Fractal-related assembly of the axial filament in the demosponge Suberites domuncula: relevance to biomineralization and the formation of biogenic silica.

The siliceous spicules of sponges (Porifera) show great variations of sizes, shapes and forms; they constitute the chief supporting framework of these animals; these skeletal elements are synthesized enzymatically by silicatein. Each sponge species synthesizes at least two silicateins, which are termed -alpha and -beta. In the present study, using the demosponge Suberites domuncula, we studied if the silicateins of the axial filament contribute to the shape formation of the spicules. For these experiments native silicateins have been isolated by a new Tris/glycerol extraction procedure. Silicateins isolated by this procedure are monomeric (24 kDa), but readily form dimers through non-covalent linkages; they show a considerable proteolytic activity that increases during the polymerization phase of the protein. The assembled silicateins (dimers, tetramers as well as hexamers) can be demonstrated in zymograms. The filament/aggregate formation from disassembled silicatein can be visualized by light microscopy and by transmission electron microscopic (TEM) analyses. Since in S. domuncula silicatein-alpha is four times more abundant in the axial filament than silicatein-beta we propose that four silicateins form a platform with serine clusters directed to the center. These serines of the con-axially arranged silicateins interact with silicatein-beta. We conclude that initially the silicateins re-assemble chaotically, and in the second phase order themselves to fractal-like structures, which subsequently form the filaments.