Silica-precipitating peptides from diatoms -: The chemical structure of silaffin-1A from Cylindrotheca fusiformis

Two silica-precipitating peptides, silaffin-1A(1) and-1A(2), both encoded by the sill gene from the diatom Cylin-drotheca 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 e-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.