1997 Volume 122 Issue 4 Pages 730-737
In response to stimulation of B-cells through cell surface IgM, the activity of the serine/ threonine protein phosphatase PP1, but not PP2A, was transiently decreased and reached a minimum 10-20 min after the stimulation. The decrease was more profound in the immature B-cell line WEHI-231, than in the mature B-cell line BAL-17. Under these conditions, PP1α, an isoform of PP1, showed unique alterations in the patterns of several spots with distinct isoelectic points in the Western blot after two-dimensional electrophoresis, whereas another isoform, PP1δ, did not show any alteration. PP1γ1 and PP1γ2 were not detected in B-cells. Similar alterations in these spots were observed in B-cells stimulated by PMA. When partially purified PP1 consisting of PP1α and PP1β was incubated with [γ-32P] ATP and PKC, radioactive spots of PP1α could be detected, but no spot of PP1δ was detected. Because differences in sequence among PP1 isoforms are mostly restricted to their C-terminals, phosphorylation rates of the C-terminal peptides containing the PKC-phosphorylation motif were compared. The C-terminal peptide of PP1α is a better substrate for PKC than those of PP1γ1 and PP1γ2, and is phosphorylated at the serine residue corresponding to Ser-325 of PP1α. The corresponding C-terminal region of PP1δ does not contain the phosphorylation site. On the other hand, there was a large difference in subcellular distribution of PP1δ, but not PP1α, between immature and mature B-cells. From these results, it was strongly suggested that PP1α is involved, via phosphorylation by PKC, in the regulation of signal transduction in response to the stimulation of B-cells through cell surface IgM.
