Mechanism of cooperative strigolactone perception by the MAX2 ubiquitin ligase-receptor-substrate complex

Strigolactones (SLs) are a group of plant hormones that regulate various aspects of plant growth and development. Additionally, SLs exuded into the soil promote symbiotic relationships with arbuscular mycorrhizal fungi and stimulate the germination of parasitic plants such as Striga hermonthica. The binding and hydrolysis of SLs by their receptors (D14 in Arabidopsis and HTL in Striga) promote the ubiquitination of transcriptional repressors by Skp1-cullin-F-box (SCF)-type E3 ubiquitin ligases. The mechanistic link between SL perception by D14/HTL and substrate recognition by the E3 remains unclear. We identified an E3-HTL-substrate complex that is sufficiently stable for cryogenic electron microscopy. This complex, composed of SKP1 (ASK1) and substrate (SMAX1) from Arabidopsis, and Striga F-box (MAX2) and SL receptor (HTL7), reveals that the substrate engages in a bidentate association through its N and D2 domains. This interaction, which is both conformationally and compositionally dynamic, directly and allosterically stabilises the MAX2-(SL)HTL7 complex and affects the positioning of ASK1 relative to MAX2. This dynamic positioning influences the proximity between the substrate D2 domain and the ubiquitin-conjugated E2 enzyme. This work advances our understanding of how E3 ligases in plants translate hormone perception into genetic adaptations.