THE 70-KILODALTON PERTUSSIS TOXIN-BINDING PROTEIN IN JURKAT CELLS

I-125-ASD photoaffinity-labeling derivatives of pertussis toxin (I-125-ASD-PT) or lipopolysaccharide (I-125-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 I-125-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 I-125-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. I-125-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. I-125-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.