👤 Mart Loog

Protein synthesis underpins cell growth and controls when cells commit to a new round of cell division at a point in late G1 of the cell cycle called Start. Passage through Start also coincides with the duplication of the microtubule-organizing centers, the yeast spindle pole bodies, which will form the 2 poles of the mitotic spindle that segregates the chromosomes in mitosis. The conserved Mps1p kinase governs the duplication of the spindle pole body (SPB) in Saccharomyces cerevisiae. Here, we show that the MPS1 transcript has a short upstream open reading frame (uORF) that represses the synthesis of Mps1p. Mutating the MPS1 uORF makes the cells smaller, accelerates the appearance of Mps1p in late G1, and promotes completion of Start. Monitoring the SPB in the cell cycle using structured illumination microscopy revealed that mutating the MPS1 uORF enabled cells to duplicate their SPB earlier at a smaller cell size. The accelerated Start of MPS1 uORF mutants depends on the G1 cyclin Cln3p and the transcriptional repressor Whi5p but not on the Cln1,2p G1 cyclins. These results identify growth inputs in mechanisms that control duplication of the microtubule-organizing center and implicate these processes in the coupling of cell growth with division. Show less

Eukaryotic cell division is driven by cyclin-dependent kinases (CDKs). Distinct cyclin-CDK complexes are specialized to drive different cell-cycle events, though the molecular foundations for these specializations are only partly understood. In budding yeast, the decision to begin a new cell cycle is regulated by three G1 cyclins (Cln1-Cln3). Recent studies revealed that some CDK substrates contain a novel docking motif that is specifically recognized by Cln1 and Cln2, and not by Cln3 or later S- or M-phase cyclins, but the responsible cyclin interface was unknown. Here, to explore the role of this new docking mechanism in the cell cycle, we first show that it is conserved in a distinct cyclin subtype (Ccn1). Then, we exploit phylogenetic variation to identify cyclin mutations that disrupt docking. These mutations disrupt binding to multiple substrates as well as the ability to use docking sites to promote efficient, multi-site phosphorylation of substrates in vitro. In cells where the Cln2 docking function is blocked, we observed reductions in the polarized morphogenesis of daughter buds and reduced ability to fully phosphorylate the G1/S transcriptional repressor Whi5. Furthermore, disruption of Cln2 docking perturbs the coordination between cell size and division, such that the G1/S transition is delayed. The findings point to a novel substrate interaction interface on cyclins, with patterns of conservation and divergence that relate to functional distinctions among cyclin subtypes. Furthermore, this docking function helps ensure full phosphorylation of substrates with multiple phosphorylation sites, and this contributes to punctual cell-cycle entry. Show less