Structure−Function Relationships in Sorcin, a Member of the Penta EF-Hand Family. Interaction of Sorcin Fragments with the Ryanodine Receptor and an Escherichia coli Model System

Abstract

Sorcin, a 21.6 kDa cytosolic EF-hand protein which undergoes a Ca²⁺-induced translocation from cytoplasm to membranes, has been assigned to the newly defined penta EF-hand family. A molecular model of the C-terminal Ca²⁺-binding domain has been generated using as a template the X-ray coordinates of the corresponding domain in the calpain light subunit, the family prototype [Lin, G., et al. (1997) Nat. Struct. Biol.4, 539−546]. The model indicates that in sorcin the three-dimensional structure is conserved and in particular that of EF1, the novel EF-hand motif characteristic of the family. On this basis, two stable fragments have been obtained and characterized. Just like the native protein, the sorcin Ca²⁺-binding domain (residues 33−198) is largely dimeric, interacts with the ryanodine receptor at physiological calcium concentrations, and undergoes a reversible, Ca²⁺-dependent translocation from cytosol to target proteins on Escherichia coli membranes. In contrast, the 90−198 fragment (residues 90−198), which lacks EF1 and EF2, does not bind Ca²⁺ with high affinity and is unable to translocate. Binding of calcium to the EF1−EF2 pair is therefore required for the activation of sorcin which uses the C-terminal calcium-binding domain for interaction with the ryanodine receptor, a physiological target in muscle cells.