The 1.55 Å resolution structure of Nicotiana alata SF11-RNase associated with gametophytic self-incompatibility

The crystal structure of Nicotiana alata (ornamental tobacco) SF11-RNase, an S-allelic glycoprotein associated with gametophytic self-incompatibility, was determined by X-ray diffraction at 1.55 Å resolution. The protein has a tertiary structure typical of members of the RNase T2 family as it consists of a variant of the (α+β) fold and has eight helices and seven strands. A heptasaccharide moiety is also present, and amino acid residues that serve as the catalytic acid and base can be assigned to His32 and His91, respectively. Two “hypervariable” regions, known as HVa and HVb, are the proposed sites of S-allele discrimination during the self-incompatibility reaction, and in the SF11-RNase these are well separated from the active site. HVa and HVb are composed of a long, positively charged loop followed by a part of an α-helix and short, negatively charged α-helix, respectively. The SF11-RNase structure shows both regions are readily accessible to the solvent and hence could participate in the process of self/non-self discrimination between the S-RNase and an unknown pollen S-gene product(s) upon pollination.