Abstract
Self-incompatibility (SI) is a mechanism that prevents self-fertilization in flowering plants. Rosaceous species such as Japanese pear and apple have gametophytic self-incompatibility (GSI) controlled by a single, multi-allelic locus, the S-locus. A pistil-specific protein encoded by the S-locus has been shown to be a ribonuclease (S-RNase) that recognizes the pollen S-allele. Three models for the S-allele-specific inhibition of pollen tube growth involving S-RNase (the S-allele-specific uptake, RNase inhibitor, and ubiquitin-dependent protein degradation models) have been proposed by researchers working on GSI, but at present there is not enough evidence to support any model. To elucidate the mechanism of the S-RNase-based GSI at molecular level, we have determined the crystal structure of Japanese pear S3-RNase at 1.5 angstrom resolution. On the basis of the tertiary structure of S3-RNase, we discuss how S-RNase discriminates between self- and nonself-pollen and what a pollen S-gene product(s) interacting with S-RNase is.
