MOUSE MAMMARY-TUMOR VIRUS SUPERANTIGENS REQUIRE N-LINKED GLYCOSYLATION FOR EFFECTIVE PRESENTATION TO T-CELLS

Mouse mammary tumor viruses (MMTVs) encode superantigens that associat e with major histocompatibility complex class II products on antigen-p resenting cells and stimulate T cells in a V beta-specific manner. Thi s T cell activation is critical for completion of the viral life cycle and vertical transmission to the next generation. To investigate the functional significance of extensive viral superantigen (Sag) glycosyl ation, we disrupted the six potential sites for N-linked carbohydrate addition in the Sag encoded by proviral integrant Mtv-1. Shifts in the apparent molecular mass of these mutant glycoproteins suggested that wild-type Mtv-l Sag is glycosylated on four of its six sites. Intracel lular and cell surface staining of the panel of mutants indicated that any decrease in glycosylation resulted in reduced levels of intracell ular protein and undetectable surface expression, suggesting that decr eased glycosylation leads to rapid Sag degradation and abates traffick ing to the plasma membrane. Nevertheless, several mutants with interme diate levels of glycosylation expressed enough Sag on the B cell surfa ce to potently stimulate reactive T cell hybrids. We show there is no specific site bearing N-linked glycosylation that is essential for act ivity, but at least one carbohydrate addition is necessary for effecti ve B cell presentation of MMTV superantigens to T cells. (C) 1997 Acad emic Press.