Immunological and Chemical Identification of Intracellular Forms of Adenovirus Type 2 Terminal Protein

Abstract Highly purified adenovirus type 2 terminal protein (TP) with an apparent Mr of 55,000 (55K) was prepared in quantities of 10 to 30 μg from guanidine hydrochloride- or sodium dodecyl sulfate-disrupted virions (60 to 120 mg). Guinea pigs were immunized with 14 to 20 injections of TP in amounts of 1 to 2 μg. Antiserum to TP was used to study the intracellular polypeptides related to adenovirus type 2 TP. By immunoprecipitation with anti-TP serum, we identified 80K and 76K polypeptides in the nucleoplasmic and cytoplasmic S100 fractions of [35S]methionine-labeled cells early and late after infection with Ad2. By immunoautoradiographic analysis which eliminates coprecipitation of unrelated proteins, we identified an 80K polypeptide (probably an 80K-76K doublet) in unlabeled, late infected cells, using anti-TP serum and 125I-labeled staphylococcal protein A. About two- to threefold-higher levels of the 80K and 76K polypeptides were present in the nucleoplasm than in the S100 fraction, and two- to threefold-higher levels were found in late infected cells than in early infected cells (cycloheximide enhanced, arabinofuranosylcytosine treated). We did not detect the 80K or 76K polypeptide in uninfected cells, indicating that these polypeptides are virus coded. Tryptic peptide map analysis showed that the 80K and 76K polypeptides are very closely related and that they share peptides with the DNA-bound 55K TP. Our data provide the first direct demonstration of intracellular 80K and 76K forms of TP. The intracellular 80K and 76K polypeptides are closely related or identical to the 80K polypeptide that Challberg and co-workers (Proc. Natl. Acad. Sci. U.S.A. 77:5105-5109, 1980) detected at the termini of adenovirus DNA synthesized in vitro and to the 87K polypeptide that Stillman and co-workers (Cell 23:497-508, 1981) translated in vitro. We did not detect the 55K TP in early or late infected cells, consistent with the proposal by Challberg and co-workers that the 80K polypeptide is a precursor to the virion-bound TP and that the conversion of the 80K polypeptide to the 55K TP occurs during virus maturation. The 80K and 76K polypeptides have many more methionine-containing tryptic peptides than does the 55K TP, and most of the tryptic peptides unique to the 80K and 76K polypeptides are very hydrophobic. Thus, the conversion of the 80K and 76K polypeptides to the 55K TP may involve the removal of a specific hydrophobic protein region.