STAPHYLOCOCCI EXPRESS A RECEPTOR FOR HUMAN TRANSFERRIN - IDENTIFICATION OF A 42-KILODALTON CELL-WALL TRANSFERRIN-BINDING PROTEIN

Staphylococcus aureus and the coagulase-negative staphylococci are com monly responsible for peritonitis in renal patients undergoing continu ous ambulatory peritoneal dialysis. To simulate growth conditions in v ivo, staphylococci isolated from peritoneal infections were cultured i n used human peritoneal dialysate (HPD). Immunoblotting experiments us ing cell wall preparations from these staphylococci revealed the prese nce of the host iron-binding glycoprotein transferrin bound to S. aure us, S. epidermidis, S. capitis, S. haemolyticus, and S. hominis but no t to S. warneri or S. saprophyticus. Similar results were obtained by incubating broth grown staphylococci with human transferrin, although, in contrast to S. aureus, the coagulase-negative staphylococci bound more transferrin after growth in iron-restricted broth. To determine w hether the staphylococci express a saturable specific receptor for hum an transferrin, the interaction of human I-125-transferrin with the st aphylococci was examined. Both S. aureus and S. epidermidis bound the radiolabelled iron-saturated ligand in a time- and concentration-depen dent manner. From competition binding assays, the affinity (K-d) and n umber of receptors were estimated for S. epidermidis (K-d, 0.27 mu M; 4,200 receptors per cell) and S. aureus (K-d, 0.28 mu M; 4,200 recepto rs per cell). S. epidermidis but not S. aureus receptor activity was p artially iron regulated. Human apotransferrin and iron-saturated trans ferrin and rabbit and rat transferrins competed equally well for the s taphylococcal receptor. Bovine and porcine transferrins and ovotransfe rrin as well as human and bovine lactoferrins were much less effective at competing with human transferrin. Treatment of whole staphylococci with proteases abolished transferrin binding, indicating the involvem ent of cell surface protein. Western blots (immunoblots) of cell wall preparations probed with human transferrin revealed the presence of a 42-kDa transferrin-binding protein common to both S. aureus and S. epi dermidis. On Western strip blots, the binding of human transferrin to this protein was blocked by labelled human transferrin but not by albu min, immunoglobulin G, or bovine transferrin or ovotransferrin. To ass ess the conservation of the 42-kDa transferrin-binding protein, cell w all proteins of S. epidermidis, S. haemolyticus, S. capitis, S. homini s, S. warneri, and S. saprophyticus were Western blotted and probed wi th human transferrin. Only S. warneri and S. saprophyticus lacked the 42-kDa wall protein, consistent with their inability to bind transferr in. These data show that the staphylococci express a specific receptor for human transferrin based at least in part on a common 42-kDa cell wall protein.