The F-Box Protein AhSLF-S2 Physically Interacts with S-RNases That May Be Inhibited by the Ubiquitin/26S Proteasome Pathway of Protein Degradation during Compatible Pollination in Antirrhinum

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Abstract
Self-incompatibility S-locus–encoded F-box (SLF) proteins have been identified in Antirrhinum and several Prunus species. Although they appear to play an important role in self-incompatible reaction, functional evidence is lacking. Here, we provide several lines of evidence directly implicating a role of AhSLF-S2 in self-incompatibility in Antirrhinum. First, a nonallelic physical interaction between AhSLF-S2 and S-RNases was demonstrated by both coimmunoprecipitation and yeast two-hybrid assays. Second, AhSLF-S2 interacts with ASK1- and CULLIN1-like proteins in Antirrhinum, and together, they likely form an Skp1/Cullin or CDC53/F-box (SCF) complex. Third, compatible pollination was specifically blocked after the treatment of the proteasomal inhibitors MG115 and MG132, but they had little effect on incompatible pollination both in vitro and in vivo, indicating that the ubiquitin/26S proteasome activity is involved in compatible pollination. Fourth, the ubiquitination level of style proteins was increased substantially after compatible pollination compared with incompatible pollination, and coimmunoprecipitation revealed that S-RNases were ubiquitinated after incubating pollen proteins with compatible but not with incompatible style proteins, suggesting that non-self S-RNases are possibly degraded by the ubiquitin/26S proteasome pathway. Fifth, the S-RNase level appeared to be reduced after 36 h of compatible pollination. Taken together, these results show that AhSLF-S2 interacts with S-RNases likely through a proposed SCFAhSLF-S2 complex that targets S-RNase destruction during compatible rather than incompatible pollination, thus providing a biochemical basis for the inhibition of pollen tube growth as observed in self-incompatible response in Antirrhinum.