A truncated HTLV-I envelope protein, lacking the hydrophobic membrane anchor domain, is associated with cellular membranes and virions.

The HTLV-I producer cell line C10/MJ2 does not induce syncytium formation with HTLV-I receptor expressing cells. Here we show that this cell line produces a truncated envelope protein, which, because of a premature stop codon, lacks the hydrophobic membrane anchor domain of the transmembrane protein (TM). Despite lacking a membrane anchor this envelope protein is expressed on the cell surface and associated with released virions. However, its incorporation into virions seems less efficient than that of a full-length envelope glycoprotein and some of its released into the cell culture supernatant as soluble surface glycoprotein (SU)-TM complexes. Small amounts of such a truncated envelope glycoprotein were also found in the fusion-competent HTLV-I producer cell line MT2. Premature truncation of HTLV-I envelope proteins in producer cell lines may result from in vitro selection for a less fusogenic phenotype. The association of truncated HTLV-I envelope proteins with virions and cell surfaces may reflect interactions between the SU domain and cellular membranes, possibly with the cellular receptor for HTLV-I.