Polymerizes silica around the axial filament during spicule formation. Homodimer. Homodimerization occurs as a result of non-covalent interactions and not through disulfide linkages between the two monomers. Belongs to the peptidase C1 family. After extraction of the protein with ammonium fluoride and hydrofluoric acid, His-137 and Cys-279 were reported to be oxidized but could be the artifactual results of sample handling. Leu-123 was also reported to be mono- and dimethylated. Despite sequence similarity with cathepsins, silicatein lacks the intra- and interchain disulfide bonds for conserved cysteines. No numeric data was provided to support the mass-spectrometric interpretations.