Figure 6.

Simultaneous expression of cyclin E and cdk2 by the X2C1 transfectants rescued entry into S phase. A: Cyclin E/cdk2 kinase activity of the X2C2 and cyclin E and X2C1 and cyclin E transfectants, in the presence and absence of the TAT-cdk2 fusion protein was determined after adhesion to type I collagen. Cyclin E/cdk2 kinase assays were performed on cyclin E immunoprecipitates from serum-starved cells after adhesion to type I collagen for 20 hours. The X2C2 and cyclin E transfectants with (+) or without (−) the TAT-cdk2 fusion protein and the X2C1 and cyclin E transfectants transduced with the TAT-cdk2 fusion protein formed active kinase complexes. B: The TAT-cdk2 fusion protein stimulated entry into S phase when expressed in X2C1 and cyclin E co-transfectants (X2C1 and E and cdk2) but not in the X2C1 transfectants (X2C1 and cdk2) adherent to type I collagen. The percentage of cells incorporating BrdU was measured during the final 2 hours of a 24-hour incubation. Neither cyclin E overexpression alone nor the TAT-cdk2 fusion protein alone was sufficient to promote entry into S phase in the X2C1-expressing cells (*, P > 0.05). However, expression of both cyclin E and TAT-cdk2 in the X2C1 transfectants was sufficient to mediate progression through G₁ when the cells were adherent to type I collagen (++, P < 0.0001).