The first structure, of the unbound SdrG protein from Staphylococcus epidermis, shows a wide cleft between two immunoglobulin-like domains. This cleft is the binding site for a peptide from the extracellular matrix protein fibrinogen.
But this is no simple binding event. The presence of the peptide, found the researchers, induces conformational changes in a COOH- terminal region of SdrG that previously extended out from the protein's N3 domain to wave in the wind. After peptide binding, the region now extends over the peptide, thus forming a hydrogen-bonded roof, and then inserts into a β-sheet in the adjacent N2 domain.
The latch may be far from unique. The region in N2 that receives the latching β strand makes up a motif that is present in many other gram-positive bacterial adhesins. These proteins, which are used by bacteria that replicate extracellularly, “are all like extending velcros,” says Narayan, “but there is specificity.” For SdrG, the binding also covers the thrombin-cleavage site in fibrinogen. This prevents the formation of fibrinopeptides that can act as chemoattractants for leukocytes and fibroblasts. ▪