The sponge silicatein-interacting protein silintaphin-2 blocks calcite formation of calcareous sponge spicules at the vaterite stage

Abstract

Ca-carbonate, the inorganic matrix of the spicules from the calcareous sponges, is formed as the result of an enzyme-catalyzed reaction with the carbonic anhydrase [CA] as a decisive component. The growth and the morphology of the spicules are genetically controlled, and are taxon-specific. In the present study it is shown that the silicatein-interacting protein silintaphin-2 is present at the surface of the siliceous spicules of the demosponge Suberites domuncula and prevents the association of calcareous crystals synthesized in vitro to these skeletal elements. Silintaphin-2 comprises a Ca²⁺-binding domain that is formed by a 22 amino acid-long peptide, N-DDDSQGEIQSDMAEEEDDDNVD-C. This very acidic 22-meric peptide, termed D/E-peptide, is shown to decelerate the in vitro synthesis of calcite through blocking the transformation of amorphous Ca-carbonate to the crystalline morph calcite at the level of vaterite. This effect is seen at a molar ratio of D/E-peptide : Ca²⁺ of 1 : 5000. During the deposition of Ca-carbonate the peptide becomes incorporated into the crystallites. Determinations of the mechanical characteristics of the formed Ca-carbonate deposits revealed a hardness of 1.98 ± 0.31 GPa (calcite) and 1.38 ± 0.39 GPa (vaterite), an elastic modulus of 72.83 ± 11.68 GPa (calcite) and 39.13 ± 8.04 GPa (vaterite) and a creep of 5.44 ± 1.15 (calcite) and 9.95 ± 1.60 (vaterite) per maximal depth (%). It is concluded that the D/E-peptide interferes with the Ca²⁺ ions within the growing vaterite crystals and freezes this unstable phase formed during calcite formation. It is postulated that proteins, like silintaphin-2 with its Ca²⁺-binding domain D/E-peptide, are involved in the taxon-specific control of the synthesis of the inorganic matrix of the sponge spicules.