X-RAY STRUCTURE OF A POKEWEED ANTIVIRAL PROTEIN, CODED BY A NEW GENOMIC CLONE, AT 0.23 NM RESOLUTION - A MODEL STRUCTURE PROVIDES A SUITABLE ELECTROSTATIC-FIELD FOR SUBSTRATE-BINDING

We have determined the crystal structure of a-pokeweed antiviral prote in, a member of ribosome-inactivating proteins, at 0.23 nm resolution, by the molecular-replacement method. The crystals belong to the space group P2(1)2(1)2 with unit-cell dimensions a = 4.71, b = 11.63 and c = 4.96 nm, and contain one protein molecule/asymmetric unit based on a crystal volume/unit protein molecular mass of 2.1 X 10(-3)nm(3)/Da. T he crystallographic residual value was reduced to 17.2% (0.6-0.23 nm r esolution) with root-mean-square deviations in bond lengths of 1.9 pm and bond angles of 2.2 degrees. The C alpha-C alpha distance map shows that alpha-pokeweed antiviral protein is composed of three modules, t he N-terminal (Ala1-Leu76), the central (Tyr77-Lys185) and the C-termi nal (Tyr186-Thr266) modules. The substrate-binding site is formed as a cleft between the central and C-terminal modules and all the active r esidues exist on the central module. The electrostatic potential aroun d the substrate-binding site shows that the central and C-terminal mod ule sides of this cleft have a negatively and a positively charged reg ion, respectively. This charge distribution in the protein seems to pr ovide a suitable interaction with the substrate rRNA.