Identification of adenovirus type 2 early region 1B proteins that share the same amino terminus as do the 495R and 155R proteins.

Adenovirus type 2 early region 1B (E1B) proteins synthesized in vitro were fractionated chromatographically and characterized by peptide and sequence analysis and by reaction with peptide-specific antisera targeted to either the N or C terminus of either of two overlapping E1B reading frames (175 or 495 codons). In addition to the previously identified E1B-495R, E1B-175R, and E1B-155R species, two other E1B proteins of similar electrophoretic mobility to the 175R protein were identified. E1B-82R is an abundant product in vitro and in vivo that has the same N terminus as that of the 495R and 155R proteins but a different C terminus. The structure of 82R is predicted by the structure of the abundant 13S (1.02-kilobase) E1B mRNA. E1B-168R is a novel minor species consisting of the 24 amino-terminal residues of the 495R protein fused to the entire polypeptide IX sequence. An additional, minor 16,000-molecular-weight polypeptide was detected that may correspond to a predicted 92R E1B protein, but definitive identification was not possible. These observations establish that the leftmost portion (78 codons) of the 495-codon reading frame, which overlaps the right half of the 175-codon reading frame, is expressed as an abundant protein that does not contain other 495R sequences. This region, which may participate in the regulation of region E1A expression, may thus constitute a functional domain distinct from the rightward portion of the 495R protein.