Movement of accessible plasma membrane cholesterol by GRAMD1 lipid transfer protein complex

Cholesterol is a major structural component of the plasma membrane (PM). The majority of PM cholesterol forms complexes with other PM lipids, making it inaccessible for intracellular transport. Transition of PM cholesterol between accessible and inaccessible pools maintains cellular homeostasis, but how cells monitor PM cholesterol accessibility remains unclear. We show that endoplasmic reticulum (ER)-anchored lipid transfer proteins, the GRAMD1s, sense and transport accessible PM cholesterol to the ER. GRAMD1s bind one another and populate at ER-PM contacts by sensing a transient expansion of the accessible pool of PM cholesterol via GRAM domains and facilitate its transport via StART-like domains. Cells lacking all three GRAMD1s exhibit striking expansion of the accessible pool of PM cholesterol due to less efficient PM to ER transport of accessible cholesterol. Thus, GRAMD1s facilitate movement of accessible PM cholesterol to the ER in order to counteract acute increase of PM cholesterol, activating non-vesicular cholesterol transport.