Cyclin Cln3 Is Retained at the ER and Released by the J Chaperone Ydj1 in Late G1 to Trigger Cell Cycle Entry

Abstract

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 critical size is set at a single-cell level by growth rate to attain homeostasis and adaptation

“…Cln3 and Ydj1 are both rate limiting for cell cycle entry 19,33 and, as expected, their overexpression decreased the average cell volume at Start (Supplementary Table S6). Both the slope of the correlation function and the coefficient of determination were also smaller in oCLN3 (Fig.…”

“…Cells deficient for Ydj1, the J-chaperone that releases the Cdc28-Cln3 complex from the ER to trigger Start 33 , were much larger than wild-type as expected but, more important, there was no correlation between cell volume at Start and volume growth rate in G1 (Fig. 4e).…”

“…Briefly, the Cdk-cyclin complex formed by Cln3 [16][17][18][19][20] and Cdc28 21 phosphorylates the transcriptional repressor Whi5 22,23 and activates sBF (swi6/swi4) and mBF (swi6/mbp1) transcription factors 14 to induce the G1/s regulon 15 and trigger cell cycle entry. Whi3 and Ydj1 act as negative and positive regulators, respectively, of release of cyclin Cln3 from the ER [31][32][33] to allow its accumulation in the nucleus. Perhaps, indirectly, Ydj1 could also be required for proper sBF and mBF function (dashed arrow).…”

A novel family of katanin-like 2 protein isoforms (KATNAL2), interacting with nucleotide-binding proteins Nubp1 and Nubp2, are key regulators of different MT-based processes in mammalian cells

“…Enlarged cell size and polyploidy have been linked [56]: it is thought that polyploidy does not affect the frequency of cell division [57] but rather, it may enlarge cell size by boosting the rate of ribosome biogenesis [58], which could provide an indicator of cellular growth and fitness for mitosis. Additionally, an overly stabilized and less dynamic MT network is a likely contributor to the acquisition of this abnormal size (up to 70 times larger than controls in our experiments).…”

Molecular basis of the functional distinction between Cln1 and Cln2 cyclins

“…The Cln3-Cdc28 complex is associated with the endoplasmic reticulum membrane until late G 1 , when it enters the nucleus to activate transcription. 11 The introduction of a nuclear export signal (NES) into Cln3 renders the Cln3-Cdc28 complex unable to trigger cell cycle initiation. As regards Cln2, it is present in both the nucleus and the cytoplasm.…”