Association of cellular PP1 isoforms with wild-type or Ser/Thr-to-Ala Rb-mutant. GST-Rb, either wild-type (WT) or carrying the Ser/Thr-to-Ala mutations (Mut) of the sites indicated in the lower part of the figure (further described in ) or GST, were bound to glutathione-Sepharose beads and used to co-precipitate the PP1 isoforms from 2.5 mg of mitotic or asynchronous Hela cells extracts. All the rest was as in Fig. 1. FAK was used to normalize the amount of mitotic and asynchronous cell extracts used, since FAK protein did not change throughout the cell cycle. The data presented are representative of two independent experiments.