Post-translational processing of the feline immunodeficiency virus envelope precursor protein

The synthesis and processing of the envelope glycoprotein precursor of the feline immunodeficiency virus (FIV) isolate FIV-UT113 was investigated in a persistently infected Crandell feline kidney cell line (CRFK) and in an eukaryotic expression system. Pulse-chase studies showed two glycoproteins after a 5 min pulse-labeling: a gp150 and a gp130 species. During a 30-min chase the gp150 species disappeared almost completely while gp130 increased proportionally; it was subsequently processed into the surface glycoprotein (SU), gp100, and the transmembrane glycoprotein (TM), gp35. This final maturation step did also occur when the env gene was expressed independently, but at a much lower rate. The results indicate that FIV-UT113 envelope glycoprotein processing involves two successive proteolytic cleavages. The first cleavage removes a protein fragment of approximately 20 kDa and takes place post-translationally, at least in part. The deduced primary translation product of the env gene lacks an N-terminal signal sequence but instead contains two internal hydrophobic regions. Cleavage is predicted to occur behind the second region which would indeed release an N-terminal 20-kDa polypeptide. Thus, FIV glycoprotein processing resembles that found in the ungulate lentiviruses but differs from that in the primate lentiviruses, the envelope proteins of which possess a short N-terminal signal sequence.