Spatial and temporal organization of protein-modifying enzymes at the bud neck of Saccharomyces cerevisiae. Gene products known or predicted to have the capacity to modify other proteins and that have been visualized at the bud neck by fluorescence microscopy are listed along with the stages of the mitotic division cycle at which they are found at the neck (orange bars), and the region of the bud neck to which they localize (green), where known. Also indicated are the time when emergence of the bud first becomes visible (dashed lined) and the time period corresponding to disassembly of the mitotic spindle and completion of the septum (grey bar). Smt3 is the yeast ortholog of SUMO. It should be noted that this list does not include certain enzymes known to act on septins with important functional consequences (e.g, Cla4 [10]) that do not stably associate with the bud neck, and instead localize to, but quickly depart from, the future site of bud emergence [78]. See Table 1 for citations of the appropriate supporting literature. Adapted from [60] with permission from Elsevier.

Models for septin organization and diffusion barrier function in the collar and split rings assemblies at the yeast bud neck. This model, based on experimental observations and considerable speculation, illustrates views of the mother-bud neck from a position within the bud (approximated by the gray plane), showing the plasma membrane (orange), globular septin G domains (white balls), and non-septin proteins (blue, green) integral to the plasma membrane and restricted to discrete cortical domains via septin-based diffusion barriers (e.g., Sec3 [6,7]). Prior to cytokinesis, the septins at the bud neck comprise a filamentous collar (left view), retaining Sec3 in the bud (blue). The beginning of cytokinesis is marked by splitting of the collar into two discrete rings (right view), followed by septin-dependent accumulation of Sec3 (green) and other cytokinesis factors within a cortical neck compartment, concomitant with actomyosin ring contraction and growth of the chitinous septum. During this transition, the C-terminal extensions (wavy lines) projecting orthogonally from the filaments in the collar rotate 90°, allowing for greater side-by-side compaction of the filaments.

Model for major transitions in septin assembly and modification state during the yeast budding cycle. Subcellular septin localization (green) during the cycle cycle is accompanied by changes in the organization and covalent modification of septin subunits (grey and white balls). (1) In the G1 phase, hetero-octamers of septin subunits (gray balls) within the "old" ring persisting from the previous cell division are subject to phosphorylation (brown dots) by G1 cyclin-activated cyclin-dependent kinases (Cdks). This modification on certain subunits (e.g., Cdc3 [50]) promotes dissolution of the old ring, permitting relocalization to a new ring at the next budding site. Newly translated septin polypeptides fold, bind GTP, and assemble into sub-octameric complexes (both Cdc11—Cdc12—Cdc3—Cdc10 and Shs1—Cdc12—Cdc3—Cdc10 tetramers, in this model; white balls) that remain stably associated throughout the lifetime of the proteins. Co-incorporation of pre-existing and newly-synthesized subcomplexes precedes (2) phosphorylation by Cla4 (purple dots) of certain subunits (e.g., Cdc10 [10]), which promotes assembly into an organized array of filaments at the neck of the emerging bud. (3) Prior to cytokinesis, SUMO (blue hexagons) is attached to certain subunits in a Siz1- and Siz2-dependent manner only on the mother side of the neck [63,76,77]. (4) During mitosis, septin phosphorylation (orange dots) by mother-side Gin4 (and presumably by its sister protein kinase Kcc4 on the bud side) promotes splitting of the septin collar. (5) Following the completion of cytokinesis and cell separation, septin filaments disassemble into hetero-octamers; residual ring-like septin deposition may reflect persistent self-reinforcing organization of PtdIns4,5P₂ and septin-binding transmembrane proteins at the cell cortex. Note that removal of each septin modification upon completion of the preceding transition is speculative, but consistent with the role ascribed, for example, to the action of the Rts1-containing isoform of PP2A [53], and with the ability of old and new subunits to populate all septin-containing structures in a given cell [42].