Sequestration of the exocytic SNARE Psy1 into multiprotein nodes reinforces polarized morphogenesis in fission yeast

Polarized morphogenesis is achieved by targeting or inhibiting growth at distinct regions. Rod-shaped fission yeast cells grow exclusively at their ends by restricting exocytosis and secretion to these sites. This growth pattern implies the existence of mechanisms that prevent exocytosis and growth along non-growing cell sides. We previously identified a set of 50-100 megadalton-sized node structures along the sides of fission yeast cells that contain the interacting proteins Skb1 and Slf1. Here, we show that Skb1-Slf1 nodes contain the syntaxin-like SNARE Psy1, which mediates exocytosis in fission yeast. Psy1 localizes in a diffuse pattern at cell tips where it likely promotes exocytosis and growth, but Psy1 is sequestered in Skb1-Slf1 nodes at cell sides where growth does not occur. Mutations that prevent node assembly or inhibit Psy1 localization to nodes lead to aberrant exocytosis at cell sides and increased cell width. Genetic results indicate that this Psy1 node mechanism acts in parallel to actin cables and Cdc42 regulation. Our work suggests that sequestration of syntaxin-like Psy1 at non-growing regions of the cell cortex reinforces cell morphology by restricting exocytosis to proper sites of polarized growth.