Abstract
A highly purified gel electrophoretic fraction of murine leukemia virus (MuLV) can confer enhanced infectivity on MuLV derived from tissue culture and can protect MuLV from neutralization by specific antiserum. Of the known viral proteins tested, a purified electrophoretic fraction (14,000 daltons) is the only active fraction and seems to consist of the group-specific antigen I and an associated lipid moiety. The action of this fraction obtained from one type of MuLV is group-specific in that it can enhance and protect serologically different types of MuLV. The effect of this fraction is exerted on viruses and not on cells. MuLV derived from tumors was not enhanced by the fraction, but became amenable to its action after a single passage through tissue culture. Reconstruction experiments suggested that the fraction may consist of an exterior protein associated with some lipids. This complex, which apparently protrudes on the viral surface, is required for infectivity and shields the type-specific antigen containing the hemagglutinating site.