title: Kinase-independent activity of Cdc2/cyclin A prevents the S phase in the Drosophila cell cycle.
abstract: BACKGROUND
  The Cdc2-dependent inhibition of S phase is required in G2 for the correct ordering of the S and M phases in yeasts and Drosophila. This function of Cdc2 has been ascribed to its ability to phosphorylate replication factors to prevent the assembly of a preinitiation complex at the origin of replication. Whether this is the sole mechanism of S phase inhibition by Cdc2 in higher metazoans is not known because the pleiotropic functions of this essential cell cycle regulator make genetic analysis difficult.
  
  
  RESULTS
  We show that Cdc2 co-expressed with Cyclin A inhibits the S phase in Drosophila salivary glands and diploid abdominal histoblasts. A kinase defective mutant of Cdc2 failed to promote mitosis, but was still able to inhibit the S phase with the same efficiency as the wild-type protein. In addition, Cdc2 and Cyclin A cooperatively inhibit transcriptional activation by the essential S phase regulator E2F. Cdc2 binds to E2F in vitro, and post-transcriptionally promotes its accumulation in vivo. Furthermore, the inhibitory effect of Cdc2 on S phase is overridden by E2F.
  
  
  CONCLUSIONS
  The inhibition of S phase by Cdc2 is achieved in part by a kinase-independent mechanism, which is likely to be mediated by the inhibition of E2F.
mesh_terms: D000005:Abdomen; D000818:Animals; D016203:CDC2 Protein Kinase / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins; D002453:Cell Cycle / Q000502:physiology*; D018797:Cell Cycle Proteins; D019925:Cyclin A / Q000378:metabolism*; D004261:DNA Replication; D004268:DNA-Binding Proteins; D004171:Diploidy; D004330:Drosophila / Q000235:genetics / Q000378:metabolism*; D029721:Drosophila Proteins; D050684:E2F Transcription Factors; D016195:G2 Phase / Q000502:physiology; D018809:Proliferating Cell Nuclear Antigen / Q000235:genetics / Q000378:metabolism; D056465:Retinoblastoma-Binding Protein 1; D016196:S Phase / Q000502:physiology*; D012469:Salivary Glands / Q000166:cytology / Q000378:metabolism; D015534:Trans-Activators; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014162:Transfection