Structure of the SH3 domain of human osteoclast‐stimulating factor at atomic resolution

Osteoclast‐stimulating factor (OSF) is an intracellular signaling protein, produced by osteoclasts themselves, that enhances osteoclast formation and bone resorption. It is thought to act via an Src‐related signaling pathway and contains SH3 and ankyrin‐repeat domains which are involved in protein–protein interactions. As part of a structure‐based anti‐bone‐loss drug‐design program, the atomic resolution X‐ray structure of the recombinant human OSF SH3 domain (hOSF‐SH3) has been determined. The domain, residues 12–72, yielded crystals that diffracted to the ultrahigh resolution of 1.07 Å. The overall structure shows a characteristic SH3 fold consisting of two perpendicular β‐­sheets that form a β‐barrel. Structure‐based sequence alignment reveals that the putative proline‐rich peptide‐binding site of hOSF‐SH3 consists of (i) residues that are highly conserved in the SH3‐domain family, including residues Tyr21, Phe23, Trp49, Pro62, Asn64 and Tyr65, and (ii) residues that are less conserved and/or even specific to hOSF, including Thr22, Arg26, Thr27, Glu30, Asp46, Thr47, Asn48 and Leu60, which might be key to designing specific inhibitors for hOSF to fight osteoporosis and related bone‐loss diseases. There are a total of 13 well defined water molecules forming hydrogen bonds with the above residues in and around the peptide‐binding pocket. Some of those water molecules might be important for drug‐design approaches. The hOSF‐SH3 structure at atomic resolution provides an accurate framework for structure‐based design of its inhibitors.