Phosphorylation of protein phosphatase-1 isoforms by cdc2-cyclin B in vitro.

Protein Phosphatase-1 is phosphorylated in vitro by cdc2-cyclin B (Villa-Moruzzi, FEBS Lett 304: 211-215, 1992). In the present study we show that all the three Phosphatase-1 isoforms, alpha, gamma 1, delta, are phosphorylated by cdc2-cyclin B. Phosphorylation is specific for this kinase and involves a C-terminal Thr. This site is most likely Thr 320 in alpha (shown by others to be phosphorylated also by cdc2-cyclin A). Such Thr is conserved in gamma 1, delta and in the testis-specific gamma 2, and is the only Thr that fits the cdc2-consensus sequence in the C-terminal region. Phosphorylation of Phosphatase-1 purified from skeletal muscle, which is a mixture of the alpha, gamma 1 and delta isoforms, is up to 0.4 mol/mol and induces 30-35% enzyme inactivation. Following tryptic proteolysis each isoform yields a distinct phosphopeptide map. This is in agreement with the different sequences of the isoforms in the C-terminal regions and may be useful to distinguish the isoforms in extracts from metabolically-labelled cells. Our results suggest that all the Phosphatase-1 isoforms may be potentially regulated at M-phase.