Silicon is the second most common element in the Earth's crust [1]; it possesses semi-metallic as well as metalloid properties. Silicon exists in nature in combination with oxygen as silicate ions or as silica; silica has no negative charge, while silicate anions carry a negative net electrical charge, which is counterbalanced by cations. Free silica ⁄ silicate is found both in the crystalline state (such as quartz) and in the amorphous state (such as opal). Silica ⁄ silicate is widely used in industry and medicine for the fabrication of poly(silicate), e.g. in amorphous glasses, ceramics, paints, adhesives, catalysts and photonic materials [2,3]. Furthermore, poly(silicate) is an important new material in nano(bio)technology [4,5]. This multidisciplinary research field is concerned with bio-and electronic engineering at nanometer, molecular and cellular levels [4]. Currently, production of silica require high temperature conditions and extremes of pH [6]. Hydrated amorphous silica, e.g. in the form of opal, has superb properties such as low density and high porosity. In nature, amorphous silica can be produced by diatoms by passive deposition onto an organic matrix. Siliceous sponges (Demospongiae) have the exceptional ability to synthesize silica enzymatically via silicatein [7,8]. Based on its protein sequence, silicatein is related to the proteinases of the cathepsin class [9].Silicatein has been isolated from a number of siliceous sponges, e.g. Tethya aurantium or Suberites domuncula [9,10]. If the enzyme is isolated from the skeletal elements of these animals, the spicules, it can be used in vitro to catalyze polycondensation of a wide variety of alkoxides, as well as ionic and organometallic Siliceous sponges can synthesize poly(silicate) for their spicules enzymatically using silicatein. We found that silicatein exists in silica-filled cell organelles (silicasomes) that transport the enzyme to the spicules. We show for the first time that recombinant silicatein acts as a silica polymerase and also as a silica esterase. The enzymatic polymerization ⁄ polycondensation of silicic acid follows a distinct course. In addition, we show that silicatein cleaves the ester-like bond in bis(p-aminophenoxy)-dimethylsilane. Enzymatic parameters for silica esterase activity are given. The reaction is completely blocked by sodium hexafluorosilicate and E-64. We consider that the dual function of silicatein (silica polymerase and silica esterase) will be useful for the rational synthesis of structured new silica biomaterials.

During evolution and with the emergence of multicellular animals, the need arose to ward off foreign organisms that threaten the integrity of the animal body. Among many different receptors that participate in the recognition of microbial invaders, toll-like receptors (TLRs) play an essential role in mediating the innate immune response. After binding distinct microbial components, TLRs activate intracellular signaling cascades that result in an induced expression of diverse antimicrobial molecules. Because sponges (phylum Porifera) are filter feeders, they are abundantly exposed to microorganisms that represent a potential threat. Here, we describe the identification, cloning, and deduced protein sequence from 3 major elements of the poriferan innate response (to bacterial lipopeptides): the TLR, the IL-1 receptor-associated kinase-4-like protein (IRAK-4l), and a novel effector caspase from the demosponge Suberites domuncula. Each molecule shares significant sequence similarity with its homologues in higher Metazoa. Sequence homologies were found in particular within the family-specific domains toll/interleukin-1 receptor/resistance (TLR family), Ser/Thr/Tyr kinase domain (IRAK family), and CASc (caspase family). In addition, in situ hybridization and immunohistological analyses revealed an abundance of SDTLR (TLR) transcripts in epithelial layers of the sponge surface (exopinacoderm and endopinacoderm). Furthermore, it is shown that both SDTLR and SDIRAK-4 like (IRAK) are expressed constitutively, regardless of treatment with synthetic triacyl lipopeptide Pam(3)Cys-Ser-(Lys)(4). In contrast, SDCASL (caspase) expression is highly Pam(3)Cys-Ser-(Lys)(4) inducible. However, blocking of the lipopeptide with recombinant TLR prior to its application completely prevented the induced expression of this poriferan caspase. These results underscore that the phylogenetically oldest extant metazoan phylum is provided already with the signaling pathways of the antimicrobial host-defense system of Metazoa.

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