Journal of Biological Chemistry
Volume 273, Issue 3, 16 January 1998, Pages 1393-1402
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PROTEIN CHEMISTRY AND STRUCTURE
The Copines, a Novel Class of C2 Domain-containing, Calciumdependent, Phospholipid-binding Proteins Conserved from Paramecium to Humans*

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In an attempt to identify proteins that might underlie membrane trafficking processes in ciliates, calcium-dependent, phospholipid-binding proteins were isolated from extracts of Paramecium tetraurelia. The major protein obtained, named copine, had a mass of 55 kDa, bound phosphatidylserine but not phosphatidylcholine at micromolar levels of calcium but not magnesium, and promoted lipid vesicle aggregation. The sequence of a 920-base pair partial cDNA revealed that copine is a novel protein that contains a C2 domain likely to be responsible for its membrane active properties. Paramecium was found to have two closely related copine genes, CPN1 andCPN2. Current sequence data bases indicate the presence of multiple copine homologs in green plants, nematodes, and humans. The full-length sequences reveal that copines consist of two C2 domains at the N terminus followed by a domain similar to the A domain that mediates interactions between integrins and extracellular ligands. A human homolog, copine I, was expressed in bacteria as a fusion protein with glutathione S-transferase. This recombinant protein exhibited calcium-dependent phospholipid binding properties similar to those of Paramecium copine. An antiserum raised against a fragment of human copine I was used to identify chromobindin 17, a secretory vesicle-binding protein, as a copine. This association with secretory vesicles, as well the general ability of copines to bind phospholipid bilayers in a calcium-dependent manner, suggests that these proteins may function in membrane trafficking.

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*

This study was initiated during the tenure of Fogarty Senior International Fellowship FO6 TWO2133 (to C. E. C.) and was supported by National Institutes of Health Grants GM53266, NS31618, and CA40042; by Ministere de l'Education Nationale, de l'Enseignement Superieur, et de la Recherche Grant ACC-SV6 9506004, and by funds from the Centre National de la Recherche Scientifique.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.