The 58000 dalton (58K) protein coded for by early region 1B of human adenovirus type 5 (Ad5) was found to be phosphorylated. At least three major tryptic phosphopeptides were identified and the average number of phosphates per 58K molecule was estimated to be between two and three. Thus, it was possible that each phosphopeptide contained just one phosphate group. The ratio of phosphoserine to phosphothreonine was about 2 to 1 on average and essentially no phosphotyrosine was detected. No evidence was found to suggest that cAMP-dependent protein kinase was involved in the phosphorylation of 58K. Previous studies have shown that 58K was phosphorylated when immunoprecipitates containing Ad5 early region 1 proteins were incubated with [γ-P]ATP. Analysis of the phosphopeptides of 58K labelled under these conditions indicated a large number of phosphorylation sites which differed from those found . Thus, the action of kinases in the phosphorylation of 58K in immunoprecipitates did not mimic the enzymic activity responsible for 58K phosphorylation .


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