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J. Biol. Chem., Vol. 278, Issue 17, 14712-14722, April 25, 2003
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From the The transferrin receptor of Neisseria
meningitidis is composed of the transmembrane protein TbpA and
the outer membrane protein TbpB. Both receptor proteins have the
capacity to independently bind their ligand human transferrin (htf). To
elucidate the specific role of these proteins in receptor function,
isothermal titration calorimetry was used to study the interaction
between purified TbpA, TbpB or the entire receptor (TbpA + TbpB) with
holo- and apo-htf. The entire receptor was shown to contain a single
high affinity htf-binding site on TbpA and approximately two lower affinity binding sites on TbpB. The binding sites appear to be independent. Purified TbpA was shown to have strong ligand preference for apo-htf, whereas TbpA in the receptor complex with TbpB
preferentially binds the holo form of htf. The orientation of the
ligand specificity of TbpA toward holo-htf is proposed to be the
physiological function of TbpB. Furthermore, the thermodynamic mode of
htf binding by TbpB of isotypes I and II was shown to be different. A
protocol for the generation of active, histidine-tagged TbpB as well as its individual N- and C-terminal domains is presented. Both domains are
shown to strongly interact with each other, and isothermal titration
calorimetry and circular dichroism experiments provide clear evidence
for this interaction causing conformational changes. The N-terminal
domain of TbpB was shown to be the site of htf binding, whereas the
C-terminal domain is not involved in binding. Furthermore, the
interactions between TbpA and the different domains of TbpB have been demonstrated.
Aventis Pasteur, 1541 avenue Marcel
Mérieux, 69280 Marcy l'Etoile, France, the ¶ Metalloprotein
Research Group, The Randall Center for Molecular Mechanisms of Cell
Function, King's College London, London SE1 1UL, United Kingdom,
and the
Centre for Applied Microbiology and Research,
Salisbury SP4 0JG, United Kingdom
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