AhSSK1, a novel SKP1‐like protein that interacts with the S‐locus F‐box protein SLF

Summary The S‐locus F‐box (SLF/SFB) protein, recently identified as the pollen determinant of S‐RNase‐based self‐incompatibility (SI) in Solanaceae, Scrophulariaceae and Rosaceae, has been proposed to serve as the subunit of an SCF (SKP1–CUL1–F‐box) ubiquitin ligase and to target its pistil counterpart S‐RNase during the SI response. However, the underlying mechanism is still in dispute, and the putative SLF‐binding SKP1‐equivalent protein remains unknown. Here, we report the identification of AhSSK1, Antirrhinum hispanicumSLF‐interacting SKP1‐like1, using a yeast two‐hybrid screen against a pollen cDNA library. GST pull‐down assays confirmed the SSK1–SLF interaction, and showed that AhSSK1 could connect AhSLF to a CUL1‐like protein. AhSSK1, despite having a similar secondary structure to other SKP1‐like proteins, appeared quite distinctive in sequence and unique in a phylogenetic analysis, in which no SSK1 ortholog could be predicted in the sequenced genomes of Arabidopsis and rice. Thus, our results suggest that the pollen‐specific SSK1 could be recruited exclusively as the adaptor of putative SCFSLF in those plants with S‐RNase‐based SI, providing an important clue to dissecting the function of the pollen determinant.