| Pertussis toxin, a virulence factor from Bordetella pertussis, has been shown to bind to a variety of glycolipids and glycoproteins, most containing sialic acid. In addition to being capable of ADP-ribosylating substrate G proteins, pertussis toxin elicits an increase in intracellular calcium concentrations when applied to lymphocytes and platelets. This rise in intracellular calcium is called the B oligomer response because only the binding subunits of the holotoxin are required; the ADP-ribosyltransferase activity of the S1 subunit is not necessary. These studies examined which binding sites of pertussis toxin on human platelets and Jurkat cells, a human T lymphocyte cell line, are involved in a biological response to the toxin.;Calcium mobilization studies in Jurkat cells were used to characterize pertussis toxin receptors. Both neuraminidase and pronase treatment of Jurkat cells decreased the B oligomer response, leading to the conclusion that a receptor for the toxin is a sialoglycoprotein. Numerous binding sites were identified for the toxin, including CD45, a transmembrane tyrosine phosphatase. While PT did bind CD45, using Jurkat cells lacking the extracellular domain of the protein, it was determined that CD45 is not a receptor for the B oligomer response. It was also found that cells which do not respond to the B oligomer, like J45.01, HSB, CHO, and EL-4, still have ADP-ribosylated G proteins after holotoxin exposure and that receptors leading to ADP-ribosylation also appear to contain sialic acid residues. Along with data showing that the B oligomer does bind to a receptor necessary for CHO cell clustering activity, it was concluded that the receptors leading to a B oligomer response are a subset of the receptors for pertussis toxin leading to the ADP-ribosylation of substrate G proteins in target cells.;Using a ligand blotting technique, pertussis toxin was found to bind to several platelet sialoglycoproteins, including GPIb and GPIIb. Platelet activation induced by the toxin, as measured by an increase in intracellular calcium levels and aggregation, was inhibited in the presence of a monoclonal antibody against GPIb. This suggested that platelet glycoprotein Ib is a receptor for the toxin on platelets. |
