The second C2-domain of copine-2, copine-6 and copine-7 is responsible for their calcium-dependent membrane association

Abstract

The copine family of proteins contains nine members with a similar domain structure, namely two N-terminal C2-domains (C2A and C2B) and a C-terminal A-domain. The former are thought to be responsible for binding to the inner face of the plasma membrane following increases in intracellular calcium levels, whereas the A-domain has been suggested to be a protein-binding structure. In this study, we examined the effects of mutagenesis of selected residues in the linker area between the C2-domains and the A-domain, and mutagenesis of the aspartates of the C2-domains, which are predicted to bind calcium and promote membrane association of the copines. We found that Lys282-Lys284 of the linker area are important for the folding of the intact protein. We showed that substitution with asparagine, single or multiple, of the aspartates in the C2A-domain had no effect on the calcium-mediated membrane association of copine-2, copine-6, or copine-7. Similar mutagenesis of a single residue in the C2B-domain of copine-6 (but not copine-2 and copine-7) was sufficient to eliminate its calcium-mediated membrane binding, and simultaneous substitution of all four of the asparagines in the C2B-domain resulted in constitutive membrane association of copine-2, copine-6 and copine-7 with the plasma membrane. These data show that the C2B-domains of copine-2, copine-6 and copine-7 are the domains responsible for the protein calcium-dependent membrane association. We examined the effects of substitution of selected aspartate residues, predicted to bind calcium and promote membrane association of the copines. We show that the C2B-domain is critical for the membrane association of copines and the linker region of copines is crucial for their folding. Our data provide new mutants and framework for mutagenesis to facilitate study of these proteins.