Purification and Characterization of a Novel Calcium-binding Protein from the Extrapallial Fluid of the Mollusc, Mytilus edulis*

Autor: Hattan, Stephen J., Laue, Thomas M., Chasteen, N. Dennis
Zdroj: Journal of Biological Chemistry; February 2001, Vol. 276 Issue: 6 p4461-4468, 8p
Abstrakt: In the bivalve mollusc Mytilus edulisshell thickening occurs from the extrapallial (EP) fluid wherein secreted shell matrix macromolecules are thought to self-assemble into a framework that regulates the growth of CaCO3crystals, which eventually constitute ∼95% of the mature shell. Herein is the initial report on the purification and characterization of a novel EP fluid glycoprotein, which is likely a building block of the shell-soluble organic matrix. This primary EP fluid protein comprises 56% of the total protein in the fluid and is shown to be a dimer of 28,340 Da monomers estimated to be 14.3% by weight carbohydrate. The protein is acidic (pI = 4.43) and rich in histidine content (11.14%) as well as in Asx and Glx residues (25.15% total). The N terminus exhibits an unusual repeat sequence of histidine and aspartate residues that occur in pairs: NPVDDHHDDHHDAPIVEHHD∼. Ultracentrifugation and polyacrylamide gel electrophoresis demonstrate that the protein binds calcium and in so doing assembles into a series of higher order protomers, which appear to have extended structures. Circular dichroism shows that the protein-calcium binding/protomer formation is coupled to a significant rearrangement in the protein's secondary structure in which there is a major reduction in β-sheet with an associated increase in α-helical content of the protein. A model for shell organic matrix self-assembly is proposed.
Databáze: Supplemental Index