HPV-16 E7 can overcome p21 Cip1 -mediated inhibition of cyclin A- and cyclin E-associated histone H1 kinase activity in vitro. Cyclin A-associated ( A,B ) and cyclin E-associated ( C ) protein kinases were immunoprecipitated from the human myeloid leukemia cell line ML-1 and kinase assays were performed in the absence (−) or presence (+) of 2 μg of purified GST–p21 Cip1 using histone H1 as a substrate. Increasing amounts of purified GST–16E7 (0.4, 1.1, 4.4, and 6.6 μg) ( A,C ) or the pRB-binding deficient mutant GST–16E7Δ21-24 (0.4, 1.1, 4.4, and 6.6 μg) ( B ) were titrated into the reaction. As a control, purified unfused GST (1.1, 4.4, and 6.6 μg) was added. Quantitations are shown underneath each assay.
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