Septins constitute a cytoskeletal structure that is conserved in eukaryotes
, In Saccharomyces cerevisiae, the Cdc3, Cdc10. Cdc11. Cdc12 and Shs1/Sep7
septins assemble as a ring that marks the cytokinetic plane throughout the
budding cycle, This structure participates in different aspects of morphoge
nesis, such as selection of cell polarity, localization of chitin synthesis
, the switch from hyperpolar to isotropic bud growth after bud emergence an
d the spatial regulation of septation. The septin cytoskeleton assembles at
the pre-bud site before bud emergence, remains there during bud growth and
duplicates at late mitosis eventually disappearing alter cell separation.
Using a septin-GFP fusion and time-lapse confocal microscopy, we have deter
mined that septin dynamics are maintained in budding zygotes and during uni
polar synchronous growth in pseudohyphae. By means of specific cell cycle a
rrests and deregulation of cell cycle controls we show that septin assembly
is dependent on G1 cyclin/Cdc28-mediated cell cycle signals and that the s
mall GTPase Cdc42, but not Rho1, are essential for this event. However, dur
ing bud growth, the septin ring shapes a bud-neck-spanning structure that i
s unaffected by failures in the regulation of mitosis, such as activation o
f the DNA repair or spindle assembly checkpoints or inactivation of the ana
phase-promoting complex (APC). At the end of the cell cycle, the splitting
of the ring into two independent structures depends on the function of the
mitotic exit network in which the protein phosphatase Cdc14 participates. O
ur data support a role of cell cycle control mechanisms in the regulation o
f septin dynamics to accurately coordinate morphogenesis throughout the bud
ding process in yeast.