title: Hierarchical composition of the axial filament from spicules of the siliceous sponge Suberites domuncula: from biosilica-synthesizing nanofibrils to structure- and morphology-guiding triangular stems.
abstract: The major structural and enzymatically active protein in spicules from siliceous sponges, e.g., for Suberites domuncula studied here, is silicatein. Silicatein has been established to be the key enzyme that catalyzes the formation of biosilica, a polymer that represents the inorganic scaffold for the spicule. In the present study, it is shown, by application of high-resolution transmission and scanning transmission electron microscopy that, during the initial phase of spicule synthesis, nanofibrils with a diameter of around 10 nm are formed that comprise bundles of between 10 and 20 nanofibrils. In intracellular vacuoles, silicasomes, the nanofibrils form polar structures with a pointed tip and a blunt end. In a time-dependent manner, these nanofibrillar bundles become embedded into a Si-rich matrix, indicative for the formation of biosilica via silicatein molecules that form the nanofibrils. These biosilicified nanofibrillar bundles become extruded from the intracellular space, where they are located in the silicasomes, to the extracellular environment by an evagination process, during which a cellular protrusion forms the axial canal in the growing spicule. The nanofibrillar bundles condense and progressively form the axial filament that becomes localized in the extracellular space. It is concluded that the silicatein-composing nanofibrils act not only as enzymatic silica bio-condensing platforms but also as a structure-giving guidance for the growing spicule.
authors.affiliation: ERC Advanced Investigator Grant Research Group at Institute for Physiological Chemistry, University Medical Center of the Johannes Gutenberg University Mainz, Mainz, Germany. wmueller@uni-mainz.de
authors.forename: Werner E G
authors.initials: WE
authors.lastname: Müller
authors.forename: Enrico
authors.initials: E
authors.lastname: Mugnaioli
authors.forename: Heinz C
authors.initials: HC
authors.lastname: Schröder
authors.forename: Ute
authors.initials: U
authors.lastname: Schloßmacher
authors.forename: Marco
authors.initials: M
authors.lastname: Giovine
authors.forename: Ute
authors.initials: U
authors.lastname: Kolb
authors.forename: Xiaohong
authors.initials: X
authors.lastname: Wang
mesh_terms: D000825:Animal Structures / Q000033:anatomy & histology* / Q000378:metabolism* / Q000648:ultrastructure; D000818:Animals; D003599:Cytoskeleton / Q000648:ultrastructure; D042541:Intracellular Space / Q000378:metabolism; D008954:Models, Biological; D057139:Nanofibers / Q000737:chemistry* / Q000648:ultrastructure; D012822:Silicon Dioxide / Q000378:metabolism*; D047233:Suberites / Q000033:anatomy & histology* / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D012822:Silicon Dioxide