125I-ASD photoaffinity-labeling derivatives of pertussis toxin (125I-ASD-PT) or lipopolysaccharide (125I-ASD-LPS) labeled similar 70-kDa proteins in Jurkat cells, a cell line derived from human CD4+ T lymphocytes. Labeling of this 70-kDa protein by 125I-ASD-PT was inhibited by underivatized PT but not by underivatized LPS. However, an immunoglobulin M monoclonal antibody with specificity for the p73 LPS receptor in murine splenocytes (S. W. Bright, T.-Y. Chen, L. M. Flebbe, M.-G. Lei, and D. C. Morrison, J. Immunol. 145:1-7, 1990) inhibited 125I-ASD-PT labeling of the 70-kDa species in Jurkat cells. Our results suggested that PT may bind to the same 70-kDa protein as LPS does in Jurkat cells but that PT and LPS bind to different sites on this receptor candidate. 125I-ASD-PT photoaffinity labeling of the 70-kDa protein was also inhibited by underivatized glycoproteins to which PT has been shown to bind, and this inhibition correlated with the relative binding affinities of the glycoproteins for PT. 125I-ASD derivatives of two sialic acid-specific plant lectins, Maackia amurensis leukoagglutinin and Sambucus nigra agglutinin, with oligosaccharide binding specificities similar to those of PT also labeled a 70-kDa protein in Jurkat cells. This suggests that the 70-kDa PT receptor candidate in Jurkat cells likely contains sialooligosaccharide sequences to which PT, M. amurensis leukoagglutinin, and S. nigra agglutinin bind. The cross-reacting epitope recognized by monoclonal antibody 5D3 in this 70-kDa species might overlap the PT- and LPS-binding sites
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