Cdc2 activation in fission yeast depends on Mcs6 and Csk1, two partially redundant Cdk-activating kinases (CAKs).
Cyclin-dependent kinases (Cdks) are fully active only when phosphorylated by a Cdk-activating kinase (CAK) [1]. Metazoan CAK is itself a Cdk, Cdk7, whereas the CAK of Saccharomyces cerevisiae is a distinct enzyme unrelated to Cdks [1]. The Mcs6-Mcs2 complex of Schizosaccharomyces pombe is a putative CAK related to the metazoan enzyme [2] [3]. Although the loss of Mcs6 is lethal, it results in a phenotype that is inconsistent with a failure to activate Cdc2, the major Cdk in S. pombe [3]. We therefore tested the ability of Csk1, a putative regulator of Mcs6 [4], to activate Cdk-cyclin complexes in vitro. Csk1 activated both the monomeric and the Mcs2-bound forms of Mcs6. Surprisingly, Csk1 also activated Cdc2 in complexes with either Cdc13 or Cig2 cyclins. When a double mutant carrying a csk1 deletion and a temperature-sensitive mcs6 allele was incubated at the restrictive temperature, Cdc2 was not activated and the cells underwent a cell division arrest prior to mitosis. Cdc2-cyclin complexes isolated from the arrested cells could be activated in vitro by recombinant CAK, whereas complexes from wild-type cells or either of the single mutants were refractory to activation. Thus, fission yeast contains two partially redundant CAKs: the Mcs6-Mcs2 complex and Csk1. Inactivation of both CAKs is necessary and sufficient to prevent Cdc2 activation and cause a cell-cycle arrest. Mcs6, which is essential, may therefore have required functions other than Cdk activation.
Lee
K M
KM
Cell Biology Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, New York 10021, USA.
Saiz
J E
JE
Barton
W A
WA
Fisher
R P
RP
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D016203:CDC2 Protein Kinase / Q000378:metabolism*; D002453:Cell Cycle / Q000235:genetics; D002455:Cell Division / Q000235:genetics; D002478:Cells, Cultured; D018844:Cyclin-Dependent Kinases; D004789:Enzyme Activation / Q000187:drug effects; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D006388:Hemagglutinins / Q000235:genetics; D009154:Mutation; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D012568:Schizosaccharomyces / Q000166:cytology / Q000201:enzymology* / Q000235:genetics; D029702:Schizosaccharomyces pombe Proteins; D013696:Temperature; D000095043:Cyclin-Dependent Kinase-Activating Kinase
D005656:Fungal Proteins; D006388:Hemagglutinins; D011993:Recombinant Fusion Proteins; D029702:Schizosaccharomyces pombe Proteins; D000255:Adenosine Triphosphate; D011494:Protein Kinases; C117097:Csk1 protein, S pombe; D017346:Protein Serine-Threonine Kinases; D016203:CDC2 Protein Kinase; D018844:Cyclin-Dependent Kinases; D000095043:Cyclin-Dependent Kinase-Activating Kinase