Protein Kinase from Rainbow‐Trout‐Testis Ribosomes

A protein kinase specifically associated with the ribosomal fraction has been isolated from rainbow trout testis. The enzyme was extracted from isolated trout‐testis ribosomes in 0.6 M KCl, and was separated from trout testis protamine kinase by hydroxylapatite chromatography. The ribosomal protein kinase is a Mg 2+ ‐dependent enzyme that will transfer the terminal phosphoryl group from ATP into O ‐phosphoseryl linkages of the substrate. The enzyme will catalyze the phosphorylation of several basic proteins, and the slightly lysine‐rich histone IIb 2 is most readily phosphorylated followed by protamine and ribosomal proteins. Histones I, IIb 1 and III are phosphorylated at an intermediate rate, while the arginine‐rich histone IV and an acidic protein such as phosvitin are poor substrates. The enzyme is stimulated by the addition of dithiothreitol, and cyclic AMP enhances enzyme activity between 10 and 25%.