Switching septins spurs sporulation

Septins are a family of GTP-binding proteins that, in most fungi and animals, form hetero-octameric rods containing two copies each of four different family members. These rods can then assemble into higher-order structures that perform a variety of cellular functions, depending on their subunit composition. The human genome, for example, encodes 13 different septins expressed by specifi c cell types at specifi c times. How distinct family members affect the overall structure and function of septin complexes remains unclear, however. Garcia et al. examine how two meiosis-specific septins alter the properties of septin complexes in order to support budding yeast sporulation (1). Mitotic budding yeast express fi ve different septins that can assemble into two types of hetero-octamer containing a Cdc12–Cdc3–Cdc10–Cdc10–Cdc3–Cdc12 core capped at both ends by either Cdc11 or Shs1. In vitro, Cdc11-capped rods assemble into tightly paired filaments that are long and straight, whereas Shs1-capped oc-tamers polymerize into spirals and rings (2, 3). In vivo, the two types of octamer combine to form a collar at the bud neck that regulates cytokinesis. Under starvation conditions , however, diploid budding yeast undergo meiosis to produce four haploid spores. Meiotic cells express two additional septins— Spr3 and Spr28—that, together with Cdc3 and Cdc10, localize to horseshoe-shaped septin structures associated with the pros-pore membranes that form around each haploid nucleus (4). One possibility, says Jeremy Thorner, from the University of California, Berkeley, is that Spr3 and Spr28 replace the mitotic septins they most resemble—Cdc12 and Cdc11/Shs1, respectively—to form a meiosis-specifi c hetero-octamer with distinct structural and functional properties. To test this idea, Thorner and colleagues expressed and purifi ed different combinations of septins and found that, indeed, Spr28 and Spr3 can replace Cdc11 and Cdc12 to form stable Spr28–Spr3–Cdc3–Cdc10– Cdc10–Cdc3–Spr3–Spr28 hetero-octamers in vitro (1). Unlike mitotic septin octamers, these meiosis-specifi c complexes didn't assemble into higher order structures in solution. In the presence of PtdIns4,5P 2-containing membranes, however, the mei-otic octamers polymerized end-on-end to form ladder-like structures consisting of two long fi laments connected together by regularly spaced " rungs. " " PtdIns4,5P 2 can electrostatically attract the septins out of solution and help organize them on the surface of the membrane, " Thorner explains. Similar events may occur in vivo, since PtdIns4,5P 2 is enriched in the prospore membrane. The incorporation of Spr3 and Spr28 therefore affects the higher-order assembly of septin octamers. But do they alter septin function? Despite …