Crystal Structure at 1.5-Å Resolution of Pyrus pyrifolia Pistil Ribonuclease Responsible for Gametophytic Self-incompatibility

Abstract

The crystal structure of the Pyrus pyrifolia pistil ribonuclease (S 3-RNase) responsible for gametophytic self-incompatibility was determined at 1.5-Å resolution. It consists of eight helices and seven β-strands, and its folding topology is typical of RNase T2 family enzymes. Based on a structural comparison of S3-RNase with RNase Rh, a fungal RNase T2 family enzyme, the active site residues of S3-RNase assigned were His33 and His88 as catalysts and Glu84 and Lys8 as stabilizers of an intermediate in the transition state. Moreover, amino acid residues that constitute substrate binding sites of the two RNases could be superimposed geometrically. A hypervariable (HV) region that has an S-allele-specific sequence comprises a long loop and short α-helix. This region is far from the active site cleft, exposed on the molecule's surface, and positively charged. Four positively selected (PS) regions, in which the number of nonsynonymous substitutions exceeds that of synonymous ones, are located on either side of the active site cleft, and accessible to solvent. These structural features suggest that the HV or PS regions may interact with a pollen S-gene product(s) to recognize self and non-self pollen.