Binding and surface exposure characteristics of the gonococcal transferrin receptor are dependent on both transferrin-binding proteins

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Abstract
Neisseria gonorrhoeae is capable of iron utilization from human transferrin in a receptor-mediated event. Transferrin-binding protein 1 (Tbp1) and Tbp2 have been implicated in transferrin receptor function, but their specific roles in transferrin binding and transferrin iron utilization have not yet been defined. We utilized specific gonococcal mutants lacking Tbp1 or Tbp2 to assess the relative transferrin-binding properties of each protein independently of the other. The apparent affinities of the wild-type transferrin receptor and of Tbp1 and Tbp2 individually were much higher than previously estimated for the gonococcal receptor and similar to the estimates for the mammalian transferrin receptor. The binding parameters of both of the mutants were distinct from those of the parent, which expressed two transferrin-binding sites. Tbp2 discriminated between ferrated transferrin and apotransferrin, while Tbp1 did not. Results of transferrin-binding affinity purification, and protease accessibility experiments were consistent with the hypothesis that Tbp1 and Tbp2 interact in the wild-type strain, although both proteins were capable of binding to transferrin independently when separated in the mutants. The presence of Tbp1 partially protected Tbp2 from trypsin proteolysis, and Tbp2 also protected Tbp1 from trypsin exposure. Addition of transferrin to wild-type but not mutant cells protected Tbp1 from trypsin but increased the trypsin susceptibility of Tbp2. These observations indicate that Tbp1 and Tbp2 function together in the wild-type strain to evoke binding conformations that are distinct from those expressed by the mutants lacking either protein.