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

Abstract 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 − α and − β . 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- α is four times more abundant in the axial filament than silicatein- β 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- β . 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.