Phosphorylation of serine residue 89 of human adenovirus E1A proteins is responsible for their characteristic electrophoretic mobility shifts, and its mutation affects biological function.

The shift in mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis that is characteristic of the adenovirus E1A proteins is the result of posttranslational modification. In the present study, we demonstrate that phosphorylation of bacterially produced E1A in higher cell extracts occurs on serine and is responsible for the mobility shift. E1A protein expressed in Saccharomyces cerevisiae also undergoes the mobility shift due to serine phosphorylation. Site-directed mutagenesis was used to identify the serine residue responsible for the mobility shift. Six serine residues were altered to glycine within E1A. Substitution at serine residue 89 was shown to selectively prevent the mobility shift of both the 289R and 243R E1A proteins. We conclude that phosphorylation at serine 89 is the specific modification responsible for the mobility shift of E1A. Moreover, we demonstrate that the Ser-89-to-Gly mutation has no effect on trans activation or complementation of an E1A-deficient adenovirus. In contrast, the mutant protein does significantly reduce both the repression and transformation efficiency of E1A. The five other Ser-to-Gly mutation were also examined for functional effects. None affected trans activation, whereas repression and transformation functions were affected. One mutant affected transformation without affecting repression, suggesting that these functions are to some degree also separable. The relevance of phosphorylation to structure and activity of E1A and other nuclear oncogene proteins is discussed.