Sterol binding mechanism of a plant START ‐like domain: A new sterol transport paradigm via an amphiphilic cavity

Redistribution of sterols among cellular compartments is crucial for the proper functions of different organelles, but how sterols are transported in plants is barely studied. Here, we identified that Arabidopsis C2 and GRAM domain-containing proteins C2GR1/2, a specialized subgroup of the lipid transfer proteins anchored at membrane contact sites (LAMs), transport sterols between membranes via their first START-like domains (SLD1s), while the SLD2 domains are inactive. Structural studies on C2GR2-SLD1/SLD2 elucidated that the sterol transport process involves the exchange of sterol and water, which requires the proper size and the amphiphilic nature of the cavity, as well as the conformational changes of the three Ω loops at the entrance. Importantly, the amphiphilicity of the cavity is shared by other SLD domains in yeast and mammals, a feature that was overlooked by previous studies. These findings not only advance our understanding of sterol transport in plants but also redefine the sterol transport paradigm for LAM proteins.