Septins spin in a polarizing microscope (top) as septin rings mature (bottom).

VRABIOIU/MACMILLAN

Septin filaments in yeast do a right-angled turn in the middle of cell division, according to Alina Vrabioiu and Tim Mitchison (Harvard Medical School, Boston, MA). The filaments initially align parallel to the spindle axis but then rotate 90° to form two circumferential rings that flank the cytokinetic furrow.

Septin filaments help cells divide, but just how they do so has remained mysterious. The direction of septin-dependent striations varied between experiments, plus the striations may have been a pattern set up by proteins that bind septins, not the septins themselves.

Now, the Boston team has directly monitored the direction of septin filaments. They attached a green fluorescent protein (GFP) to septin by linking together two rigid α-helices. Polymerizing the septin–GFP molecules in a filament lined up all the GFPs. Polarized light would effectively excite these GFPs only when the light's electromagnetic oscillations were aligned with the GFPs' dipoles—the direction along which an excited electron preferentially moves to the higher energy state.

The team established in vitro what direction of polarized light best excited a filament of known orientation. Applying this to in vivo data, in which polarized light excited septin–GFP in cells whose orientation was carefully controlled, they could deduce the direction of septin filaments in vivo.

The septins are initially aligned parallel to the spindle axis, in an hourglass shape that spans the bud neck. Dephosphorylation has been implicated in reshaping the septins into two rings; if it acts selectively in the middle of the hourglass it might detach septins at one end with the other end providing a pivot point. The force driving turning might then come from membrane insertion between the two rings.

Reference:

Vrabioiu, A.M., and T.J. Mitchison.
2006
.
Nature.
443
:
466
–469.