Removal of phosphate from phosphohistidine in proteins

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.

Phosphohistidine is found in basic nuclear proteins of Physarum polycephalum

Nuclear extracts of the true slime mold, Physarum polycephalum, show protein histidine kinase activity towards exogenous histones [(1985) J. Biol. Chem. 260, 16106-16113]. Physarum microplasmodia were labeled with [32P]phosphate in vivo and two basic proteins containing alkali-stable phosphate were detected. The labeled proteins comigrated with Physarum histones H1 (approximately) and H2A and phosphoamino acid analysis showed that each protein contained [32P]-phosphohistidine. The H2A-like protein was also labeled in isolated nuclei incubated with [35S]thio-ATP. We conclude that some Physarum nuclear proteins contain phosphohistidine.

Histone acetylation in replication and transcription: turnover at specific acetylation sites in histone H4 from Physarum polycephalum

Histone H4 from growing cells is partially acetylated at lysines 5, 8, 12, and 16. The turnover rate at each of these sites was investigated by pulse-labeling plasmodia of Physarum polycephalum with [3H]acetate for 55 min in either S phase or G2 phase of the cell cycle. Labeled histone H4 was purified and digested with a protease which cleaves on the carboxyl side of arginine residues. The peptide containing the acetylation sites was purified by high-performance liquid chromatography. Subfractions of the peptide were obtained due to differences in acetyllysine content. Each subfraction was subjected to automated Edman degradation and the radioactivity released after each cycle was determined. Histone H4 was acetylated uniformly in vitro and acetylated peptide 1-23 was used as a control. The results show a very striking preference for turnover on lysine-5 in the "low acetyl" subfraction from cells in S phase; the "high acetyl" subfraction showed turnover at all four sites. The peptides labeled in G2 phase showed turnover mainly at positions -8, -12, and -16. The data imply that the patterns of histone acetyl turnover associated with replication and transcription are nonrandom and distinct. The results have implications for nucleosome structure particularly the possible role of lysine-5 in chromosome maturation and for the design of experiments to test chromatin function in vitro.