Three-dimensional structure and ligand-binding site of carp Fishelectin (FEL)

Carp FEL (fishelectin or fish egg lectin) is a 238 amino acid lectin that can be purified from the fish eggs exploiting its selective binding to Sepharose followed by elution with N-acetyl glucosamine. We previously reported its amino acid sequence and other biochemical properties. The glycoprotein has four disulphide bridges and the structure of the oligosaccharides linked to Asn 27 was described (1). The poster will describe the three-dimensional structure of apo carp FEL (cFEL) and of its complex with N-acetyl glucosamine determined by X-ray crystallography to resolutions of 1.35 and 1.70 Å respectively (2). The molecule folds as a six-blade β-propeller and internal short consensus amino acid sequences have been identified in all the blades. A calcium atom binds at the bottom of the funnel shaped tunnel located in the centre of the propeller. Two ligand binding sites,αand β are present in each of the two protomers in the dimer. The first site, α, is closer to the N terminus of the chain and is located in the crevice between the second and the third blade while the second site, β, is located between the fourth and the fifth blade. The amino acids that participate in the contacts have been identified as well as the conserved water molecules in all the sites. Both sites can bind the two anomers, αandβ of N-acetyl glucosamine, clearly recognizable in the electron density maps. The lectin presents sequence homology to members of the tachylectin family, known to have a function in the innate immune system of arthropods and homologous genes are present in the genome of other fish and amphibians (3). This structure is the first of a protein of this group and, given the degree of homology with other members of the family, we expect it will be useful to experimentally determine other crystal structures using the coordinates of cFEL as search probe in molecular replacement.