Degradation of cyclin A does not require its phosphorylation by CDC2 and cyclin-dependent kinase 2.
Many cyclins are degraded by the ubiquitination/proteasome pathways involving the anaphase-promoting complex and SCF complexes. These degradations are frequently dependent on phosphorylation by cyclin-dependent kinases (CDKs), providing a self-limiting mechanism for CDK activity. Here we present evidence from in vitro and in vivo assay systems that the degradation of human cyclin A can be inhibited by kinase-inactive mutants of CDK2 and CDC2. One obvious interpretation of these results is that like other cyclins, CDK-dependent phosphorylation of the cyclin A may be involved in cyclin A degradation. Our data indicated that CDK2 can phosphorylate cyclin A on Ser-154. Site-directed mutagenesis of Ser-154 abolished the phosphorylation by recombinant CDK2 in vitro and the majority of cyclin A phosphorylation in the cell. Activation of CDK2 and binding to SKP2 or p27(KIP1) were not affected by the phosphorylation of Ser-154. Surprising, in marked contrast to cyclin E, where phosphorylation of Thr-380 by CDK2 is required for proteolysis, degradation of cyclin A was not affected by Ser-154 phosphorylation. It is likely that the stabilization of cyclin A by the kinase-inactive CDKs was mainly due to a cell cycle effect. These data suggest an important difference between the regulation of cyclin A and cyclin E.
Yam
C H
CH
Department of Biochemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
Siu
W Y
WY
Lau
A
A
Poon
R Y
RY
D016203:CDC2 Protein Kinase / Q000378:metabolism*; D042846:CDC2-CDC28 Kinases; D002453:Cell Cycle; D019925:Cyclin A / Q000378:metabolism*; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D004789:Enzyme Activation; D006367:HeLa Cells; D006801:Humans; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*
D019925:Cyclin A; D017346:Protein Serine-Threonine Kinases; D016203:CDC2 Protein Kinase; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases