The envelope
glycoprotein (gp70) of a molecularly cloned, replication-defective feline leukemia virus (FeLV-
FAIDS clone 61C) carries determinants for induction of fatal immunodeficiency disease, whereas the gp70 of its companion replication-competent, probably parent virus (clone 61E) does not. Immunoprecipitation analysis of the extracellular
glycoproteins of 61E and EECC, a replication-competent viral construct composed of the 61C env and 3' long terminal repeat fused to the 61E gag-pol genes, demonstrated that the gp70 of EECC could be distinguished from that of 61E by both feline immune serum and a murine
monoclonal antibody. Molecular weights of both the envelope precursor
polyprotein (gp80) and the mature extracellular
glycoprotein (gp70) of 61E were smaller than the corresponding
proteins from the pathogenic EECC. Both the molecular weight disparity and
monoclonal antibody discrimination of the two gp80s were abolished by inhibition of envelope protein glycosylation with
tunicamycin, whereas the apparent gp70 size differences were resolved by enzymatic removal of N-linked
oligosaccharides. Pulse-chase studies in EECC-infected cells demonstrated that processing of gp80 to gp70 was delayed and that this retardation of envelope
glycoprotein processing could be simulated in 61E-infected cells by treatment with the
glucosidase inhibitor
N-methyldeoxynojirimycin, a compound that causes retention of
oligosaccharides in the high-
mannose form. The resultant 61E gp70 then could be recognized by sera from EECC-immunized cats. The presence of a higher content of
sialic acid on the apathogenic 61E gp70 indicated that
oligosaccharides of 61E and EECC gp70 were processed differently. These data suggested that the unique biochemical properties which distinguish the envelope
glycoproteins of the FeLV-
FAIDS variant from its companion apathogenic parent virus were responsible for T-cell cytopathicity and induction of immunodeficiency disease. Further biochemical characterization of these
glycoproteins should be useful in understanding the pathogenic mechanisms of immunodeficiency disease induced by retroviruses.