Many species in the Rosaceae, Solanaceae and Plantaginaceae exhibit the S-RNase-based self-incompatibility (SI). Their S loci contain the genes encoding S-ribonuclease (S-RNase) and F-box protein(s) for the pistil and the pollen specificities, respectively. The pollen S specificity in the Solanaceae is determined by multiple S locus F-box proteins (SLFs), while that in Prunus (Rosaceae) is by a single S haplotype-specific F-box protein (SFB). In addition to SFB, three other pollen-expressed F-box genes called S locus F-box protein like 1-3 (SLFL1-3) are located in the flanking regions of the Prunus S locus, although it is not clear whether they are involved in SI. An F-box protein generally forms the SCF complex with Skp1, Cul1 and Rbx1. The SCF complex is known to polyubiquitinate substrate proteins to be degraded by the ubiquitin proteasome system. Although solanaceous SLFs have been shown to form the SCF complex and to participate in the SI system, it remained to be elucidated whether Prunus SFB and SLFLs would form the SCF complex. We previously identified SFB-interacting Skp1-like protein 1 (SSK1), which showed the binary protein interactions with SFB, SLFLs and the pollen-expressed Cul1 homologs (Cul1A and Cul1B). Here, we report the ternary protein interactions of GST-fused SFB/SLFLs, SSK1 and Cul1A/Cul1B, in pull-down assay, indicating that SFB and SLFLs could form the SCF complex with either of the Cul1 homologs and polyubiquitinate unidentified substrate proteins in pollen tube.
