RECEPTOR-MEDIATED RECOGNITION AND UPTAKE OF IRON FROM HUMAN TRANSFERRIN BY STAPHYLOCOCCUS-AUREUS AND STAPHYLOCOCCUS-EPIDERMIDIS

Staphylococcus aureus and Staphylococcus epidermidis both recognize an d bind the human iron-transporting glycoprotein, transferrin, via a 42 -kDa cell surface protein receptor. In an iron-deficient medium, staph ylococcal growth can be promoted by the addition of human diferric tra nsferrin but not human apotransferrin, To determine whether the staphy lococcal transferrin receptor is involved in the removal of iron from transferrin, we employed 6 M urea-polyacrylamide gel electrophoresis, which separates human transferrin into four forms (diferric, monoferri c N-lobe, and monoferric C-lobe transferrin and apotransferrin), S, au reus and S, epidermidis but not Staphylococcus saprophyticus (which la cks the transferrin receptor) converted diferric human transferrin int o its apotransferrin form within 30 min. During conversion, iron was r emoved sequentially from the N lobe and then from the C lobe, Metaboli c poisons such as sodium azide and nigericin inhibited the release of iron from human transferrin, indicating that it is an energy-requiring process. To demonstrate that this process is receptor rather than sid erophore mediated, we incubated (i) washed staphylococcal cells and (i i) the staphylococcal siderophore, staphyloferrin A, with porcine tran sferrin, a transferrin species which does not bind to the staphylococc al receptor. While staphyloferrin A removed iron from both human and p orcine transferrins, neither S, aureus nor S. epidermidis cells could promote the release of iron from porcine transferrin, In competition b inding assays, both native and recombinant N-lobe fragments of human t ransferrin as well as a naturally occurring human transferrin variant with a mutation in the C-lobe blocked binding of I-125-labelled transf errin, Furthermore, the staphylococci removed iron efficiently from th e iron-loaded N-lobe fragment of human transferrin, These data demonst rate that the staphylococci efficiently remove iron from transferrin v ia a receptor-mediated process and provide evidence to suggest that th ere is a primary receptor recognition site on the N-lobe of human tran sferrin.