Calcite-specific coupling protein in barnacle underwater cement.

The barnacle relies for its attachment to underwater foreign substrata on the formation of a multiprotein complex called cement. The 20 kDa cement protein is a component of Megabalanus rosa cement, although its specific function in underwater attachment has not, until now, been known. The recombinant form of the protein expressed in bacteria was purified in soluble form under physiological conditions, and confirmed to retain almost the same structure as that of the native protein. Both the protein from the adhesive layer of the barnacle and the recombinant protein were characterized. This revealed that abundant Cys residues, which accounted for 17% of the total residues, were in the intramolecular disulfide form, and were essential for the proper folding of the monomeric protein structure. The recombinant protein was adsorbed to calcite and metal oxides in seawater, but not to glass and synthetic polymers. The adsorption isotherm for adsorption to calcite fitted the Langmuir model well, indicating that the protein is a calcite-specific adsorbent. An evaluation of the distribution of the molecular size in solution by analytical ultracentrifugation indicated that the recombinant protein exists as a monomer in 100 mm to 1 m NaCl solution; thus, the protein acts as a monomer when interacting with the calcite surface. cDNA encoding a homologous protein was isolated from Balanus albicostatus, and its derived amino acid sequence was compared with that from M. rosa. Calcite is the major constituent in both the shell of barnacle base and the periphery, which is also a possible target for the cement, due to the gregarious nature of the organisms. The specificity of the protein for calcite may be related to the fact that calcite is the most frequent material attached by the cement.

Identification and functional characterization of a novel barnacle cement protein.

Barnacle attachment to various foreign materials in water is guided by an extracellular multiprotein complex. A 19 kDa cement protein was purified from the Megabalanus rosa cement, and its cDNA was cloned and sequenced. The gene was expressed only in the basal portion of the animal, where the histologically identified cement gland is located. The sequence of the protein showed no homology to other known proteins in the databases, indicating that it is a novel protein. Agreement between the molecular mass determined by MS and the molecular weight estimated from the cDNA indicated that the protein bears no post-translational modifications. The bacterial recombinant was prepared in soluble form under physiologic conditions, and was demonstrated to have underwater irreversible adsorption activity to a variety of surface materials, including positively charged, negatively charged and hydrophobic ones. Thus, the function of the protein was suggested to be coupling to foreign material surfaces during underwater attachment. Homologous genes were isolated from Balanus albicostatus and B. improvisus, and their amino acid compositions showed strong resemblance to that of M. rosa, with six amino acids, Ser, Thr, Ala, Gly, Val and Lys, comprising 66-70% of the total, suggesting that such a biased amino acid composition may be important for the function of this protein.