Structural basis for lipid transport at membrane contact sites by the Ist2/Osh6 complex

Membrane contact sites (MCS) are hubs for inter-organellar lipid transport within eukaryotic cells. As one of the principal tethers bridging the ER and the plasma membrane in Saccharomyces cerevisiae , the protein Ist2 plays a major role during lipid transport between both compartments. Here, we present a comprehensive investigation elucidating structural and mechanistic properties of Ist2 and its interaction with the soluble lipid transport protein Osh6. The ER-embedded transmembrane domain of Ist2 is homologous to the TMEM16 family and acts as constitutively active lipid scramblase. The extended C-terminus binds to the plasma membrane and the PS/PI ₄ P exchanger Osh6. Through cellular growth assays, biochemical and structural studies, we have elucidated the interaction between both proteins and provide evidence that Osh6 remains associated with Ist2 during lipid shuttling between membranes. These results highlight the role of the Ist2/Osh6 complex in lipid trafficking and offer new insights into the relevance of scramblases at MCSs.