Proteasome activity is reduced in senescent WI-38 cells. (A) Cell lysates were prepared from proliferating or senescent WI-38 cells. Protein concentrations were normalized and proteasome activity was determined using a synthetic fluorogenic substrate (± standard error; n = 4). The reduction in proteasome activity was found to be significant in the older cells (**, p < 0.01). (B) Polyubiquitinated proteins found in the cytosolic extracts of proliferating and senescent WI-38 cells. Sample concentrations were normalized and the level of polyubiquitin-protein conjugates was determined by immunoblot using an antibody specific for ubiquitin.