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 b y the sill gene from the diatom Cylin-drotheca fusiformis, were extracted f rom cell walls and purified to homogeneity, The chemical structures were de termined by protein chemical methods combined with mass spectrometry, Silaf fin-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 foun d to be post-translationally modified. In silaffin-1A(2) the lysine residue s are clustered in two pairs in which the e-amino group of the first residu e is linked to a linear polyamine consisting of 5 to 11 N-methylated propyl amine units, whereas the second lysine is converted to epsilon -N,N-dimethy llysine. Silaffin-1A(1) contains only a single lysine pair exhibiting the s ame structural features. One of the two remaining lysine residues was ident ified 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 show n to contain epsilon -N,N,N-trimethyl-delta -hydroxylysine. In vitro, both peptides precipitate silica nanospheres within seconds when added to a mono silicic acid solution.