Resistance of native, oligomeric envelope on simian immunodeficiency virusto digestion by glycosidases

Stocks of simian immunodeficiency virus (SM from the supernatants of infect ed cell cultures were used to examine the sensitivity of envelope glycoprot ein gp120 to enzymatic deglycosylation and the effects of enzyme treatment on infectivity. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophor esis and Western blot analysis revealed little or no change in the mobility of virion-associated gp120 after digestion with high concentrations of N-g lycosidase F, endoglycosidase F, endoglycosidase H, and endo-beta -galactos idase. Soluble gp120, which was not pelletable after the enzymatic reaction , was sensitive to digestion by the same enzymes within the same reaction m ix and was only slightly less sensitive than gp120 that had been completely denatured by boiling in the presence of SDS and beta -mercaptoethanol. Dig estion by three of the seven glycosidases tested significantly changed the infectivity titer compared to that of mock-treated virus. Digestion by endo -beta -galactosidase increased infectivity titers by about 2.5-fold, and ne uraminidase from Newcastle disease virus typically increased infectivity ti ters by 8-fold. Most or all of the increase in infectivity titer resulting from treatment with neuraminidase could be accounted for by effects on the virus, not the cells; SIV produced in the presence of the sialic acid analo g 2,3-dehydro-2-deoxy-N-acetylneuraminic acid also exhibited increased infe ctivity, and the effects could not be duplicated by neuraminidase treatment of cells. Digestion with mannosidase reduced infectivity by fivefold. Our results indicate that carbohydrates on native oligomeric gp120 as it exists on the surface of virus particles are largely occluded and are refractory to digestion by glycosidases. Furthermore, the sialic acid residues at the ends of carbohydrate side chains significantly reduce the inherent infectiv ity of SIV.