Abstract

Of a total of 17 monoclonal antibodies (MAb) directed against structural proteins of the lymphocytic choriomeningitis (LCM) virus, 3 were specific for the viral nucleoprotein (p63) and attached to the plasma membrane of infected cells, as disclosed by the indirect immunofluorescence procedure and complement-mediated cytolysis. We had previously demonstrated that a portion of the nucleoprotein (p63E) was part of the envelope of the intact virion (M. Bruns, W. Zeller, H. Rohdewohld, and F. Lehmann-Grube (1986) Virology 151, 77-85), and we now show that after external iodination of virions followed by limited proteolysis the label was attached to the smallest peptide thus obtained. If purified nucleocapsids were labeled with 125I, digested as before, and incubated with an anti-p63 MAb that has the ability to bind the surface of the infected cell, a similarly small peptide was precipitated; an antibody specific for p63 but not recognizing it on the cell surface precipitated the largest peptide and failed to bring down the small one. We conclude that the epitopes complementary to a few of our anti-p63 MAb are represented on both the virion and the surface of virus-infected cells.