Evidence for a Protein Kinase Activity Associated with Purified Particles of Cauliflower Mosaic Virus

Summary A cyclic nucleotide-independent protein kinase (PK) activity has been found to be associated with purified particles of cauliflower mosaic virus. The main acceptors of phosphorylation were proteins with mol. wt. of 42000 (the capsid protein), 58000 (which may be the capsid protein precursor) and 110000 (of unknown function). Acid hydrolysis and phosphoamino acid analysis of nucleocapsid proteins phosphorylated in vitro showed that the PK catalyses the transfer of phosphate to both serine and threonine residues. Activation of the PK made the DNA more accessible to DNase I, suggesting that a modification of the structure of the capsid had occurred.