Fig. 2. Expression of proteasomal components in the cytosolic and membrane fractions of various tissues from wild-type and PA28α^–/–/β^–/– mice. Cytosolic (C) and membrane (M) proteins were isolated from the liver, spleen and brain of wild-type and knockout mice as described in Materials and methods. The ratios of protein content in cytosolic and membrane fractions were 2.4:1 in the liver, 5.6:1 in the spleen and 3.0:1 in the brain. Samples (10 µg of proteins) were analyzed by SDS–PAGE and western blotting using antibodies against PA28α, PA28β, PA28γ, X, LMP2 and Mss1. Asterisks indicate artifact bands produced by the PA28α and PA28β antibodies.