Identification of a Staphylococcus aureus extracellular matrix-binding protein with broad specificity

Abstract

A staphylococal surface protein capable of binding several extracellular matrix glycoproteins was purified as a result of our attempts to identify a receptor(s) for bone sialoprotein (BSP) on Staphylococcus aureus cells. Proteins from different staphylococcal strains were solubilized in sodium lauryl sulfate, separated by polyacrylamide gel electrophoresis, blotted onto Immobilon P membranes, and probed with 125I-BSP. Several bacterial proteins bound the radiolabeled ligand, and various strains expressed different repertoirs of BSP-binding proteins. Major BSP-binding proteins with apparent M(r)s of 72,000 or 60,000 were present on most strains, and these proteins were further studied. The 72- and 60-kDa proteins were preferentially expressed when bacteria were cultured in Luria broth compared with when they were cultured on tryptic soy broth, and the abundance of the proteins could be correlated to an increased 125I-BSP binding. Both the 72-kDa and the 60-kDa proteins were solubilized by extraction of cells with 1 M LiCl and were purified by cation-exchange chromatography. Amino acid composition analysis of the purified 72-kDa protein indicated a high content of lysine (11.9%) and hydrophobic amino acids (28.0% combined). In Western ligand blotting (immunoblotting) experiments, the 72-kDa protein bound not only BSP but also radiolabeled fibronectin, fibrinogen, vitronectin, thrombospondin, and, to some extent, collagen. Addition of the purified 60-kDa protein to S. aureus cells did not inhibit binding of the different ligands but in some cases resulted in an augmentation of the binding of 125I-ligand. Purified 60-kDa protein could hemagglutinate sheep erythrocytes at a concentration of 61 micrograms/ml. The agglutination reaction was inhibited by high concentrations of fucose, mannose, or melibiose. These data suggest that the purified proteins may serve as bacterial receptors with broad specificity for matrix glycoproteins and that the proteins may act as carbohydrate-binding proteins.