The structure of SALSA/DMBT1 SRCR domains reveal the conserved ligand-binding mechanism of the ancient SRCR-fold

The scavenger receptor cysteine-rich (SRCR) family of proteins comprise more than 20 membrane-associated and secreted molecules. Characterised by the presence of one or more copies of the ~110 amino acid SRCR domain, this class of proteins have widespread functions as anti-microbial molecules, scavenger- and signalling-receptors. Despite the high level of structural conservation of SRCR domains, no molecular basis for ligand interaction has been described. The SRCR protein SALSA, also known as dmbt1/gp340, is a key player in mucosal immunology. Based on detailed structures of the SALSA SRCR domains 1 and 8, we here reveal a novel universal ligand binding mechanism for SALSA ligands. The binding interface incorporates a dual cation binding site, which is highly conserved across the SRCR super family. Along with the well-described cation dependency on most SRCR domain-ligand interactions, our data suggest that the binding mechanism described for the SALSA SRCR domains is applicable to all SRCR domains. We thus propose to have identified in SALSA a conserved functional mechanism for ligand recognition by the SRCR class of proteins.