Characterization of two new proteins from the silica of hexactinellid sponges
Metazoans use silicon traces but rarely develop extensive silica skeletons, except for an early-diverging lineage: sponges. The mechanisms underlying metazoan silicification remain incipiently deciphered, despite raising enormous biotechnological and evolutionary expectations. Two new proteins (hexaxilin, perisilin) characterized from hexactinellid sponge silica suggest that the three classes of siliceous sponges (Hexactinellida, Demospongiae, and Homoscleromorpha) use independent protein machineries to build their skeletons, which become non-homologous structures. Hexaxilin forms the axial filament to intracellularly pattern the main symmetry of the skeletal parts, while perisilin appears to operate in their thickening, guiding extracellular deposition of peripheral silica, as does glassin —another hexactinellid silicifying protein.  Distant hexaxilin homologs occur in some bilaterians with siliceous parts, suggesting that this protein may have maintained (or regained) silicifying activity along metazoan evolution. The findings also support that ancestral Porifera were non-skeletonized, acquiring silica skeletons only after diverging into major classes, what reconciles molecular-clock dating and the fossil record.