Figure 5.

Relative amounts of CDK4 and CDK6 complexed with p16 and p21 in senescent MRC-5. Extracts of senescent MRC-5 fibroblasts were immunodepleted for either p16 or p21 by three serial precipitations with p16 and p21 antisera. The protein levels of CDK4, CDK6 and MAP kinase (A) or p16 and p21 (B) were examined. (A) Lanes 1 and 2 represent the same number of early passage and senescent cells while lanes 3–6 represent 150 μg of the untreated (lane 3), p16-depleted (lane 4), and p21-depleted extracts (lane 5). (B) Protein (150 μg) was analyzed for total extracts of early passage (lane 1), senescent cells (lane 2), and untreated (lane 4), p16-depleted (lane 5) and p21-depleted (lane 6) extracts. The extracted cell pellet was resuspended in a similar volume of Laemmli sample buffer and a fraction representing the same fraction as loaded for the untreated extract representing an undefined amount of protein (lane 3) was analyzed.