The Aster Proteins: Key Mediators of Plasma Membrane to ER Cholesterol Transport

Individual cells maintain tight control over their cholesterol content and its intracellular distribution. Abnormal cholesterol accumulation is cytotoxic and promotes coronary artery disease. How cholesterol is synthesized and exchanged between cells was elucidated in the past century. However, cholesterol transport between intracellular membranes is poorly understood. Most cellular cholesterol is found at the plasma membrane (PM), yet the endoplasmic reticulum (ER) is key for cholesterol sensing, biosynthesis, and storage. The pathway between cholesterol deposition at the plasma membrane and its subsequent transport to the ER represents a major gap in our understanding. We sought to address this by using genome-wide screens to identify novel target genes of the cholesterol-responsive transcription factor LXR. We discovered Gramd1b as a�previously uncharacterized LXR target gene. Through bioinformatic analyses we determined that Gramd1b has two paralogs, Gramd1a and Gramd1c, that together encode a family of GRAM domain containing proteins. Secondary structural analyses of this family revealed remarkable similarity to StAR-family proteins despite little primary sequence conservation. We named the Gramd1a-c protein products as the Aster (Greek, aster for “star”) proteins. Through binding experiments and x-ray crystallography we determine the Aster proteins contain a unique cholesterol-binding ASTER domain. Next, we used live cell super-resolution microscopy and determined that the Aster proteins are tethered to the ER by a single pass transmembrane domain. Remarkably, cholesterol loading of the plasma membrane results in the dynamic recruitment of Aster-A, -B, and –C to ER-PM contact sites. The N-terminal GRAM domain showed high affinity for phosphatidylserine in binding experiments and mediates cholesterol-dependent Aster recruitment to the PM. To determine the biological role of Aster proteins we knocked down Aster-A in cells and generated Aster-B knockout mice. Cells lacking Aster-A have slowed PM to ER cholesterol transport. Mice lacking Aster-B are deficient in adrenal cholesterol ester storage and steroidogenesis due to defective cholesterol transport from SR-BI to the ER. Our findings identify a nonvesicular pathway for plasma membrane to ER cholesterol transport that is critical in cellular uptake of HDL-derived cholesterol