DIFFERENTIAL BINDING OF APO AND HOLO HUMAN TRANSFERRIN TO MENINGOCOCCI AND COLOCALIZATION OF THE TRANSFERRIN-BINDING PROTEINS (TBPA AND TBPB)

Apo-transferrin (apo-hTf) and holo-transferrin (holo-hTf) were separat ely conjugated to 15-nm colloidal gold. Iron-restricted Neisseria meni ngitidis strain SD (B:15:P1.16) bound up to three-fold more holo-hTf t han apo-hTf (p<0.001). The ability of meningococcal mutants lacking ei ther transferrin-binding protein A (TbpA) or TbpB to discriminate betw een apo-hTf and holo-hTf was also investigated. There was no significa nt difference between the amount of gold-labelled apo-transferrin boun d by the isogenic TbpA mutant (expressing TbpB) and the parent strain, whereas an isogenic TbpB mutant (expressing TbpA) bound significantly less gold-labelled apo-hTf. The isogenic TbpA and TbpB mutants and th e parent strain all bound significantly more holo-hTf than apo-hTf, wh ereas the double `knock-out' mutant failed to bind hTf irrespective of the iron-loading. In the isogenic mutants, TbpB was more effective in binding either apo- or holo-hTf than TbpA. Monoclonal antibodies agai nst TbpA and TbpB were used to colocalise the transferrin-binding prot eins on strain SD. The ratio of TbpA:TbpB was approximately 1:1. TbpA and TbpB were occasionally observed in close proximity to each other, but the two proteins were generally quite separate, which may indicate that they do not usually form a complex to act as a transferrin recep tor.