Shs1 rounds out the septin repertoire

Shs1 rounds out the septin reper toire S eptins are a family of GTPases that assemble into filaments and direct a variety of cellular processes , including cytokinesis (1). Garcia et al. describe how the architecture of these filaments is modulated by phos-phorylation and by the incorporation of different septin subunits (2). Budding yeast express fi ve septins during mitosis, all of which accumulate in a collar around the bud neck that recruits proteins required for cell division. In vitro, four of these mitotic septins—Cdc3, Cdc10, Cdc11, and Cdc12—form a rod-shaped octamer with a Cdc11 subunit on each end. Interactions between the terminal Cdc11 subunits help polymerize these septin octamers into long filaments (3), and yeast lacking any of these four septins have severe diffi culty assembling a collar at the bud neck and completing cytokinesis (4). The fi fth mitotic septin, Shs1, isn't an essential protein, however, and its function in cytokinesis remains enigmatic. " Shs1 is one of the most phosphorylated septins, " says Eva Nogales, from the University of California, Berkeley. " So we thought it might have a regulatory role that could change the properties of septin fi laments. " Nogales and colleagues, led by graduate student Galo Garcia, found that Shs1 could substitute for Cdc11 in vitro and cap the ends of octameric septin rods containing Cdc3, Cdc10, and Cdc12 (2). Rather than polymerizing end-to-end to form long filaments, however, Shs1-containing octamers bundled together into large ring-shaped structures. " So Shs1 is a regulator of septin assembly and organization, " Nogales says. In budding yeast lacking Shs1, the remaining septins still accumulated between mother and daughter cells, but they formed disorganized, randomly oriented fi laments instead of continuous rings encircling the bud neck. The mutant yeast still complete cytokinesis, but, says Nogales, " they do it badly. The process becomes very slow. " Because all five septins normally colocalize at the bud neck, Galo Garcia wanted to see what would happen if he mixed different proportions of Shs1-and Cdc11-containing octamers in vitro. " I found that, as I titrated in more complexes containing Cdc11, the diameter [of the septin fi lament rings] increased, " Garcia recalls. This immediately put Garcia in mind of the hourglass-shaped septin collar that surrounds the yeast bud neck. " If you think of the hourglass structure as a series of stacked septin rings, " Garcia explains, " the rings in …