The E3-6.7K protein of adenovirus is an Asn-linked integral membrane glycoprotein localized in the endoplasmic reticulum

Abstract

We have previously shown that the early region E3 of adenovirus type 2 encodes a 6700 MW (6.7K) protein. This protein is immunoprecipitated from infected cells as two series of bands, a doublet at 7-8K and a doublet or triplet at 15-16K. The predicted amino acid sequence of the 6.7K indicates that there are three potential Asn-linked glycosylation sites near the N-terminus of the protein. Studies done using tunicamycin, endoglycosaminidase H (endo H), and endo F demonstrate that 6.7K has exclusively high mannose oligosaccharides and that only one of the three potential glycosylation sites of 6.7K is glycosylated. [3H]-Mannose labeling confirmed that the upper species of 6.7K were glycosylated. Since the oligosaccharides are not processed from high mannose to the complex type, this implies that the 6.7K protein is retained in the endoplasmic reticulum (ER). This was confirmed with immunofluorescence. Based on the predicted amino acid sequence, 6.7K does not have a classical N-terminus signal sequence, but it does have a 22 amino acid hydrophobic domain, located at amino acids 15-36, that could function as a signal-anchor domain. We demonstrate that the 6.7K protein is an integral membrane ER protein. Considering that all of the potential Asn-glycosylation sites are near the N-terminus of 6.7K, it must be oriented in the membrane with its N-terminus in the lumen of the ER and its C-terminus in the cytoplasm. Pulse-chase studies performed to examine the temporal appearance of the different 6.7K moieties suggests that this protein may translocate into the ER membrane post-translationally, or may be glycosylated post-translationally.