Recently, using hybrid Ad5/12 E1A-transformed rat cells, we identified at least two regions of Ad12 E1A which influence tumorigenicity. In this report, again using the hybrid Ad5/12 E1A-transformants, we show that expression of these same two Ad12 E1A regions not only correlates with down-regulation of cell surface MHC class I expression, but coincides with the presence of an altered form of the E1A-associated cellular protein, p300, detected as a more slowly migrating species in SDS-polyacrylamide gels. The decreased electrophoretic mobility of p300 from hybrid Ad5/12 E1A- and Ad12 E1A-transformants can be abolished by protein phosphatase treatment suggesting that the change in mobility results from differential phosphorylation of p300. We suggest that differential phosphorylation of p300 may be functionally significant in the context of phenotypic differences between Ad5 E1- and Ad12 E1-transformed cells.

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