Purification, characterization, and cloning of a novel pro-inflammatory secreted protein from Staphylococcus aureus

Staphylococcus aureus is a human pathogen, causing infections ranging from benign skin and soft tissue infections to life-threatening pneumonia, sepsis/infective endocarditis, and toxic shock syndrome (TSS). The organisms cause infections through production of both cell-surface and secreted virulence factors. We focus primarily on secreted virulence factors, having done scans for such proteins for many years. We have purified and characterized TSS toxin-1; staphylococcal enterotoxin-like superantigens K, L, and Q; an operon of six serine proteases, epsilon cytotoxin; and enterotoxin-associated ampicillin resistance protein. The current study clones and characterizes another novel secreted factor that we tentatively refer to as methionine-rich protein (MRP) because its gene encodes a protein with ten methionine residues and eight in the small mature protein. The MRP gene encodes a protein of 199 amino acids with the first 28 being a signal peptide. The mature protein has 171 amino acids and has a molecular weight of approximately 18,800. MRP has an unusual isoelectric point (pI) for staphylococcal secreted virulence factors of approximately 5.3, whereas other secreted virulence factors have neutral or basic pIs. MRP was listed as a hypothetical protein in the NCBI database. We tested MRP for biological activities that characterize S. aureus and observed that the protein has no detectable superantigen, cytolytic (hemolytic), lipase, or protease activity. MRP induced chemokine (interleukin-8) production from human vaginal epithelial cells. This activity potentially facilitates S. aureus provoking harmful mucosal and skin inflammatory responses.