A conformational switch in the SCF-D3/MAX2 ubiquitin ligase facilitates strigolactone signaling

Strigolactones (SLs) are a class of plant hormones that regulate numerous processes of growth and development. SL perception and signal activation involves interaction between F-box E3 ubiquitin ligase D3/MAX2 with D14 α/β hydrolase in a SL-dependent manner and targeting of D53/SMXL6/7/8 transcriptional repressors (SMXLs) for proteasome-mediated degradation. D3/MAX2 has been shown to exist in multiple conformational states where the C-terminal helix (CTH) undergoes a closed-to-open dynamics and regulates D14 binding and SL perception. Despite the multiple modes of D3-D14 interactions found in vitro, the residues that regulate the conformational switch of D3/MAX2 CTH in targeting D53/SMXLs and the subsequent effect on SL signaling remain unclear. Here, we elucidate the functional dynamics of ASK1-D3/MAX2 in SL-signaling by leveraging conformational switch mutants in vitro and in plants. We report the crystal structure of a dislodged CTH of ASK1-D3 mutant and demonstrate that disruptions in CTH plasticity via either CRISPR/Cas9 genome editing, or expression of point mutation mutants, result in impairment of SL signaling. We show that the conformational switch in ASK1-D3/MAX2 CTH directly regulates the ubiquitin-mediated protein degradation process. A dislodged conformation involved in D53/SMXLs SL-dependent recruitment and ubiquitination, and an engaged conformation is required for the release of polyubiquitinated D53/SMXLs and subsequently D14 for proteasomal degradation. Finally, we uncovered an organic acid metabolite that can directly trigger the D3/MAX2 CTH conformational switch. Our findings unravel a new regulatory function of an SCF ubiquitin ligase in plant signaling.