Brassinolide and β-Sitosterol Interleaflet Diffusion in an Asymmetric Plant Model Membrane.

Brassinosteroids (BR) are an essential steroid phytohormone found throughout the plant kingdom. Synthesis of BR occurs at the endoplasmic reticulum, and the mechanism of transport through the membrane to the apoplast is not well understood. The ABCB1 and ABCB19 transporters enhance BR transport through the plasma membrane, but the available cryo-EM structures do not provide a clear understanding of the steroid's diffusion process. Sphingolipid and sterol enriched domains improve the translocation of BR through localization of ABCB1 and ABCB19. The similarity of BR to sterols suggests that BR localizes differently between the more disordered inner leaflet and the tightly packed outer leaflet which contains sphingolipid and sterol domains. The presented work utilizes all-atom molecular dynamics (MD) simulations to compare the diffusion properties of BR with β-sitosterol. Our results show key thermodynamic differences between BR and sitosterol passive transport and a pathway to exposure of BR to ABCB entry sites.