Silicatein expression in the hexactinellid Crateromorpha meyeri:: the lead marker gene restricted to siliceous sponges

The siliceous spicules of sponges (Porifera) are synthesized by the enzyme silicatein. This protein and its gene have been identified so far in the Demospongiae, e.g., Tethya aurantium and Suberites domuncula. In the Hexactinellida, the second class of siliceous sponges, the mechanism of synthesis of the largest bio-silica structures on Earth remains obscure. Here, we describe the morphology of the spicules (diactines and stauractines) of the hexactinellid Crateromorpha meyeri. These spicules are composed of silica lamellae concentrically arranged around a central axial canal and contain proteinaceous sheaths (within the siliceous mantel) and proteinaceous axial filaments (within the axial canal). The major protein in the spicules is a 24-kDa protein that strongly reacts with anti-silicatein antibodies in Western blots. Its cDNA has been successfully cloned; the deduced hexactinellid silicatein comprises, in addition to the characteristic catalytic triad amino acids Ser-His-Asn and the conventional serine cluster, a hexactinellid C. meyeri-specific Ser cluster. We show that anti-silicatein antibodies react specifically with the proteinaceous matrix of the C. meyeri spicules. The characterization of silicatein at the genetic level should contribute to an understanding of the molecular/biochemical mechanism of spiculogenesis in Hexactinellida. These data also indicate that silicatein is an autapomorphic molecule common to both classes of siliceous sponges.