Solid-state protein kinase. A tool for post-synthetically modifying and radioactively labeling proteins in vitro

The capacity of partially purified rat muscle protein kinase coupled to cyanogen-bromide-activated Sepharose 4B to (radio-)phosphorylate proteins in vitro was evaluated using histones from calf thymus and rat liver and certain proteins as substrates. Data are presented which point to a low substrate specificity of this enzyme. It is demonstrated that even within a short time period histones are efficiently phosphorylated without the introduction of contaminating (phospho-)proteins. Therebye phosphoserine residues are formed. The phosphorylation reaction usually performed at 30 degrees C is shown to function quite efficiently also at 4 degrees C. It proceeds even at 30 degrees C for several hours at pH values close to the physiological range without the release of proteins from the solid matrix. The phosphorus transfer can be largely increased with the use of high ATP concentrations. The stability of the substrates is sufficient to suggest a wide applicability of this solid-state protein kinase in the phosphorylation of proteins either for labeling or as a tool to modify proteins post-synthetically under gentle conditions. The solid enzyme seems to be suitable for radioactively labeling proteins of more complex biological structures, such as membrane surfaces.