yeast

“…32 This defect is tolerated by otherwise wild-type cells, and, accordingly, the persistence of the diffusion barrier formed by septins is not essential for execution of cytokinesis and for cell viability. 33 So, Vhs2 plays a role in mantaining septin structure stability after mitotic entry, but how Vhs2 participates in septin organization is not fully understood. Vhs2 could control septin structure stabilization by interfering with septin post-translational modifications.…”

Microcompartments within the yeast plasma membrane

“…The septin rings are essential for cytokinesis because they act as a permeability barrier, which prevents free diffusion of lipids and proteins between the mother cell and the daughter cell (McMurray and Thorner , 2009b ). However, the diffusion barrier is apparently not essential for yeast cytokinesis but rather it may recruit proteins to the bud neck, a function which is primarily exerted by the AMR (Wloka et al , 2011 ). By allowing specific factors to accumulate at the division site, they create a unique cortical compartment at the bud neck.…”