The silicatein propeptide acts as inhibitor/modulator of self‐organization during spicule axial filament formation

Abstract

Silicateins are crucial enzymes that are involved in formation of the inorganic biosilica scaffold of the spicular skeleton of siliceous sponges. We show that silicatein acquires its structure-guiding and enzymatically active state by processing of silicatein from pro-silicatein to the mature enzyme. A recombinant propeptide (PROP) of silicatein from the siliceous demosponge Suberites domuncula was prepared, and antibodies were raised against the peptide. In sponge tissue, these antibodies reacted with both surface structures and the central region of the spicules. Using phage display expression, spicule-binding 12-mer peptides were identified that are rich in histidine residues. In the predicted tertiary structure of PROP, these histidine residues are only present in the a-helical region. The recombinant PROP was found to inhibit self-assembly of silicatein molecules. By light scattering, it was shown that, in the presence of 4 M urea, the recombinant silicatein is obtained in the mono/oligomeric form with a hydrodynamic radius of 4 nm, while lower urea concentrations promote self-aggregation and assembly of the protein. Finally, it is shown that the enzymatic activity of silicatein is abolished by PROP in silicatein samples that predominantly contain mono-or oligomeric silicatein particles, but the enzyme is not affected if present in the filamentous aggregated form. It is concluded that the functions of silicatein, acting as a structural template for its biosilica product and as an enzyme, are modulated and controlled by its propeptide. Structured digital abstractalpha-silicatein and alpha-silicatein bind by transmission electron microscopy (View interaction)[Structured digital abstract was added on 27 March 2013 after original online publication]

Biomimetic Magnetite Formation: From Biocombinatorial Approaches to Mineralization Effects

“…10,11 Recent examples are selections for the binding of demosponge spicule silica, 12 synthetic silica, 13 ZnO, 14 and GdO. 15 Because the selections can be performed under close-to-physiological conditions, the question has arisen as to whether natural and synthetic selection evolves molecules with similar characteristics and whether the biomineralizing functionality might be encoded in homologue structures for materials also found in organisms.…”

Characterization and osteogenic activity of a silicatein/biosilica-coated chitosan-graft-polycaprolactone

“…The height and roughness analyses of the surface topologies demonstrated that silicatein immobilization had taken place: whereas the uncoated CHS-g-PCL-NH 2 surface appeared flat with the fibrous patterns of microcrystals (Fig. 4E), the coated surface was covered by a smooth film with a thickness of up to 500 nm, consisting of silicatein that typically self-assembles to aggregates with sizes of several hundred nanometers [42].…”

Biomedical Inorganic Polymers

“…It is assumed that the silicatein propeptide, either directly by association or indirectly by separation from the mature silicatein, controls the folding and function (structure-forming and enzymatic activity) of the mature protein (Müller et al 2013d). The autocatalytic cleavage of pro-silicatein to assembly-competent and enzymatically active silicatein might allow the development of a bio-inspired molecular switch of the biomineralization process.…”

“…The thrombin-mediated cleavage of the silicatein-trigger factor fusion protein results in the liberation of the pro-silicatein molecule which is immediately split by autocatalytic cleavage into the propeptide and the mature silicatein [Modified after silicatein propeptide, while in the cathepsin L propeptide, this amino acid is only present in the C-terminal β-sheet region (Müller et al 2013d). (b) Construct used for the expression of recombinant silicatein-trigger factor fusion protein and cleavage products.…”