Frustulins: Domain Conservation in a Protein Family Associated with Diatom Cell Walls

Abstract

The outstanding feature of a diatom is the species-specific design and ornamentation of the silicabased cell wall, termed frustulum. A new frustulum is shaped in a specialized organelle (silica deposition vesicle) and secreted. Proteins in the lumen of this organelle may control the biomineralization process and are likely to remain associated with the mature cell wall. Therefore a study of the structures of proteins associated with the diatom cell wall was initiated. The complete primary structures of three cell wall proteins (denoted as frustulins) have been determined. In addition, partial amino acid sequences from two more cell wall components were obtained. From these data, a highly conserved domain has been identified as a common building block of diatom cell wall proteins that is repeated several time; per polypeptide chain together with polyproline/hydroxyproline or polyglycine spacers. All frustulins characterized so far, are synthesized as preproteins with a novel type of N-terminal presequence.Keywords: diatom ; cell wall; hydroxyproline-rich glycoprotein ; presequence; Cylindrotheca fusiformis.The cell wall of diatoms is composed of amorphous, hydrated silica and organic components (Volcani, 1981). The species-specific design and ornamentation of the cell wall's silica skeleton is the most fascinating aspect of diatom biology and the basis for diatom systematics. It was estimated that there are more than 10 000 recent diatom species each displaying a different cell wall structure (Pickett-Heaps et al., 1990). The diatom cell wall (also termed frustulum or frustle) consists of two parts, the epitheca and the hypotheca. Each theca is composed of a valve and several silica strips (girdle bands). It is the girdle band region in which epitheca and hypotheca overlap (see Fig. 1).New valves are produced after cell division and cytokinesis of the mother protoplast. The resulting daughter protoplasts are trapped within the mother cell wall. Each daughter protoplast produces a new valve in a specialized intracellular organelle, the silica deposition vesicle (Volcani, 1981). In this organelle the polymerization of silicate takes place. The vesicle develops closely beneath the plasma membrane and extends during the process of silica deposition. Finally, it spans the whole area beneath the plasma membrane, containing a new valve. The latter is exocytosed and the daughter cells separate, each possessing a maternal epitheca and a newly formed hypotheca. It is suggested that cytoskeleton and cytoplasmic organells take part in shaping the silica deposition vesicle and thus are determinants of cell wall morphology . In addition it is assumed that an organic matrix within the vesicle may exert influence on the fine patterning of the cell wall (Pickett-Heaps et al., 1990). This is an attractive hypothesis, since certain organic macromolecules are known to function as templates and regulators for inorganic crystal growth in vitro (Weiner and Addadi, 1991).The species-specific structure of a diatom cell wall demands genet...

Molecular Interactions in Natural and Synthetic Self-Assembly Systems

“…This is mediated by silicon transporters, or SITS, whose expression and activity also show tight regulation during the cell cycle (Kröger et al, 1996(Kröger et al, , 1997. Intracellular concentrations of dissolved silicon exceed the solubility of silicic acid (Kröger et al, 2002), suggesting the presence of an organic silicon complex that may maintain silicon in solution and shepherd it to the silicon deposition vesicle (SDV).…”

“…Accordingly, silintaphin-1 might not only enhance the enzymatic activity of silicatein-, but also accelerate the nonenzymatic polycondensation of the silica product before releasing the fully synthesized biosiliceous polymer (Kröger et al, 1996;1997;.…”

“…Diatom transporters have been sequenced and characterized and have been shown to interact directly with silicon to actively transport silicic acid against a large concentration gradient, although the mechanism for intracellular storage of soluble silicic acid is not known (Kröger et al, 1996;1997).…”

Comprehensive computational analysis of leucine-rich repeat (LRR) proteins encoded in the genome of the diatom Phaeodactylum tricornutum

“…These secreted proteins provide a barrier responsible for trapping and removing bacteria (Dekker et al, 2002;McGuckin et al, 2011). One protein (ID 35777) features four acidic and cysteine rich (ACR) regions, typical for the diatom specific frustulin proteins, which have been described to function as structural elements covering the diatom cell wall (Kröger et al, 1996). While ACR domains are as sumed to have a globular structure, potentially highly glycosylated PST rich domains are proposed to be stiff linear stalks retaining the pro teins in the cellular vicinity of the EPS matrix (Dekker et al, 2002;Williamson, 1994).…”

Learning from Biosilica: Nanostructured Silicas and Their Coatings on Substrates by Programmable Approaches

“…Three families of proteins have been identified in the cell wall of marine diatom C. fusiformis, termed frustulins, pleuralins and silaffins (Pohnert, 2002). Frustulins are a group of calcium-binding glycoproteins being assumed to play a role in cell adhesion to surfaces, gliding of pennate diatoms, and protection against desiccation (Kröger et al, 1994;1996). The pleuralins (HF-extractable www.intechopen.com Learning from Biosilica: Nanostructured Silicas and Their Coatings on Substrates by Programmable Approaches 161 proteins) are highly acidic molecules, comprising a proline-rich region and several repeats of a proline-, serine-, cysteine, aspartate-rich domain (Kröger et al, 1997).…”