Interaction of pRb or RbCT with PP1 catalytic subunit. PP1 catalytic subunit (purified from muscle as a mixture of the three isoforms) was applied to a 0.5 ml column prepared with either GST-Rb-Sepharose or GST-RbCT-Sepharose or GST-Sepharose (further described in the Methods section). The PP1-containing fractions were pooled and concentrated by precipitation in the presence of 7% TCA. One aliquot of each pool was subjected to electrophoresis and immunoblotting to detect PP1 (PP1 blot, using a mixture of the three isoform-specific antibodies) and GST-Rb, GST-RbCT or GST (GST blot). PP1 catalytic subunit (30 ng loaded on the electrophoresis) is also shown as positive control. The data presented are representative of two independent experiments.