Evidence for a 58-kilodalton polypeptide as precursor of the coat protein of cauliflower mosaic virus.

Purified cauliflower mosaic virus subjected to agarose gel electrophoresis separated into two zones, slow and fast, with the latter consisting of less than 5-10% of the total. When these two types of virions were excised from gels and subjected to analysis in dodecyl sulfate impregnated gels, the fast component consisted mainly of the 58- and 44-kilodalton (kd) phosphorylated forms of the coat protein whereas the slow component contained only lower molecular weight forms of the same protein. Pulse labeling experiments with 32P (as orthophosphate) or tritiated thymidine showed the fast electrophoretic form of the virus to become labeled more rapidly than the slow form. With long labeling periods the slow form accumulated more label than the fast form suggesting a precursor-product relationship between the fast and slow electrophoretic forms. With [32P]orthophosphate or [35S]methionine labeling, the 58-kd phosphorylated protein showed a much higher relative level of labeling than that obtained with protein stains such as Coomassie blue. With longer labeling periods greater amounts of activity appeared in the 44-kd and lower molecular weight forms of the coat protein. These results supported previous results suggesting that the 58-kd protein is the primary translation product of the coat protein gene and that this is the form used in encapsidation to produce mature virions.