G1 cyclin Cln3 plays a key role in linking cell growth and proliferation in budding yeast. It is generally assumed that Cln3, which is present throughout G1, accumulates passively in the nucleus until a threshold is reached to trigger cell cycle entry. We show here that Cln3 is retained bound to the ER in early G1 cells. ER retention requires binding of Cln3 to the cyclin-dependent kinase Cdc28, a fraction of which also associates to the ER. Cln3 contains a chaperone-regulatory Ji domain that counteracts Ydj1, a J chaperone essential for ER release and nuclear accumulation of Cln3 in late G1. Finally, Ydj1 is limiting for release of Cln3 and timely entry into the cell cycle. As protein synthesis and ribosome assembly rates compromise chaperone availability, we hypothesize that Ydj1 transmits growth capacity information to the cell cycle for setting efficient size/ploidy ratios.

The G1 cyclin Cln3 is a key activator of cell-cycle entry in budding yeast. Here we show that Whi3, a negative G1 regulator of Cln3, interacts in vivo with the cyclin-dependent kinase Cdc28 and regulates its localization in the cell. Efficient interaction with Cdc28 depends on an N-terminal domain of Whi3 that is also required for cytoplasmic localization of Cdc28, and for proper regulation of G1 length and filamentous growth. On the other hand, nuclear accumulation of Cdc28 requires the nuclear localization signal of Cln3, which is also found in Whi3 complexes. Both Cln3 and Cdc28 are mainly cytoplasmic during early G1, and become nuclear in late G1. However, Whi3-deficient cells show a distinct nuclear accumulation of Cln3 and Cdc28 already in early G1. We propose that Whi3 constitutes a cytoplasmic retention device for Cln3-Cdc28 complexes, thus defining a key G1 event in yeast cells.

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 G 1 . 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.