High-resolution structure of the conger eel galectin, congerin I, in lactose-liganded and ligand-free forms: emergence of a new structure class by accelerated evolution

Background: Congerin I is a member of the galectin (animal beta-galactoside -binding lectin) family and is found in the skin mucus of conger eel. The g alectin family proteins perform a variety of biological activities. Because of its histological localization and activity against marine bacteria and starfish embryos, congerin I is thought to take part in the eels' biologica l defense system against parasites. Results: The crystal structure of congerin I has been determined in both la ctose-liganded and ligand-free forms to 1.5 Angstrom and 1.6 Angstrom resol ution, respectively. The protein is a homodimer of 15 kDa subunits. Congeri n I has a beta-sheet topology that is markedly different from those of know n relatives. One of the beta-strands is exchanged between two identical sub units. This strand swap might increase the dimer stability. Of the known ga lectin complexes, congerin I forms the most extensive interaction with lact ose molecules. Most of these interactions are substituted by similar intera ctions with water molecules, including a pi-electron hydrogen bond, in the ligand-free form. This observation indicates an increased affinity of conge rin I for the ligand. Conclusions: The genes for congerin I and an isoform, congerin II, are know n to have evolved under positive selection pressure. The strand swap and th e modification in the carbohydrate-binding site might enhance the cross-lin king activity, and should be the most apparent consequence of positive sele ction. The protein has been adapted to functioning in skin mucus that is in direct contact with surrounding environments by an enhancement in cross-li nking activity. The structure of congerin I demonstrates the emergence of a new structure class by accelerated evolution under selection pressure.