Comparison of the Saccharomyces cerevisiae G1 cyclins: Cln3 may be an upstream activator of Cln1, Cln2 and other cyclins.

Abstract

In the budding yeast Saccharomyces cerevisiae, the G1 cyclins Cln1, Cln2 and Cln3 regulate entry into the cell cycle (Start) by activating the Cdc28 protein kinase. We find that Cln3 is a much rarer protein than Cln1 or Cln2 and has a much weaker associated histone H1 kinase activity. Unlike Cln1 and Cln2, Cln3 is not significantly cell cycle regulated, nor is it down‐regulated by mating pheromone‐induced G1 arrest. An artificial burst of CLN3 expression early in G1 phase accelerates Start and rapidly induces at least five other cyclin genes (CLN1, CLN2, HCS26, ORFD and CLB5) and the cell cycle‐specific transcription factor SWI4. In similar experiments, CLN1 is less efficient than CLN3 at activating Start. Strikingly, expression of HCS26, ORFD and CLB5 is dependent on CLN3 in a cln1 cln2 strain, possibly explaining why CLN3 is essential in the absence of CLN1 and CLN2. To explain the potent ability of Cln3 to activate Start, despite its apparently weak biochemical activity, we propose that Cln3 may be an upstream activator of the G1 cyclins which directly catalyze Start. Given the large number of known cyclins, such cyclin cascades may be a common theme in cell cycle control.

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.…”

“…As protein synthesis seems to increase exponentially during the cell (a) scheme of the start network with components and interactions. 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.…”

“…As they contained a MET3p-CLN2 construct to avoid accumulation of aberrantly shaped cells, cln1,2 cells were grown in SC medium without methionine, which was added immediately before the time-lapse experiment 27 . Newly born cells were obtained by elutriation 19 . Coverslips (25 mm) were precoated at a small central area with 0.1 ml 0.1mg ml − 1 concanavalin A type V (Sigma-Aldrich) in water for 60 min in a humid chamber, washed twice with water and kept at 4 °C in water for up to 24 h before use 39 .…”

“…We next analysed the role of Cln3, the most upstream G1 cyclin that activates Start [16][17][18][19][20] . We observed a clear dependence of cell size at Start on growth rate in Cln3-deficient cells (Fig.…”

A role for the Pcl9‐Pho85 cyclin–cdk complex at the M/G1 boundary in Saccharomyces cerevisiae

“…pACTPCL9 encodes both an HA tag and a portion of the Gal-4 activation domain (AD) fused to the N-terminus of Pcl9. We used an ␣-HA monoclonal antibody to immunoprecipitate AD-Pcl9 from crude yeast extracts (Tyers et al, 1993;Tennyson et al, 1997). Immunoprecipitation of the Gal-4 activation domain alone expressed from the pACTII vector was used as a control in these experiments.…”