Characterization of the molecular function of MGST, the pollen-part modifier of the gametophytic self-incompatibility in Prunus species

Some of the Prunus species (Rosaceae) such as almond (P. dulcis), sweet cherry (P. avium), apricot (P. armeniaca), Japanese apricot (P. mume), and Japanese plum (P. salicina) exhibit S-RNase (S-ribonuclease) based gametophytic self-incompatibility (SI), a major obstacle for stable and efficient fruit production. Clarification of the molecular mechanism of the SI reaction would contribute to the future improvement of their productivity. S-RNase-based SI system is shared among Rosaceae, Solanaceae, and Plantaginaceae, where self/non-self discrimination is fulfilled by the interaction between S-RNase, pistil-part factor, and F-box protein, pollen-part factor. Among the species with S-RNase-based SI system, Prunus species developed a specific mechanism. However, the details have not been clarified fully. Recently, Prunus-specific pollen-part SI modifier gene, MGST (M-locus encoded glutathione S-transferase-like), was identified. Molecular function of MGST may give us important clues to elucidate Prunus-specific mechanism. In our previous study, comprehensive identification of the sweet cherry pollen tube proteins co-immunoprecipitated with the recombinant S-RNase indicated the possible interaction between MGST and S-RNase. In the present study, we analyzed the enzymatic activity of the recombinant MGST expressed in Escherichia coli and its interaction with S-RNase. The recombinant MGST did not exhibit glutathione conjugation activity to 2,4-dinitrochlorobenzene, nor protein disulfide isomerase activity to recombinant human insulin. The interaction between the endogenous MGST in pollen tube extracts and the silkworm-expressed recombinant S-RNases from three alleles was confirmed by co-immunoprecipitation assay, while the interaction was not detected when recombinant MGST was used. Furthermore, co-incubation with recombinant MGST had no effect on the ribonuclease activity of S-RNase. These results may imply that unidentified additional factors that exist in the environment of pollen tube are required for the enzymatic activity of MGST and its interaction with S-RNase. Further analysis of the molecular function of MGST is necessary to elucidate the underlying mechanism of Prunus-specific SI reaction.