Structural Plasticity of D3-D14 Ubiquitin Ligase in Strigolactone Signaling

The plant hormone strigolactones (SLs) regulate many aspects of plant physiology. In shoot branching inhibition, the SL-metabolizing α/β hydrolase D14 interacts with the F-box protein D3 to ubiquitinate and degrade the transcription repressor D53. Despite multiple modes of D14-SL interactions determined recently, how the hydrolase functions with D3 to mediate hormone-dependent D53 ubiquitination remains elusive. Here we show that D3 features a C-terminal α-helix (CTH), which can switch between two conformational states. Distinct from its engaged form, which facilitate the binding of D3 and D14 with a hydrolyzed SL intermediate, the dislodged D3 CTH can recognize unmodified D14 in an open conformation and inhibits its enzymatic activity. In an SL-dependent manner, the D3 CTH enables D14 to recruit D53, which in turn activates the hydrolase. By unraveling an unexpected structural plasticity in SCF(D3-D14) ubiquitin ligase, our results suggest an intricate mechanism by which the E3 coordinates SL signaling and metabolism.