Similarity between protein-protein and protein-carbohydrate interactions, revealed by two crystal structures of lectins from the roots of pokeweed.
The roots of pokeweed (Phytolacca americana) are known to contain the lectins designated PL-A, PL-B, PL-C, PL-D1, and PL-D2. Of these lectins, the crystal structures of two PLs, the ligand-free PL-C and the complex of PL-D2 with tri-N-acetylchitotriose, have been determined at 1.8A resolution. The polypeptide chains of PL-C and PL-D2 form three and two repetitive chitin-binding domains, respectively. In the crystal structure of the PL-D2 complex, one trisaccharide molecule is shared mainly between two neighboring molecules related to each other by a crystallographic 2(1)-screw axis, and infinite helical chains of complexed molecules are generated by the sharing of ligand molecules. The crystal structure of PL-C reveals that the molecule is a dimer of two identical subunits, whose polypeptide chains are located in a head-to-tail fashion by a molecular 2-fold axis. Three putative carbohydrate-binding sites in each subunit are located in the dimer interface. The dimerization of PL-C is performed through the hydrophobic interactions between the carbohydrate-binding sites of the opposite domains in the dimer, leading to a distinct dimerization mode from that of wheat-germ agglutinin. Three aromatic residues in each carbohydrate-binding site of PL-C are involved in the dimerization. These residues correspond to the residues that interact mainly with the trisaccharide in the PL-D2 complex and appear to mimic the saccharide residues in the complex. Consequently, the present structure of the PL-C dimer has no room for accommodating carbohydrate. The quaternary structure of PL-C formed through these putative carbohydrate-binding residues may lead to the lack of hemagglutinating activity.
Hayashida
Minoru
M
Institute for Chemical Research, Kyoto University, Uji, 611-0011, Kyoto, Japan.
Fujii
Tomomi
T
Hamasu
Mika
M
Ishiguro
Masatsune
M
Hata
Yasuo
Y
D000595:Amino Acid Sequence; D001665:Binding Sites; D002236:Carbohydrate Conformation; D050260:Carbohydrate Metabolism; D002241:Carbohydrates / Q000737:chemistry; D018360:Crystallography, X-Ray; D019281:Dimerization; D008958:Models, Molecular; D008969:Molecular Sequence Data; D018517:Plant Roots / Q000737:chemistry*; D011043:Pokeweed Mitogens / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D054788:Ribosome Inactivating Proteins; D017386:Sequence Homology, Amino Acid
D002241:Carbohydrates; D011043:Pokeweed Mitogens; C095648:pokeweed lectin C; C104076:pokeweed lectin D; D054788:Ribosome Inactivating Proteins