Region E3 of subgroup B human adenoviruses encodes a 16-kilodalton membrane protein that may be a distant analog of the E3-6.7K protein of subgroup C adenoviruses

There is an open reading frame in the E3 transcription unit of adenovirus type 3 (Ad3) and Ad7 that could encode a protein of 16 kDa (16K protein). Ad3 and Ad7 are members of subgroup B of human adenoviruses. Using a rabbit antipeptide antiserum, we show that the 16K protein is expressed in Ad3- and Ad7-infected cells at early and late stages of infection; it is not expressed in cells infected with an Ad7 mutant that deletes the 16K gene. The 16K protein was also transcribed and translated in vitro from DNA containing the open reading frame for the 16K protein. The 16K protein has two hydrophobic domains typical of integral membrane proteins; consistent with this, we detected 16K in the crude membrane but not the cytosol cellular fractions. Although 16K has two potential sites for Ash-linked glycosylation, the protein is not glycosylated. The 16K gene is located in the same position in region E3 as the gene for the 6.7K protein of subgroup C adenoviruses (Ad2 and Ad5). E3-6.7K is an Asn-linked integral membrane glycoprotein, localized in the endoplasmic reticulum, whose function is unknown. The 16K protein has a putative transmembrane domain located in the same place in 16K as is the transmembrane domain in 6.7K, and the C-terminal portion of 16K is partially homologous to the C-terminal cytoplasmic domain of 6.7K; we suggest that these domains in 16K and 6.7K may have a similar function. The N-terminal 102 residues in 16K are not found in 6.7K; these residues may have a function that is unique to the 16K protein. In common with all known E3 proteins, the 16K protein is dispensable for virus replication in cultured cells; this suggests that the 16K protein may function in virus-host interactions.