A bone sialoprotein-binding protein from Staphylococcus aureus: a member of the staphylococcal Sdr family

Staphylococcus aureus bacteria, isolated from bone and joint infections, sp ecifically interact with bone sialoprotein (BSP), a glycoprotein of bone an d dentine extracellular matrix, via a cell-surface protein of M-r 97000 [Ya coub, Lindahl, Rubin, Wendel, Heinegard and Ryden, (1994) fur. J, Biochem. 222, 919-925]. Amino acid sequences of seven trypsin fragments from the 970 00-M-r BSP-binding protein were determined. A gene encoding a protein encom passing all seven peptide sequences was identified from chromosomal DNA iso lated from S. aureus strain O24. This gene encodes a protein with 1171 amin o acids, called BSP-binding protein (Bbp), which displays similarity to rec ently described proteins of the Sdr family from S. aureus. SdrC, SdrD and S drE encode putative cell-surface proteins with no described ligand specific ity. Bbp also shows similarity to a fibrinogen-binding protein from S. epid ermidis called Fbe. A serine-aspartic acid repeat sequence was found close to the cell-wall-anchoring Leu-Pro-Xaa-Thr-Gly sequence in the C-terminal e nd of the protein. Escherichia coli cells were transformed with an expressi on vector containing a major part of the bbp gene fused to the gene for glu tathione S-transferase. The affinity-purified fusion protein bound radiolab elled native BSP, and inhibited the binding of radiolabelled BSP to staphyl ococcal cells. Serum from patients suffering from bone and joint infection contained antibodies that reacted with the fusion protein of the BSP-bindin g protein, indicating that the protein is expressed during an infection and is immunogenic. The S. aureus Bbp protein may be important in the localiza tion of bacteria to bone tissue, and thus might be of relevance in the path ogenicity of osteomyelitis.