STRUCTURE OF A NOVEL EXTRACELLULAR CA2-BINDING MODULE IN BM-40()

The EF-hand is a highly conserved Ca2+-binding motif found in many cyt osolic Ca2+-modulated proteins. Here we report the crystal structure a t 2.0 Angstrom resolution of the carboxy-terminal domain of human BM-4 0 (SPARC, osteonectin), an extracellular matrix protein containing an EF-hand pair. The two EF-hands interact canonically but their detailed structures are unusual. In the first EF-hand a one-residue insertion is accommodated by a cis-peptide bond and by substituting a carboxylat e by a peptide carbonyl as a Ca2+ ligand. The second EF-hand is stabil ized by a disulphide bond. The EF-hand pair interacts tightly with an amphiphilic aminoterminal helix, reminiscent of target peptide binding by calmodulin. The present structure defines a novel protein module o ccurring in several other extracellular proteins.