Membrane-bound vesicles in diatoms build the organism's mineral shell by secreting proteins that template the design and trigger silica deposition.

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The diatom's silica shell, or frustule, has a top half (epitheca) and bottom half (hypotheca) that fit together like a micro-sized 18th century snuff box. One or more overlapping silica girdle bands surround the rims of each theca like tamper-proof seals on the outside of food containers. While humans have mastered the art of glass-making (silica is the major component of glass), we depend on high temperatures to manifest our design ideas in this medium. Diatoms, on the other hand, have mastered the intricate art of "glass-making" without heating their surroundings. Instead, information contained in the specific 3-dimensional shape of proteins associated with still mysterious membrane-bound silica deposition vesicle, facilitates the formation of silica nanospheres that make up the diatom's silica shell. Researchers have recently discovered a set of proteins they've named "cingulins" because they appear to self-assemble into micro-scale rings that conform to the shape and pattern of the silica girdle band, a region of the diatom called the "cingulum." Cingulins are thought to mediate formation of the girdle band by templating the design and triggering silica deposition.

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"Diatoms are eukaryotic microalgae that produce species-specifically structured cell walls made of SiO2 (silica). Formation of the intricate silica structures of diatoms is regarded as a paradigm for biomolecule-controlled self-assembly of three-dimensional, nano- to microscale-patterned inorganic materials. Silica formation involves long-chain polyamines and phosphoproteins (silaffins and silacidins), which are readily soluble in water, and spontaneously form dynamic supramolecular assemblies that accelerate silica deposition and influence silica morphogenesis in vitro. However, synthesis of diatom-like silica structure in vitro has not yet been accomplished, indicating that additional components are required. Here we describe the discovery and intracellular location of six novel proteins (cingulins) that are integral components of a silica-forming organic matrix (microrings) in the diatom Thalassiosira pseudonana. The cingulin-containing microrings are specifically associated with girdle bands, which constitute a substantial part of diatom biosilica. Remarkably, the microrings exhibit protein-based nanopatterns that closely resemble characteristic features of the girdle band silica nanopatterns." (Scheffel et al. 2011: 3175)

Journal article
Nanopatterned protein microrings from a diatom that direct silica morphogenesisProceedings of the National Academy of SciencesFebruary 8, 2011
A. Scheffel, N. Poulsen, S. Shian, N. Kroger

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