THE GLYCOSYLATION OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 TRANSMEMBRANE GLYCOPROTEIN (GP41) IS IMPORTANT FOR THE EFFICIENT INTRACELLULAR-TRANSPORT OF THE ENVELOPE PRECURSOR GP160

The role of the glycans of the human immunodeficiency virus type 1 tra nsmembrane glycoprotein (gp41) in the intracellular events of Env prec ursor (gp160) biosynthesis has been examined by the use of a mutant gp 160 in which the cluster of conserved glycosylation sites within the g p41 domain (Asn-621, -630 and -642) has been mutated. Expression of th e wild-type and mutant forms of gp160 in BHK-21 cells using recombinan t vaccinia viruses has shown that the kinetics of the events occurring in the endoplasmic reticulum (ER) were normal: both Env proteins had similar kinetics of disulphide bond formation, as determined by the ac quisition of CD4-binding capability, and both had similar kinetics of oligomer formation. However, in contrast to the parental molecule, mut ated gp160 displayed relatively slow transport from the cis to the med ial Golgi where it was retained in the oligomeric state. Transport to the trans Golgi was impaired, as determined by the sensitivity of gp16 0 to glycosidases. Cleavage of mutated gp160 at the gp120/gp41 junctio n was substantially reduced but this was apparently not due to the inv olvement of the gp41 glycosylation in the cleavage reaction by furin i nasmuch as, in the baculovirus system, mutated gp160 could be cleaved when recombinant furin was co-expressed. The reduced cleavage in mamma lian cells may thus reflect the impaired routing of mutated Env to the compartment where cleavage occurs. The glycan component of gp41 is, t herefore, important for the efficient intracellular transport and proc essing of gp160. gp160 lacking gp41 carbohydrates is an additional exa mple, among few others, of a protein lacking glycans that is arrested in the Golgi rather than the ER following its biosynthesis.