Septins (red) corral division proteins (Spa2; green) at the bud neck.

Barral/AAAS

Septin filaments in budding yeast make a corral that restricts certain division proteins to the site of cytokinesis, according to Jeroen Dobbelaere and Yves Barral (ETHZ, Zurich, Switzerland).

The corral is formed when the septin ring at the bud neck splits to form two parallel fences. The Swiss researchers had earlier noted that during this period septin becomes transiently mobile before reverting to a static organization.

They have now extended this investigation of mobility to other cytokinesis proteins. Three membrane-associated proteins involved in polarizing the actin cytoskeleton (Spa2), secretion (Sec3), and delivering cell wall to the division site (Chs2) were all extremely mobile at the division site, but did not diffuse to regions outside of the division site. Disruption of the septin barriers via a temperature-sensitive mutant led to rapid loss of the division proteins from the bud neck and cytokinesis failure.

This trapping function for septins contrasts with their function earlier in the budding yeast cell cycle, when septins act as a scaffold that recruits other division proteins. But the later, membrane-associated division proteins do not colocalize with septins. Instead, they bounce around between the two wall-like septin rings. The split rings of fission yeast septin and even the septin patches seen in mammalian cells may perform a similar function.

Dobbelaere suspects that the division proteins may be endocytosed and then delivered via vesicles to the site between the septin rings. The alternative is that membrane-associated proteins slip through the septin rings when the rings are flexible and then are trapped when the rings regain their solidity. ▪

Reference:

Dobbelaere, J., and Y. Barral.
2004
.
Science.
305
:
393
–396.