Nbl. Powell et al., DIFFERENTIAL BINDING OF APO AND HOLO HUMAN TRANSFERRIN TO MENINGOCOCCI AND COLOCALIZATION OF THE TRANSFERRIN-BINDING PROTEINS (TBPA AND TBPB), Journal of Medical Microbiology, 47(3), 1998, pp. 257-264
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.