A sequence-function analysis of the silica precipitating silaffin R5 peptide.

The R5 peptide is derived from silaffin peptides naturally occurring in the diatom Cylindrotheca fusiformis and exhibits outstanding activity in silica precipitation. Because of its ability to cause silicification under mild conditions, several biotechnological applications based on R5-mediated biomimetic silica formation have already been reported. Yet a more detailed understanding of the R5 peptide and its intrinsic silica precipitation activity will help the rational design of R5 peptide variants as efficient agents for defined silica precipitation. The herein presented analysis of the relationship between the R5 amino acid sequence and its activity in silica precipitation emphasizes the essential role of the lysine residues in mediating silica polycondensation. Furthermore, a tetra amino acid motif (RRIL) has to be present within the R5 sequence, but in contrast to previous reports, we demonstrate that localization of the RRIL motif shows minor impact on silica precipitation activity but rather on morphology of the resulting silica material. The amino acid sequence of silaffin peptides is a well-balanced arrangement in terms of charges, functional groups and distances. The impact of this pattern of charges and functionalities was highlighted by the disturbed morphology of silica spheres resulting from R5 variants with scrambled sequences. A detailed understanding of the highly evolved silaffin sequence(s) will contribute to unravel the intriguing process of silica biomineralization in diatoms.