Analysis of adenovirus early region 4-encoded polypeptides synthesized in productively infected cells

Peptide-specific antisera were developed to analyze the products encoded by adenovirus type 5 early region 4 (E4) open reading frames 6 and 7. Reading frame 6 previously was shown to encode a 34-kilodalton polypeptide (34K polypeptide) that forms a complex with the early region 1B (E1B)-55K antigen and is required for efficient viral growth in lytic infection. Antisera that were generated recognized the E4-34K protein as well as a family of related polypeptides generated by the fusion of open reading frames 6 and 7. These polypeptides shared amino-terminal sequences with the 34K protein. Short-pulse analysis suggested that the heterogeneity observed with the 6/7 fusion products resulted from differential splicing patterns of related E4 mRNAs. An antiserum directed against the amino terminus of reading frame 6 recognized only the free form of the 34K antigen that was not associated with the E1B-55K protein. This observation allowed the determination of the stability of the free and complexed form of this polypeptide. Pulse-chase analyses demonstrated that both forms of the 34K protein had half-lives greater than 24 h, suggesting that complex formation did not result in stabilization of this gene product. The half-lives of the 6/7 fusion products were approximately 4 h. The 34K protein also was shown to have a nuclear localization within infected cells. Finally, analysis of a mutant carrying deletions in both the E4-34K and E1B-55K polypeptides indicated that the complex formed between these two proteins was a functional unit in lytic infection.