A filter-based protein kinase assay selective for alkali-stable protein phosphorylation and suitable for acid-labile protein phosphorylation.

Alkali-stable phosphorylation of proteins, particularly phosphotyrosine and phosphohistidine, is an important phenomenon in cells. In the case of phosphohistidine and some other phosphoamino acids, the phosphorylation is acid-labile and in these cases studies have been severely limited by the absence of a rapid assay suitable for acid-labile phosphorylation. The assay presented here involves a conventional kinase assay reaction followed by mild alkaline hydrolysis and adsorption of the product to washed Nytran paper at high pH. After further washing, at pH 9, the radioactivity on the papers is determined by liquid scintillation counting. Hence, acid-labile phosphorylation is preserved. The assay is selective for alkali-stable phosphorylation but not fully specific, mainly due to the need to balance the severity of the partial alkaline hydrolysis with the stability of the protein-peptide bonds. The assay has been used for the purification and characterization of a protein histidine kinase from Saccharomyces cerevisiae.