The role of the silicatein-a interactor silintaphin-1 in biomimetic biomineralization q Matthias Wiens a, * , Melanie Bausen b , Filipe Natalio a , Thorben Link a , Ute Schlossmacher a , Werner E.G. Mu ¨ ller a a Institute for Physiological Chemistry and Pathobiochemistry, Johannes Gutenberg University, Medical School, Duesbergweg 6, 55099 Mainz, Germany b Max Planck Institute for Medical Research, Jahnstrasse 29, 69120 Heidelberg, Germany article info Article history: Received 10 October 2008 Accepted 4 December 2008 Available online 1 January 2009 Keywords: Biosilicification Silicatein interactor Silintaphin-1 Sponges g-Fe 2 O 3 nanoparticles abstract Biosilicification in sponges is initiated by formation of proteinaceous filaments, predominantly consisting of silicateins. Silicateins enzymatically catalyze condensation of silica nanospheres, resulting in symmetric skeletal elements (spicules). In order to create tailored biosilica structures in biomimetic approaches it is mandatory to elucidate proteins that are fundamental for the assembly of filaments. Silintaphin-1 is a core component of modularized filaments and also part of a spicule-enfolding layer. It bears no resemblance to other proteins, except for the presence of an interaction domain that is fundamental for its function as scaffold/template. In the presence of silicatein silintaphin-1 facilitates the formation of in vitro filaments. Also, it directs the assembly of g-Fe 2 O 3 nanoparticles and surface- immobilized silicatein to rod-like biocomposites, synthetic spicules. Thus, silintaphin-1 will contribute to biomimetic approaches that pursue a controlled formation of patterned biosilica-based materials. Its combination with g-Fe 2 O 3 nanoparticles and immobilized silicatein will furthermore inspire future biomedical applications and clinical diagnostics. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Only very few extant taxa integrate silicic acid as a main component in their skeleton, and of these sponges (phylum Por- ifera, classes Demospongiae and Hexactinellida) are singular in that they mediate the biomineralization process enzymatically [1]. This biologically controlled silicification inspired several nano- biotechnological applications that are based on biomimetic approaches (reviewed in Ref. [2]). During biomineralization demosponges deposit amorphous silica in discrete skeletal elements (spicules) within highly specialized cells (sclerocytes) [3]. Spicules lend structural stability to the sponge body, deter predators, and transmit light similar to optic fibers. Their astonishing diversity has received great attention in phylogenetic studies. According to their size, spicules have been grouped into two classes, micro- and megascleres. The latter class of skeletal elements enfolds a central proteinaceous structure, the axial filament. During the initial stage of spiculogenesis in the poriferan model organism Suberites domuncula (Demospongiae) silicatein mono- mers assemble into the rod-like structure of the axial filament. However, the mode of action resulting in this multimerous struc- ture is unknown so far. The organic filament consists to a large extent of two silicateins (silicatein-a,-b) [4], though isoforms have been found in different sponge taxa [5]. According to significant sequence homologies silicateins belong to the protein family of cathepsins, even if they are distinguished from these proteases by replacement of a cysteine residue by serine at the catalytic center. Similar to cathepsins silicateins are expressed as pre-proenzymes containing an N-terminal leader sequence. Posttranslational modifications involve phosphorylation and proteolytic processing, either through autolysis or mediated by the proteolytic activity of a yet unknown enzyme [6]. The axial filament comprises only the processed, matured form of silicatein [7]. In marine environments silicon is predominately available in form of unionized silicic acid [SA; Si(OH) 4 ] [8]. Multimerized sili- cateins mediate the condensation of soluble SA to amorphous hydrated silica, the main constituent of spicules. During this process, silicateins initially act as a template for the formation of q Data deposition: The Suberites domuncula silintaphin-1 cDNA sequence has been deposited (EMBL/GenBank accession number AM902264). * Corresponding author. Tel.: þ49 6131 39 25961; fax: þ49 6131 39 25243. E-mail address: wiens@uni-mainz.de (M. Wiens). Contents lists available at ScienceDirect Biomaterials journal homepage: www.elsevier.com/locate/biomaterials 0142-9612/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2008.12.021 Biomaterials 30 (2009) 1648–1656