BINDING AND SURFACE EXPOSURE CHARACTERISTICS OF THE GONOCOCCAL TRANSFERRIN RECEPTOR ARE DEPENDENT ON BOTH TRANSFERRIN-BINDING PROTEINS

Neisseria gonorrhoeae is capable of iron utilization from human transf errin in a receptor-mediated event. Transferrin-binding protein 1 (Tbp 1) and Tbp2 have been implicated in transferrin receptor function, but their specific roles in transferrin binding and transferrin iron util ization have not yet been defined. We utilized specific gonococcal mut ants lacking Tbp1 or Tbp2 to assess the relative transferrin-binding p roperties of each protein independently of the other. The apparent aff inities of the wild-type transferrin receptor and of Tbp1 and Tbp2 ind ividually were much higher than previously estimated for the gonococca l receptor and similar to the estimates for the mammalian transferrin receptor. The binding parameters of both of the mutants a ere distinct from those of the parent, which expressed two transferrin-binding sit es. Tbp2 discriminated between ferrated transferrin and apotransferrin , while Tbp1 did not. Results of transferrin-binding, affinity purific ation, and protease accessibility experiments were consistent with the hypothesis that Tbp1 and Tbp2 interact in the wild-type strain, altho ugh both proteins were capable of binding to transferrin independently when separated in the mutants. The presence of Tbp1 partially protect ed Tbp2 from trypsin proteolysis, and Tbp2 also protected Tbp1 from tr ypsin exposure. Addition of transferrin to wild-type but not mutant ce lls protected Tbp1 from trypsin but increased the trypsin susceptibili ty of Tbp2, These observations indicate that Tbp1 and Tbp2 function to gether in the wild-type strain to evoke binding conformations that are distinct from those expressed by the mutants lacking either protein.