Protein phosphorylation in Rhodospirillum rubrum: purification and characterization of a water-soluble B873 protein kinase and a new component of the B873 complex, Ω, which can be phosphorylated

Abstract

A water-soluble protein kinase (the B873 kinase), which can phosphorylate isolated B873 complexes, was purified from the water-soluble fraction of crude extracts from Rhodospirillum rubrum G9. The B873 kinase is monomeric with a molecular mass of approx. 20 kDa. Apart from B873 complexes, the enzyme is able to phosphorylate histones and ovalbumin. Although Mg 2+ and Ca 2+ are active as metal cofactors, a higher activity is observed in the presence of the transition metals Cu 2+ and Mn 2+. The kinase activity is only weakly inhibited by ADP and AMP but no effect is observed upon the addition of cAMP. The B873 kinase is specific for serine residues. In vitro phosphorylation of the B873 complexes revealed that besides the α- and β-polypeptides, a third, new component, Ω, becomes labelled. This Ω component, which is not stained with Coomassie blue but by silver, is a polypeptide with a molecular mass of approx. 4 kDa.