Silica-precipitating peptides from diatoms. The chemical structure of silaffin-A from Cylindrotheca fusiformis.

Two silica-precipitating peptides, silaffin-1A(1) and-1A(2), both encoded by the sil1 gene from the diatom Cylindrotheca fusiformis, were extracted from cell walls and purified to homogeneity. The chemical structures were determined by protein chemical methods combined with mass spectrometry. Silaffin-1A(1) and -1A(2) consist of 15 and 18 amino acid residues, respectively. Each peptide contains a total of four lysine residues, which are all found to be post-translationally modified. In silaffin-1A(2) the lysine residues are clustered in two pairs in which the epsilon-amino group of the first residue is linked to a linear polyamine consisting of 5 to 11 N-methylated propylamine units, whereas the second lysine is converted to epsilon-N,N-dimethyllysine. Silaffin-1A(1) contains only a single lysine pair exhibiting the same structural features. One of the two remaining lysine residues was identified as epsilon-N,N,N-trimethyl-delta-hydroxylysine, a lysine derivative containing a quaternary ammonium group. The fourth lysine residue again is linked to a long-chain polyamine. Silaffin-1A(1) is the first peptide shown to contain epsilon-N,N,N-trimethyl-delta-hydroxylysine. In vitro, both peptides precipitate silica nanospheres within seconds when added to a monosilicic acid solution.