ALTERATIONS IN TYPE-1 SERINE THREONINE PROTEIN PHOSPHATASE PP1-ALPHA IN RESPONSE TO B-CELL RECEPTOR STIMULATION/

In response to stimulation of B-cells through cell surface IgM, the ac tivity of the serine/threonine protein phosphatase PP1, but not PP2A, was transiently decreased and reached a minimum 10-20 min after the st imulation. The decrease was more profound in the immature B-cell line WEHI-231, than in the mature B-cell line BAL-17. Under these condition s, PP1 alpha, an isoform of PP1, showed unique alterations in the patt erns of several spots with distinct isoelectic points in the Western b lot after two-dimensional electrophoresis, whereas another isoform, PP 1 delta, did not show any alteration. PP1 gamma 1 and PP1 gamma 2 were not detected in B-cells. Similar alterations in these spots were obse rved in B-cells stimulated by PMA. When partially purified PP1 consist ing of PP1 alpha and PP1 delta was incubated with [gamma-P-32]ATP and PKC, radioactive spots of PP1 alpha could be detected, but no spot of PP1 delta was detected. Because differences in sequence among PP1 isof orms are mostly restricted to their C-terminals, phosphorylation rates of the C-terminal peptides containing the PKC-phosphorylation motif w ere compared. The C-terminal peptide of PP1 alpha is a better substrat e for PKC than those of PP1 gamma 1 and PP1 gamma 2, and is phosphoryl ated at the serine residue corresponding to Ser-325 of PP1 alpha. The corresponding C-terminal region of PP1 delta does not contain the phos phorylation site. On the other hand, there was a large difference in s ubcellular distribution of PP1 delta, but not PP1 alpha, between immat ure and mature B-cells. From these results, it was strongly suggested that PP1 alpha is involved, aia phosphorylation by PRC, in the regulat ion of signal transduction in response to the stimulation of B-cells t hrough cell surface IgM.