Fig. 1

Sterol import into yeast under anaerobic growth conditions. The two ABC transporters, Aus1 and Pdr11 import sterol (brown ellipses) into the plasma membrane (PM; a). Once in the PM, sterol can reach the endoplasmic reticulum (ER) via non-vesicular transport (red arrows) bound to sterol transfer proteins through the cytosol or at membrane contact sites (MCS). In the ER, sterol can be esterified by Are1/2 and stored in lipid droplets (LD). b Transport steps considered in the model in Eq. 1; sterol import by Aus1/Pdr11 with rate v₁, sterol flip-flop with rate v₂ and non-vesicular sterol transport with rate v₃. c sterol in the PM visualized using dehydroergosterol (DHE) showing that most sterol resides in the inner compared to the outer PM leaflet