Monoclonal antibody to the amino-terminal L sequence of murine leukemia virus glycosylated gag polyproteins demonstrates their unusual orientation in the cell membrane.

To analyze cell surface murine leukemia virus gag protein expression, we have prepared monoclonal antibodies against the spontaneous AKR T lymphoma KKT-2. One of these antibodies, 43-13, detects an AKR-specific viral p12 determinant. A second monoclonal antibody, 43-17, detects a novel murine leukemia virus-related antigen found on glycosylated gag polyproteins (gp95gag, gp85gag, and gp55gag) on the surface of cells infected with and producing ecotropic endogenous viruses, but does not detect antigens within these virions. The 43-17 antibody immunoprecipitates the precursor of the cell surface gag protein whether in its glycosylated or unglycosylated state, but does not detect the cytoplasmic precursor of the virion gag proteins (Pr65gag). Based on these findings, we have localized the 43-17 determinant to the unique amino-terminal part of the glycosylated gag polyprotein (the L domain). We have determined that gp95gag contains L-p15-p12-p30-p10 determinants, whereas gp85gag lacks the carboxyterminal p10 determinant, and gp55gag lacks both p30 and p10 carboxy terminal determinants. Analysis of cell surface gag expression with the 43-17 antibody leads us to propose that the L domain plays a crucial role in (i) the insertion and orientation of murine leukemia virus gag polyproteins in the cell membrane and (ii) the relative abundance of expression of AKR leukemia virus versus Moloney murine leukemia virus glycosylated gag polyproteins in infected cells.

Magnetic resonance studies of the binding site interactions between 19F-labeled nitrophenyl haptens and specific mouse myeloma immunoglobulin MOPC-315.

The interactions between MOPC-315, a mouse myeloma protein with specificity for nitrophenyl haptens, and 19F-substituted haptens have been investigated using nuclear magnetic resonance (NMR) spectroscopy. The haptens studied are mono- or dinitrophenyl derivatives of gamma-aminobutyric acid, lysine, or glycine which have trifuoromethyl groups attached to the phenyl rings. Upon binding to immunoglobulin, the 19F nucleus experiences a downfield shift whose magnitude depends on the position of the trifluoromethyl group on the phenyl ring but is independent of other structural changes in the hapten such as the number of nitro groups attached to the phenyl ring. Further, the chemical shift of bound hapten is not influenced by the amount of the constant region attached to the binding site; we accordingly conclude that the presence of the distal, constant regions of the immunoglobulin molecule does not influence binding site interactions.