Cyclin A potentiates maturation-promoting factor activation in the early Xenopus embryo via inhibition of the tyrosine kinase that phosphorylates cdc2.
We have produced human cyclin A in Escherichia coli and investigated how it generates H1 kistone kinase activity when added to cyclin-free extracts prepared from parthenogenetically activated Xenopus eggs. Cyclin A was found to form a major complex with cdc2, and to bind cdk2/Eg1 only poorly. No lag phase was detected between the time when cyclin A was added and the time when H1 histone kinase activity was produced in frog extracts, even in the presence of 2 mM vanadate, which blocks cdc25 activity. Essentially identical results were obtained using extracts prepared from starfish oocytes. We conclude that formation of an active cyclin A-cdc2 kinase during early development escapes an inhibitory mechanism that delays formation of an active cyclin B-cdc2 kinase. This inhibitory mechanism involves phosphorylation of cdc2 on tyrosine 15. Okadaic acid (OA) activated cyclin B-cdc2 kinase and strongly reduced tyrosine phosphorylation of cyclin B-associated cdc2, even in the presence of vanadate. 6-dimethylamino-purine, a reported inhibitor of serine-threonine kinases, suppressed OA-dependent activation of cyclin B-cdc2 complexes. This indicates that the kinase(s) which phosphorylate(s) cdc2 on inhibitory sites can be inactivated by a phosphorylation event, itself antagonized by an OA-sensitive, most likely type 2A phosphatase. We also found that cyclin B- or cyclin A-cdc2 kinases can induce or accelerate conversion of the cyclin B-cdc2 complex from an inactive into an active kinase. Cyclin B-associated cdc2 does not undergo detectable phosphorylation on tyrosine in egg extracts containing active cyclin A-cdc2 kinase, even in the presence of vanadate. We propose that the active cyclin A-cdc2 kinase generated without a lag phase from neo-synthesized cyclin A and cdc2 may cause a rapid switch in the equilibrium of cyclin B-cdc2 complexes to the tyrosine-dephosphorylated and active form of cdc2 during early development, owing to strong inhibition of the cdc2-specific tyrosine kinase(s). This may explain why early cell cycles are so rapid in many species.
Devault
A
A
Centre National de la Recherche Scientifique UPR 8402, Montpellier, France.
Fesquet
D
D
Cavadore
J C
JC
Garrigues
A M
AM
Labbé
J C
JC
Lorca
T
T
Picard
A
A
Philippe
M
M
Dorée
M
M
D000595:Amino Acid Sequence; D000818:Animals; D016203:CDC2 Protein Kinase / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism*; D004789:Enzyme Activation; D004988:Ethers, Cyclic / Q000494:pharmacology; D006801:Humans; D007399:Interphase; D016200:Maturation-Promoting Factor / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D019319:Okadaic Acid; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism; D014638:Vanadates / Q000494:pharmacology; D014981:Xenopus
D016213:Cyclins; D004988:Ethers, Cyclic; D019319:Okadaic Acid; D014638:Vanadates; D014443:Tyrosine; D011505:Protein-Tyrosine Kinases; D016203:CDC2 Protein Kinase; D016200:Maturation-Promoting Factor