Hypothetical cathepsin-like protein from Nematostella vectensis and its silicatein-like cathepsin mutant for biosilica production

• Hypothetical cathepsin-like protein from Nematostella vectensis (CAT) was expressed in E. coli.
• CAT displayed silica-condensing activity with tetraethoxy silane substrate.
• CAT was evolved to silicatein-like protein (SLC) by mutating some residues into conserved silicatein residues.
• SLC exhibited protein stability and an increased silica-condensing activity compared to CAT.

Silicatein has high sequence identity and similarity with that of cathepsin L. In silicatein, serine replaces the active-site cysteine that is found in cathepsin L. Here, we obtained hypothetical cathepsin-like protein (CAT) from Nematostella vectensis which is 55% identical and 75% similar to mature silicatein alpha (SIL) of Suberites domuncula. When this protein was expressed in Escherichia coli, it displayed protease activity with both N-carbobenzoxy-l-phenylalanyl-l-arginine-4-methylcoumaryl-7-amide (Z-FR-AMC) and gelatin substrates, as well as silica-condensing activity using the tetraethoxy silane (TEOS) substrate. To increase its silica-forming activity and stability, some residues including the active site cysteine, were mutated into conserved silicatein residues, resulting in a mutant with 65% identity and 79% similarity to SIL. The mutant silicatein-like cathepsin (SLC) had increased expression levels in E. coli, and silica-forming activity comparable to that of SIL. In addition, SLC exhibited decreased protease activity as compared to that of CAT. Both CAT and SLC produced silica particles of sizes smaller than 50 nm, which increased to 200–300 nm in the presence of a structure-directing agent, such as Triton X-100. In conclusion, CAT was evolved to function as a biosilica-forming protein, and SLC was engineered by mutating CAT residues into conserved SIL residues to produce various silica-based materials.