Silicateins are proteins with catalytic, structure-directing activity that are responsible for silica biosynthesis in certain sponges; they are the constituents of macroscopic protein filaments that are found occluded within the silica needles made by Tethya aurantia. Self-assembly of the silicatein monomers and oligomers is shown to form fibrous structures by a mechanism that is fundamentally different from any previously described filament-assembly process. This assembly proceeds through the formation of diffusionlimited, fractally patterned aggregates on the path to filament formation. The driving force for this self-assembly is suggested to be entropic, mediated by the interaction of hydrophobic patches on the surfaces of the silicatein subunits that are not found on highly homologous congeners that do not form filaments. Our results are consistent with a model in which silicatein monomers associate into oligomers that are stabilized by intermolecular disulfide bonds. These oligomeric units assemble into a fractal network that subsequently condenses and organizes into a filamentous structure. These results represent a potentially general mechanism for protein fiber self-assembly.here is considerable interest in the mechanisms governing the supramolecular assembly of proteins in biological systems (1-6). This interest stems from the dual desire to understand the fundamental mechanisms and harness them for use in technological applications (7-10). A recently discovered family of proteins, called silicateins, are interesting candidates for use in nanobiotechnology because of their dual roles as structural and enzymatic proteins that catalyze and structurally direct the formation of silica, silsesquioxanes, and metal oxide semiconductors (11-16). Thus, silicateins offer the potential for controlled site-directed assembly of nanofibers, which could serve as templates for the enzymatic formation of inorganic materials for use in semiconductor and optoelectronic devices.Silicatein proteins govern the enzymatic and structurally controlled synthesis of silica in marine demosponges. Typically, silicateins are axial protein filaments (2 m ϫ 2 mm) that are occluded within, and run the entire length of, the silica spicules (20 m ϫ 2 mm) that constitute 75% of the dry weight of the sponge Tethya aurantia (11,12). The filament is composed of three related proteins (silicatein ␣, ␤, and ␥ in an apparent ratio of 12:6:1), constituting Ϸ95% of the mass of the filament (11). Preliminary x-ray fiber-diffraction experiments reveal a crystalline order in the filament, although the arrangement of proteins has not been determined because of the complexity of the diffraction pattern (17). In vitro, the silicatein filament is capable of catalyzing the hydrolysis and polycondensation of silicon alkoxides and certain molecular precursors of metal oxides to yield a coating of the filament with silica, titanium dioxide, or gallium oxide (depending on the precursor that is used) (12,15,16). Also, in certain cases, the resulting inorganic ...

) and Mn 3 (PO 4 ) 2 ?7H 2 O thin films have been prepared using a kinetically controlled vapor-diffusion method. Vectorial control by diffusion of ammonia as a base catalyst into an aqueous metal salt solution yields large area (2 cm 2 ) metal hydroxide and metal phosphate films with unique structures. No supporting substrate for growth of the films is necessary in this approach. The films were characterized using X-ray powder diffraction and scanning electron microscopy. The cobalt containing films were studied in more detail using transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption near edge structure and various chemical analysis techniques. For the first time the electronic properties and crystal structure of these materials could be studied in thin films not influenced by the presence of an underlying substrate. For Co 5 (OH) 8 (NO 3 ) 2 ?2H 2 O films, which crystallize in a layered hydrotalcite-like structure that is homogeneous from the nanoscale to the macroscale, unprecedented photoconductivity properties were observed. Resistivity measurements show that this material is a p-type semiconductor with an unusually long minority carrier lifetime and high carrier density.

We introduce a novel method of inorganic synthesis using the catalytic and structure-directing properties of the demosponge enzyme silicatein-alpha. Recombinant silicatein-alpha was displayed at the surface of Escherichia coli cells by fusion to outer membrane protein A and used to biocatalytically direct the formation of layered and amorphous titanium phosphates from a small water-soluble precursor at near-neutral pH at 16 degrees C. Synthesis of titanium phosphates, with potential applications in catalysis and separation technology, previously has required prolonged reactions with phosphoric acid at elevated temperatures. Additionally, we use library screening to isolate a 15-mer with affinity toward the silicatein active site (Kd ca. 50 nM) and introduce this new approach to demonstrate the success of our display strategy. Considering our previous findings with native silicatein filaments, we suggest that this scalable, efficient, cell-based system may have a broad utility for the synthesis of a range of structured metallophosphates and other inorganic materials.

No abstract

No abstract