The budding yeast lyt1 mutation causes cell lysis, We report here that lyt1
is an allele of cdc15, a gene which encodes a protein kinase that function
s late in the cell cycle, Neither cdc15-1 nor cdc15-lyt1 strains are able t
o septate at 37 degrees C, even though they may manage to rebud. Cells lyse
after a shmoo-like projection appears at the distal pole of the daughter c
ell. Actin polarizes towards the distal pole but the septins remain at the
mother-daughter neck. This morphogenetic response reflects entry into a new
round of the cell cycle: the preference for polarization from the distal p
ole was lost in bud1 cdc15 double mutants; double cdc15-lyt1 cdc28-4 mutant
s, defective for START, did not develop apical projections and apical polar
ization was accompanied by DNA replication. The same phenomena were caused
by mutations in the genes CDC14, DBF2, and TEM1, which are functionally rel
ated to CDC15. Apical polarization was delayed in cdc15 mutants as compared
with budding in control cells and this delay was abolished in a septin mut
ant. Our results suggest that the delayed M/G1 transition in cdc15 mutants
is due to a septin-dependent checkpoint that couples initiation of the cell
cycle to the completion of cytokinesis.