Important in regulating the uptake, storage, and metabolism of retinoids, cellular retinol-binding protein 1 (CRBP1) is essential for trafficking vitamin A through the cytoplasm. However, the molecular details of ligand uptake and targeted release by CRBP1 remain unclear. Here we report the first structure of CRBP1 in a ligand-free form as well as ultra-high resolution structures of this protein bound to either all-trans-retinol or retinylamine, the latter a therapeutic retinoid that prevents light-induced retinal degeneration. Superpositioning of human apo-and holo-CRBP1 revealed major differences within segments surrounding the entrance to the retinoid-binding site. These included ␣-helix II and hairpin turns between -strands C-D and E-F as well as several side chains, such as Phe-57, Tyr-60, and Ile-77, that change their orientations to accommodate the ligand. Additionally, we mapped hydrogen bond networks inside the retinoid-binding cavity and demonstrated their significance for the ligand affinity. Analyses of the crystallographic B-factors indicated several regions with higher backbone mobility in the apoprotein that became more rigid upon retinoid binding. This conformational flexibility of human apo-CRBP1 facilitates interaction with the ligands, whereas the more rigid holoprotein structure protects the labile retinoid moiety during vitamin A transport. These findings suggest a mechanism of induced fit upon ligand binding by mammalian cellular retinol-binding proteins.Effective distribution of vitamin A (all-trans-retinol) and its derivatives (called "retinoids") throughout the body and cellular compartments determines their essential functions in embryonic development, growth, immunology, reproduction, and vision (1-4). However, because of their lipophilic nature, free retinoids exist only at extremely low concentrations in plasma or cytosol, making diffusion kinetics between sites of action inefficient. This solubility limitation is overcome by specialized retinoid-binding proteins that regulate retinoid metabolism and physiological functions (5). Thus, an appreciation of the molecular basis for ligand uptake and targeted release by retinoid-binding proteins is critical for understanding retinoid homeostasis, yet current knowledge of this process is very limited.Three classes of retinoid-binding proteins named for their selectivity (i.e. cellular retinol-binding protein (CRBP), 3 cellular retinoic acid-binding protein, and cis-retinoid-specific cellular retinal-binding proteins) carry vitamin A and its metabolites within cells (5-7). All representatives of CRBPs and cellular retinoic acid-binding proteins are members of intracellular lipid-binding proteins (8), whereas cellular retinal-binding protein belongs to the Sec14 protein family (9). Four CRBPs (CRBP1, -2, -3, and -4) are encoded in the human genome (10, 11). Among them, CRBP1 is the most widely expressed in numerous tissues, with the highest abundance in the liver, kidney, lung, and retinal pigment epithelium cells of the eye (12)(13)(14)...
The ability to store and distribute vitamin A inside the body is the main evolutionary adaptation that allows vertebrates to maintain retinoid functions during nutritional deficiencies and to acquire new metabolic pathways enabling light-independent production of 11-cis retinoids. These processes greatly depend on enzymes that esterify vitamin A as well as associated retinoid binding proteins. Although the significance of retinyl esters for vitamin A homeostasis is well established, until recently, the molecular basis for the retinol esterification enzymatic activity was unknown. In this review, we will look at retinoid absorption through the prism of current biochemical and structural studies on vitamin A esterifying enzymes. We describe molecular adaptations that enable retinoid storage and delineate mechanisms in which mutations found in selective proteins might influence vitamin A homeostasis in affected patients.
Cellular retinol-binding proteins (CRBPs) facilitate the uptake and intracellular transport of vitamin A. They integrate retinoid metabolism, playing an important role in regulating the *
Lipids secreted by the meibomian glands (MGs) of the eyelids are essential to the protection of the eye's surface. An altered meibum composition represents the primary cause of evaporative dry eye disease (DED). Despite the critical importance of the meibum, its biosynthetic pathways and the roles of individual lipid components remain understudied. Here, we report that the genetic deletion of Acyl‐CoA:wax alcohol acyltransferase 2 (AWAT2) causes the obstruction of MGs and symptoms of evaporative DED in mice. The lipid composition of the meibum isolated from Awat2−/− mice revealed the absence of wax esters, which was accompanied by a compensatory overproduction of cholesteryl esters. The resulting increased viscosity of meibum led to the dilation of the meibomian ducts, and the progressive degeneration of the MGs. Overall, we provide evidence for the main physiological role of AWAT2 and establish Awat2−/− mice as a model for DED syndrome that can be used in studies on tear film‐oriented therapies.
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