Synthesis and processing of the transmembrane envelope protein of equine infectious anemia virus

The transmembrane (TM) envelope protein of lentiviruses, including equine infectious anemia virus (EIAV), is significantly larger than that of other retroviruses and may extend in the C-terminal direction 100 to 200 amino acids beyond the TM domain. This size difference suggests a lentivirus-specific function for the long C-terminal extension. We have investigated the synthesis and processing of the EIAV TM protein by immune precipitation and immunoblotting experiments, by using several envelope-specific peptide antisera. We show that the TM protein in EIAV particles is cleaved by proteolysis to an N-terminal glycosylated 32- to 35-kilodalton (kDa) segment and a C-terminal nonglycosylated 20-kDa segment. The 20-kDa fragment was isolated from virus fractionated by high-pressure liquid chromatography, and its N-terminal amino acid sequence was determined for 13 residues. Together with the known nucleotide sequence, this fixes the cleavage site at a His-Leu bond located 240 amino acids from the N terminus of the TM protein. Since the 32- to 35-kDa fragment and the 20-kDa fragment are not detectable in infected cells, we assume that cleavage occurs in the virus particle and that the viral protease may be responsible. We have also found that some cells producing a tissue-culture-adapted strain of EIAV synthesize a truncated envelope precursor polyprotein. The point of truncation differs slightly in the two cases we have observed but lies just downstream from the membrane-spanning domain, close to the cleavage point described above. In one case, virus producing the truncated envelope protein appeared to be much more infectious than virus producing the full-size protein, suggesting that host cell factors can select for virus on the basis of the C-terminal domain of the TM protein.