Structural basis of receptor-mediated cellular vitamin A uptake

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Vitamin A is an essential nutrient for all mammals. Many biological processes, including and foremost vision, are crucially dependent on its adequate supply for proper function. Alterations of vitamin A metabolism can result in a wide spectrum of ocular defects and lead to blindness. Retinol (vitamin A alcohol) is the predominant circulating vitamin A form in the fasting state. In times of need (i.e. in the absence of dietary vitamin A intake), in order to distribute vitamin A to the target peripheral tissues, retinol is released in the bloodstream from the liver, the main body storage site of the vitamin, bound to retinol-binding protein (RBP). Inside the cells, retinol binds specific intracellular carriers, namely cellular retinol-binding proteins, and it serves as a precursor for the active vitamin A forms: retinaldehyde, critical for vision, and retinoic acid, the ligand for specific nuclear receptors that regulate the transcription of hundreds of target genes. How retinol is released from the retinol-RBP complex and internalized by the cell has been subject of debate for decades. STRA6, the putative plasma membrane receptor for RBP, was identified in 2007. However, its mechanism of action has remained elusive, not least due to the absence of any structural information. Here we present the structure of STRA6 determined to 4.2 Å resolution by single-particle cryo-electron microscopy. STRA6 is a dimer, with each protomer contributing nine transmembrane and a horizontal intramembrane helix that is positioned at the core of the dimer interface. Unexpectedly, the C-terminus of each protomer is tightly bound to calmodulin in a compact, non-canonical arrangement. The structure suggests possible sites for interaction with extracellular and intracellular carriers for retinol, and modes for internalization of retinol. The atomic model of STRA6 provides a template to guide our understanding at a molecular level on how this protein may function, and to further investigate its physiological role.