Blocking Clb2p nuclear export delays mitotic exit and the isotropic
switch to promote filamentous growth.
Diego Rua, Stephen
Kron
Center for Molecular Oncology, University of Chicago, 924 E
57th St Rm 322, Chicago, IL 60637, USA
Saccharomyces cerevisiae can
perform a dimorphic switch from its vegetative yeast form to filamentous
growth (FG) when cultured on nitrogen-limited agar media. The polarized
bud shape, clustered cortical actin and mitotic delay characteristic of
FG are each consistent with deficient function of the Clb2/Cdc28 mitotic
cyclin-dependent kinase. CLB2 expression, Clb2p stability and Clb2/Cdc28
kinase activity are unchanged during FG. Strikingly, we find that Clb2p
in filamentous cells accumulates in the nucleus and persists into late
mitosis. We created a mutation in a Clb2p nuclear export sequence (NES)
and found that it is sufficient to induce constitutive FG. In cells also
lacking Clb1p, nuclear sequestration of Clb2p does not hinder anaphase
progression but delays mitotic exit. Inasmuch as Clb2p proteolysis is a
key aspect of cell cycle progression, blocking Clb2p nuclear export
prevents destruction of Clb2p and completion of telophase. Indeed, the
Clb2p NES mutant resembles the Clb2p destruction box mutant in delaying
cytokinesis and disassembly of the mitotic spindle, even though the
Clb2p NES mutant becomes polyubiquitinated. Furthermore, mutants lacking
cytoplasmic Clb2p fail to depolarize cortical actin on schedule in
mitosis, reflecting the marked decrease in hyperphosphorylated forms of
the PAK-like kinase Cla4p. Our data suggest down-regulation of Clb2p
export to delay mitotic exit and activation of Cla4p links cell cycle
progression to morphogenesis in FG.
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