Silicification: The Processes by Which Organisms Capture and Mineralize Silica - Biomineralization - page 309

pleuralin, HEP200 investigated, it is localized only where the epitheca overlaps the
hypotheca. It is important to note that the molecule could not be localized during the
developmental stages leading to silicification of the girdle bands suggesting that the role of
this family of molecules is not with silicification but that it may be involved in establishing
a reversible connection between the epitheca and hypotheca, Figure 23. During cell
reproduction, this connection must be broken for the new hypotheca and epitheca to form
but when these new valves have been formed the connection must again be closed so that
the mechanical stability of the daughter cell can be maintained.
The third group of proteins, the low molecular weight (ca. 4 17 kDa (although these

molecular weights will have been revised recently when a milder extraction agent was used
(Kröger et al. 2002) silaffins (1A, 1B and 2) can also be released from the diatom cell wall
using hydrogen fluoride and the sil-1-gene has been sequenced (Kröger et al. 1999, 2001).
Structural information obtained for the sil-1 encoded polypeptide displays a modular
primary structure that comprises an N-terminus signal peptide, an acidic region having high
negative charge and a series of repeat units with a high content of post-translationally
modified lysine and serine. Although the sil-1 polypeptide contains several different
functional domains, only the repeat units containing high levels of post-translationally
modified lysine and serine are found in the polypeptides isolated from diatoms, Figure 24.
The structure of these low molecular weight species varies according to the extraction
method used (Kröger et al. 2002). The use of a milder extractant, ammonium fluoride,
previously used by others (Swift and Wheeler 1992; Harrison 1996) has yielded the most
comprehensive information on this family of molecules (Kröger et al. 2002) with both
lysine and serine showing post-translational modifications. A range of detailed structural
methods including high resolution NMR (proton and multinuclear) and mass spectrometric
techniques were required to provide unequivocal evidence for the proposed structures
(Kröger et al. 1999, 2001). The chemical modifications of lysine involve the

−

ε -amino group

and give rise to ε -N-dimethyllysine or δ -hydroxy- ε -N,N,N-trimethyllysine residues or a

modification that has a methyl group on the -amino group together with an oligomeric

structure consisting of N-methylpropylamine units where between 5 and 10
methylpropylamine groups are attached to the lysine residue (Kröger et al. 1999, 2001).
The

ε

ε -N-trimethyl- δ -hydroxylysine residue or its phosphorylated derivative as identified by

Figure 23 . A model scheme showing the
overlap of girdle bands and their
separation during cell wall development.
Published with permission of Urban &
Fischer Verlag from Kröger and
Wetherbee 2000. (A) intact diatom, (B)
cleavage of daughter cells, (C) separation
of daughter cells, and (D) beginning of
interphase. Pleuralins are involved in the
opening and closing of the two theca
during reproduction and the development
of daughter cells.