The protein
kinase Hsl1 binds to the yeast mitotic septins.
Björn Gullbrand (1),
Matthias Versele (1), Victor Cid (2), Jeremy Thorner (1)
(1) Molecular and Cell Biology, UC Berkeley, 401 Barker Hall, Berkeley, CA
94720 3202, USA (bjorng@uclink.berkeley.edu); (2) Departamento de Microbiologia
II, Universidad Complutense de Madrid, 28040 Madrid, Spain
Septins are
GTPases involved in cytokinesis and exocytosis and are found in fungi as well
as in animals. The five mitotic septins in Saccharomyces cerevisiae, Cdc3, Cdc10, Cdc11,
Cdc12 and Shs1, assemble in G1 to form a ring at the future bud
site. The septin ring is then maintained at the bud neck throughout the cell
cycle, serving as a scaffold for proteins involved in cytokinesis. Mutants with
a disturbed septin organisation are delayed in entry into mitosis. This delay
is mediated by Swe1, a Wee1-related protein kinase that downregulates Cdc28
bound to mitotic cyclins. In cells with a normal septin ring, Swe1 is recruited
to the ring where it becomes targeted for destruction. Hsl1, a large
Nim1-related protein kinase, is required for both the recruitment and efficient
degradation of Swe1. We are investigating how Hsl1 is recruited to the septin
ring and how Hsl1 is activated. We found that Hsl1 binds in vitro to Cdc10, Cdc11 and
Cdc12 but, surprisingly, not to Cdc3. A segment of the non-catalytic C-terminus
of Hsl1 was found to be important for both septin binding in vitro and bud neck
localisation, strongly suggesting that Hsl1 is recruited to the bud neck via a
direct septin interaction. Only Cdc12 could bind the N-terminal catalytic
domain of Hsl1. We speculate that as Hsl1 binds to the septins, its catalytic
and non-catalytic domains are separated, allowing activity. The C-terminal
coiled coil of the septins were found to be dispensable for Hsl1 binding.