Purification and partial characterization of a lectin protein complex, the clathrilectin, from the calcareous sponge Clathrina clathrus

• A new lectin, the clathrilectin, was purified for the first time from a calcareous sponge, Clathrina clathrus. This N-acetyl-glucosamine-binding protein is a C-type heteropolymeric lectin displaying diagnostic haemagglutination activity toward rabbit erythrocytes in the presence of calcium. This protein shares partial amino-acid sequence homologies with a conserved domain of plant lectins, suggesting structural conservation among highly divergent organisms.

Carbohydrate-binding proteins were purified from the marine calcareous sponge Clathrina clathrus via affinity chromatography on lactose and N-acetyl glucosamine-agarose resins. Proteomic analysis of acrylamide gel separated protein subunits obtained in reducing conditions pointed out several candidates for lectins. Based on amino-acid sequence similarity, two peptides displayed homology with the jack bean lectin Concanavalin A, including a conserved domain shared by proteins in the L-type lectin superfamily. An N-acetyl glucosamine - binding protein complex, named clathrilectin, was further purified via gel filtration chromatography, bioguided with a diagnostic rabbit erythrocyte haemagglutination assay, and its activity was found to be calcium dependent. Clathrilectin, a protein complex of 3200 kDa estimated by gel filtration, is composed of monomers with apparent molecular masses of 208 and 180 kDa estimated on 10% SDS-PAGE. Nine internal peptides were identified using proteomic analyses, and compared to protein libraries from the demosponge Amphimedon queenslandica and a calcareous sponge Sycon sp. from the Adriatic Sea. The clathrilectin is the first lectin isolated from a calcareous sponge and displays homologies with predicted sponge proteins potentially involved in cell aggregation and interaction with bacteria.