Optimizing thiophosphorylation in the presence of competing phosphorylation with MALDI-TOF-MS detection.

Thiophosphorylation provides a metabolically stable, chemically reactive phosphorylation analogue for analyzing the phosphoproteome in vitro and in vivo. We developed a MALDI-TOF-MS based assay for optimizing thiophosphopeptide production by a kinase even in the presence of Mg(2+) and ATP. We found that Abl kinase thiophosphorylation rates can be "rescued" using Mn(2+) in the presence of Mg(2+). Under our ideal conditions, titration of Mn(2+) and ATPgammaS in the presence of Mg(2+) allowed relatively rapid, highly specific thiophosphorylation by Abl tyrosine kinase, both as purified enzyme and in complex cell extracts.