The nature of the association between the murine leukemia virus envelope proteins

The amount of MuLV gp70 and p15(E) present as a disulfide-linked complex, ([gp90]), was assayed for several MuLV preparations. Upon dissociation of AKR MuLV with 1% SDS under nonreducing conditions, ∼10% of the gp70 molecules were found in the form of [gp90]; disruption with 0.5% NP-40 prior to addition of SDS increased this percentage to 39%. For Rauscher MuLV, [gp90] was not observed after disruption directly with SDS; upon incubation with NP-40 prior to SDS treatment, increasing amounts of the complex were formed. Pretreatment with iodoacetamide did not inhibit the subsequent formation of [gp90] in the presence of NP-40. Exposure of virions to the thiol-specific reagents, 2,2′-dithiobis(m-nitropyridine) or N-ethylmaleimide, resulted in quantitative conversion of gp70 to [gp90]. When these latter reactions were used to test for the noncovalent association of gp70 and p15(E), it was demonstrated that the two proteins maintained their native association in the presence of 0.5% NP-40, both in high and low salt concentrations. These results indicate that (a) a stable, intermolecular disulfide-linked gp70-p15(E) complex can form spontaneously after disruption of virions with nonionic detergents, and after activation of sulfhydryl groups of the membrane proteins, and (b) the complex is not present in large quantities in the intact virions.