Figure 1

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.