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The Whi3 protein is associated with the endoplasmic reticulum,
interacts with Cdc28, the budding-yeast Cdk, binds
the mRNA of cyclin CLN3 and prevents accumulation of the
Cdc28-Cln3 in the nucleus until late G1. Besides its function
as a cell size regulator, Whi3 is strictly required for filamentous
growth. Here we show that emerging buds in Whi3-deficient
cells are considerably rounder than in wild-type cells, indicating
that Whi3 is required to maintain apical growth during S phase.
This defect was not suppressed by deletion of CLB2, which is
involved in switching from polar to isotropic bud growth, indicating
that the observed phenotype is not the result of Whi3
acting solely as a negative regulator of cyclin Clb2. However,
Cdc28 did not properly accumulate at the bud tip during S phase
in whi3Δ cells, and their elongation defects were suppressed by
CLN2 overexpression, suggesting a positive function for Whi3
in a Cdk-cyclin-dependent step required for apical growth.
Additionally, the actin cytoskeleton was perturbed in Whi3-
deficient cells, and WHI3 showed genetic interactions with actin
patch components. Our results point to Whi3 as a key modulator
of apical growth effectors to coordinate cell cycle events and
morphogenesis. We propose that Whi3 is required for the apical
localization of Cdc28-Cln1,2 complexes during bud growth and
thereby, to promote the activation of Cdc42 and its effectors in
the bud apex.
We thank Sònia Rius and Isis Navarro for their technical assistance. We gratefully acknowledge Yoshimi Takai, Enrico Cabib, Erin O’Shea, Michael Hall, Vladimir Voynov, Gerald Fink, Humberto Torres, María Molina, Barbara Winsor, Carlos Vázquez de Aldana, Kelly Tedrick and Gary Eitzen for yeast strains and plasmids. We also thank Jordi Torres and Carme Gallego for critically reading the manuscript. Thanks also go to the members of CYC lab for helpful discussions. This work was funded by the Ministry of Science and Innovation of Spain (Consolider-Ingenio 2010), Fundació La Caixa, and the European Union (FEDER). N.C. and F.F. are researchers of the Ramon y Cajal program. E.V. received a fellowship from Generalitat de Catalunya. |
