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Nanoscale control of the polymerization of silicon and oxygen determines the structures and properties of a wide range of siloxane-based materials, including glasses, ceramics, mesoporous molecular sieves and catalysts, elastomers, resins, insulators, optical coatings, and photoluminescent polymers. In contrast to anthropogenic and geological syntheses of these materials that require extremes of temperature, pressure, or pH, living systems produce a remarkable diversity of nanostructured silicates at ambient temperatures and pressures and at near-neutral pH. We show here that the protein filaments and their constituent subunits comprising the axial cores of silica spicules in a marine sponge chemically and spatially direct the polymerization of silica and silicone polymer networks from the corresponding alkoxide substrates in vitro, under conditions in which such syntheses otherwise require either an acid or base catalyst. Homology of the principal protein to the well known enzyme cathepsin L points to a possible reaction mechanism that is supported by recent site-directed mutagenesis experiments. The catalytic activity of the ''silicatein'' (silica protein) molecule suggests new routes to the synthesis of silicon-based materials.Silicon, the second most abundant element in the earth's crust, interacts with living systems by mechanisms that have remained poorly understood (1-6). Evidence suggests that silicon is essential for normal growth and biological function in a diversity of plant, animal, and microbial systems (2); silicon compounds have been shown to modulate these activities (1, 2), but the molecular mechanisms of these effects have proved elusive. Studies of marine organisms that produce relatively large masses of silicified structures (3) show the underlying molecular and genetic mechanisms by which living systems process silicon. Hildebrand et al. (7) recently characterized the cDNAs coding for a family of silicon transporters in diatoms and showed that the transporters are likely transmembrane proteins. We have found that the marine sponge Tethya aurantia produces copious silica spicules (1-2 mm in length and 30 m in diameter) that constitute 75% of the dry weight of the organism and that each of these spicules contains a central axial filament of protein (1-2 mm in length and 2 m in diameter) consisting of three very similar subunits that we have named silicateins (for silica proteins; ref. 8). Characterization of silicatein ␣ (the subunit comprising nearly 70% of the mass of the filaments) and its cloned cDNA indicated that it is homologous to members of the cathepsin L subfamily of the papain family of proteolytic enzymes (8).We show here that, at neutral pH, the silicatein filaments and their constituent subunits catalyze the in vitro polymerization of silica and silsesquioxanes from tetraethoxysilane and organically modified silicon triethoxides, respectively. These substrates were chosen because of their stability at neutral pH and the similarity of their chemical reactivity to that of...

Earth's biota produces vast quantities of polymerized silica at ambient temperatures and pressures by mechanisms that are not understood. Silica spicules constitute 75% of the dry weight of the sponge Tethya aurantia, making this organism uniquely tractable for analyses of the proteins intimately associated with the biosilica. Each spicule contains a central protein filament, shown by x-ray diffraction to exhibit a highly regular, repeating structure. The protein filaments can be dissociated to yield three similar subunits, named silicatein ␣, ␤, and ␥. The molecular weights and amino acid compositions of the three silicateins are similar, suggesting that they are members of a single protein family. The cDNA sequence of silicatein ␣, the most abundant of these subunits, reveals that this protein is highly similar to members of the cathepsin L and papain family of proteases. The cysteine at the active site in the proteases is replaced by serine in silicatein ␣, although the six cysteines that form disulfide bridges in the proteases are conserved. Silicatein ␣ also contains unique tandem arrays of multiple hydroxyls. These structural features may help explain the mechanism of biosilicification and the recently discovered activity of the silicateins in promoting the condensation of silica and organically modified siloxane polymers (silicones) from the corresponding silicon alkoxides. They suggest the possibility of a dynamic role of the silicateins in silicification of the sponge spicule and offer the prospect of a new synthetic route to silica and siloxane polymers at low temperature and pressure and neutral pH.

Properties of the organic matrix of bone as well as its function in the microstructure could be the key to the remarkable mechanical properties of bone. Previously, it was found that on the molecular level, calcium-mediated sacrificial bonds increased stiffness and enhanced energy dissipation in bone constituent molecules. Here we present evidence for how this sacrificial bond and hidden length mechanism contributes to the mechanical properties of the bone composite, by investigating the nanoscale arrangement of the bone constituents and their interactions. We find evidence that bone consists of mineralized collagen fibrils and a non-fibrillar organic matrix, which acts as a 'glue' that holds the mineralized fibrils together. We believe that this glue may resist the separation of mineralized collagen fibrils. As in the case of the sacrificial bonds in single molecules, the effectiveness of this mechanism increases with the presence of Ca2+ ions.