Quantified levels of the β5 proteasome subunit and actin protein in proliferating and senescent WI-38 cells. Cytosolic extracts were prepared and equivalent protein levels were analyzed for β5 subunit expression by immunoblotting. Actin levels served as a loading control. The graphical display indicates the relative intensities of β5 and actin levels determined by optical densitometry from corresponding immunoblots (± standard error; n = 3). The decline in β5 levels was found to be significant in senescent cells (**, p < 0.01).