Lake Baikal endemic sponge Lubomirskia baikalensis: structure and organization of the gene family of silicatein and its role in morphogenesis

Lake Baikal is known for its abundant endemic fauna and flora. The photosymbiotic sponges are the most numerous sessile animals in the Baikal's littoral zone. These endemic sponges are grouped to the family Lubomirskiidae and, based on molecular data, they are separated from the cosmopolitan family Spongillidae. The endemic sponges are monophyletic and originate from a common ancestor with the freshwater sponges Ephydatia fluviatilis / Spongilla lacustris. They are grouped to the class of Demospongiae having spicules that are composed of hydrated, amorphous, noncrystalline silica. With the Baikalian sponge Lubomirskia baikalensis it could be shown that silica is deposited around an organic filament. A major step forward to elucidate the formation of the siliceous spicules on molecular level was the finding that the "axial organic filament" of siliceous spicules is an enzyme, silicatein, which mediates the apposition of amorphous silica and hence the formation of spicules. The formation of siliceous spicules is certainly genetically controlled; this process initiates the morphogenesis phase and involves primarily silicatein. In the present study the existence of silicatein-α genes in the fresh-water Lake Baikal sponge L. baikalensis is demonstrated. The intron-exon structure of the full-size silicatein-α gene was determined. A comprehensive phylogenetic analysis with new sequences sheds light upon the evolution of the cathepsin L and silicatein families which occurred at the base of the metazoan phyla. It is concluded, that in parallel with the emergence of these enzymes at first the number of introns increased, especially in the coding region of the mature enzyme. Later in evolution the number of introns decreased again. We postulate that modification of the catalytic triad, especially of its first amino acid, is a suitable target for a chemical modulation of enzyme function of the silicateins/cathepsins L.