Cyclin D1/cdk2 kinase is present in a G1 phase-specific protein complex Yi1 that binds to the mouse thymidine kinase gene promoter.

Our studies provide evidence for the presence of cyclin D1 in an early G1 cycle-specific DNA binding complex Yi1. Previously we identified several complexes including Yi and E2F that at different times during G0 to S transition bind to three distinct DNA sequences (MT1, MT2, MT3) located in the mouse thymidine kinase upstream promoter. These various complexes contain DNA binding proteins (Sp1, E2F, p110, p60), cyclins A and E, cyclin-dependent kinase 2 (cdk2), and retinoblastoma-related proteins (pRB, p107). Here we report that Yi1 is different from the E2F complexes. Yi1 contains cyclin D1/cdk2 kinase as shown by using specific antibodies to cyclins, cdks and the Yi1 DNA-binding protein in gel retardation, western blotting, and immunoprecipitation assays. Yi1 binding is specific to a consensus sequence different from that of E2F.