We used the combination of preparative electrophoresis and immunological detection to isolate two new proteins from the shell calcitic prisms of Pinna nobilis, the Mediterranean fan mussel. The amino acid composition of these proteins was determined. Both proteins are soluble, intracrystalline, and acidic. The 38-kDa protein is glycosylated; the 17-kDa one is not. Ala, Asx, Thr, and Pro represent the dominant residues of the 38-kDa protein, named calprismin. An N-terminal sequence was obtained from calprismin. This sequence, which comprises a pattern of 4 cysteine residues, is not related to any known protein. The second protein, named caspartin, exhibits an unusual amino acid composition, since Asx constitutes by far the main amino acid residue. Preliminary sequencing surprisingly suggests that the first 75 N-terminal residues are all Asp. Caspartin self-aggregates spontaneously into multimers. In vitro tests show that it inhibits the precipitation of calcium carbonate. Furthermore, it strongly interferes with the growth of calcite crystals. A polyclonal antiserum raised against caspartin was used to localize this protein in the shell by immunogold. The immunolocalization demonstrates that caspartin is distributed within the prisms and makes a continuous film at the interface between the prisms and the surrounding insoluble sheets. Our finding emphasizes the prominent role of aspartic acid-rich proteins for the building of calcitic prisms among molluscs.The secretion of a shell by molluscs is a striking example of a selfassembling process performed outside living tissues. When a mollusc builds its shell, the calcifying epithelium of its mantle extrudes mineral ions, mainly calcium and bicarbonate. In addition, it secretes an extracellular matrix composed of proteins, glycoproteins, proteoglycans, and polysaccharides (1). This mixture is released into the extrapallial space, a microvolume delimited by the epithelium, the growing shell, and the leathery periostracum (2). In this supersaturated space, the released macromolecules interact with bicarbonate and calcium to form biocrystallites that self-aggregate in an orderly manner. The end product is a densely packed organomineral assembly, the shell, in which the mineral phase represents more than 95% by weight (3). As shell proteins exert a control on the biomineralization process, they can be used in the synthesis of biomimetic materials (4 -6). However, the resolution of the primary structure of molluscan shell proteins has been seriously impaired for decades by their polydispersity, their polyanionic properties, and finally their post-translational modifications (3, 7).Despite these obstacles, investigations on molluscan shell proteins have made significant advances in the last 6 years, with the characterization of 16 proteins and, for most of them, the identification of the corresponding transcript (8, 9). Among the different shell textures that molluscs use to produce their shell, mother-of-pearl has received the greatest deal of attention. Its intrinsic bea...
