The Role of Proteins in Biosilicification

Table 1

	Diatoms	Sponges
Silica structure	Cell wall	Body skeleton (multilamellar spicules of varying sizes)
Silica storage organelle	Silicon deposition vesicle with silicalemma	Silicasomes
Major silicifying protein	Small silaffin peptides (mainly 2.5–3 kDa)	Large silicatein protein (36 kDa in S. domuncula), homologous to cathepsin.
Precursor in vitro	Silicic acid (occurs naturally though orthosilicate has never been isolated in vivo)	Silicon alkoxides such as tetraethoxysilane (not identified in vivo)
Mechanism of silicification	General stimulation of polycondensation by electrostatic interactions	Catalysis of condensation with well-defined catalytic residues
Post-translational modifications	Lys hydroxylation, methylation, long chain amines; Ser phosphorylation (kinase identified); Hydroxy(phospho)prolines; Also glycosylation and sulfation	Phosphorylation is required for silicatein oligomerization.
Long chain polyamines	Covalently attached to silaffins and in some cases free in solution. Play a major role in silicification.	No LCPAs (except for Axinyssa aculeate where they can deposit silica and are associated with spicules [77])
Additional protein components	Cingulins, silacidins	Collagen, galectin, and silintaphins
Protein scaffold assembly	Unclear how the silaffins assemble in vivo	Silicatein forms an axial filament at the core of the spicules and coats the spicule surface to promote growth by apposition