Phosphorylation of chicken protein tyrosine phosphatase 1 by casein kinase II in vitro

The phosphorylation and dephosphorylation of proteins on tyrosyl residues are key regulatory mechanisms of cell growth and signal transduction and are controlled by opposing activities of protein tyrosine kinases and phosphotyrosyl phosphatases (PTPs). We have previously cloned and characterized a nontransmembrane chicken protein tyrosine phosphatase 1 (CPTP1) similar to human placental PTP1B (HPTP1B). CPTP1 contains several phosphorylation sequence motifs (S/T-X-X-D/E) for casein kinase II (CKII), [(I > E > V)-Y-(E > G)-(E > D > P > N)-(I/V > L)] for p56(1ck), and (P-E-S-P) for MAP kinase. To examine whether phosphatase activity of CPTP1 could be controlled by phosphorylation, CPTP1 and HPTP1B fusion proteins purified from E. coli were subjected to the in vitro phosphorylation by CKII. Phosphoamino acid analysis revealed that CPTP1 was phosphorylated on both serine and threonine residues by CKII in vitro. In addition, the degree of the phosphorylation of CPTP1 by CKII was shown to be five times higher than that of HPTP1B. Phosphorylation on both serine and threonine residues of CPTP1 in vitro results in an inhibition of its phosphatase activity. This result suggests that phosphorylation of CPTP1 and HPTP1B by CKII might be implicated in the regulation of their catalytic activities in the cell.