Kinetic constants of KM = 0.8 microM, 3 microM and 1.6 microM, and kcat = 9 s-1, 7 s-1 or 9 s-1 were determined for histidine dephosphorylation by protein phosphatases 1, 2A and 2C respectively. IC50 values were determined for the inhibition of protein phosphatase 1 by inhibitor 1 (IC50 = 1 nM), inhibitor-2 (IC50 = 3 nM) and okadaic acid (IC50 = 30 nM) and for the inhibition of protein phosphatase 2A by okadaic acid (IC50 = 0.02 nM) and microcystin-LR (IC50 = 1 nM). Inhibitor-1 (Ki = 0.7 nM) and okadaic acid (Ki = 32 nM) are noncompetitive with protein phosphatase 1. Some of the IC50 values were low enough to violate the assumptions of the usual inhibition equations and a more general approach to the analysis of the data was used. On the basis of these kinetic parameters and the presence of phosphohistidine, the major cellular protein serine/threonine phosphatases are likely to act as protein histidine phosphatases in the cell.

No abstract

Nuclear extracts of the true slime mold, Physurum polycephulum, show protein histidine kinase activity towards exogenous histones [(1985) J. Biol. Chem. 260, 1610616113]. Physarum microplasmodia were labeled with P*P]phosphate in vivo and two basic proteins containing alkali-stable phosphate were detected. The labeled proteins comigrated with Physarum histones Hl (approximately) and H2A and phosphoamino acid analysis showed that each protein contained P'P]-phosphohistidine. The H2A-like protein was also labeled in isolated nuclei incubated with P%]thio-ATP. We conclude that some Physarum nuclear proteins contain phosphohistidine.Protein kinase; Phosphohistidine; Alkali-stable protein phosphorylation; Histone; (Physarum polycephalum)