title: Phosphorylations of cyclin-dependent kinase 2 revisited using two-dimensional gel electrophoresis.
abstract: To control the G1/S transition and the progression through the S phase, the activation of the cyclin-dependent kinase (CDK) 2 involves the binding of cyclin E then cyclin A, the activating Thr-160 phosphorylation within the T-loop by CDK-activating kinase (CAK), inhibitory phosphorylations within the ATP binding region at Tyr-15 and Thr-14, dephosphorylation of these sites by cdc25A, and release from Cip/Kip family (p27kip1 and p21cip1) CDK inhibitors. To re-assess the precise relationship between the different phosphorylations of CDK2, and the influence of cyclins and CDK inhibitors upon them, we introduce here the use of the high resolution power of two-dimensional gel electrophoresis, combined to Tyr-15- or Thr-160-phosphospecific antibodies. The relative proportions of the potentially active forms of CDK2 (phosphorylated at Thr-160 but not Tyr-15) and inactive forms (non-phosphorylated, phosphorylated only at Tyr-15, or at both Tyr-15 and Thr-160), and their respective association with cyclin E, cyclin A, p21, and p27, were demonstrated during the mitogenic stimulation of normal human fibroblasts. Novel observations modify the current model of the sequential CDK2 activation process: (i) Tyr-15 phosphorylation induced by serum was not restricted to cyclin-bound CDK2; (ii) Thr-160 phosphorylation engaged the entirety of Tyr-15-phosphorylated CDK2 associated not only with a cyclin but also with p27 and p21, suggesting that Cip/Kip proteins do not prevent CDK2 activity by impairing its phosphorylation by CAK; (iii) the potentially active CDK2 phosphorylated at Thr-160 but not Tyr-15 represented a tiny fraction of total CDK2 and a minor fraction of cyclin A-bound CDK2, underscoring the rate-limiting role of Tyr-15 dephosphorylation by cdc25A.
authors.affiliation: Institute of Interdisciplinary Research and Protein Chemistry Department, Faculté de Médecine, Université Libre de Bruxelles, Campus Erasme, B-1070 Brussels, Belgium. kcoulonv@ulb.ac.be
authors.forename: Katia
authors.initials: K
authors.lastname: Coulonval
authors.forename: Laurence
authors.initials: L
authors.lastname: Bockstaele
authors.forename: Sabine
authors.initials: S
authors.lastname: Paternot
authors.forename: Pierre P
authors.initials: PP
authors.lastname: Roger
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D015153:Blotting, Western; D042846:CDC2-CDC28 Kinases / Q000378:metabolism*; D002453:Cell Cycle; D002460:Cell Line; D019927:Cyclin E / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000378:metabolism; D015180:Electrophoresis, Gel, Two-Dimensional / Q000379:methods*; D016193:G1 Phase; D006801:Humans; D008934:Mitogens / Q000737:chemistry / Q000378:metabolism; D008954:Models, Biological; D010766:Phosphorylation; D011233:Precipitin Tests; D011485:Protein Binding; D016196:S Phase; D013912:Threonine / Q000737:chemistry; D013997:Time Factors; D014443:Tyrosine / Q000737:chemistry; D020687:cdc25 Phosphatases / Q000378:metabolism
chemical_list: C493956:CDKN1A protein, human; D019927:Cyclin E; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D008934:Mitogens; D013912:Threonine; D014443:Tyrosine; D000255:Adenosine Triphosphate; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; C517004:CDC25A protein, human; D020687:cdc25 Phosphatases