Functions as retinol transporter (PubMed:23839944, PubMed:24852372). Accepts all-trans retinol from the extracellular retinol-binding protein RBP4, facilitates retinol transport across the cell membrane, and then transfers retinol to the cytoplasmic retinol-binding protein RBP1. Retinol uptake is enhanced by LRAT, an enzyme that converts retinol to all-trans retinyl esters, the storage forms of vitamin A (By similarity). Contributes to the activation of a signaling cascade that depends on retinol transport and LRAT-dependent generation of retinol metabolites that then trigger activation of JAK2 and its target STAT5, and ultimately increase the expression of SOCS3 and inhibit cellular responses to insulin (PubMed:21368206, PubMed:23839944). Important for the homeostasis of vitamin A and its derivatives, such as retinoic acid and 11-cis-retinal (PubMed:22467576, PubMed:24852372). STRA6-mediated transport is particularly important in the eye, and under conditions of dietary vitamin A deficiency (PubMed:22467576, PubMed:23839944, PubMed:24852372). Does not transport retinoic acid (By similarity). Homodimer (By similarity). Interacts with JAK2 and STAT5. Interacts (via extracellular domains) with RBP4. Interacts (via cytoplasmic domains) with RBP1 (By similarity). In the retinal pigment epithelium localizes to the basolateral membrane (PubMed:22467576, PubMed:24852372). Plasma membrane of the basal pole of Sertoli cells, absent in plasma membrane of neighboring spermatogonia (PubMed:9203140). Widely expressed in the embryo (PubMed:9203140, PubMed:23839944). Detected in adult in the retinal pigment epithelium in the eye (PubMed:9203140, PubMed:22467576, PubMed:23839944, PubMed:24852372). In the adult, is highly expressed in cells that compose blood-organ barriers in the brain (choroid plexus and the brain microvascular), in testis (the basal layer of the seminiferous epithelium), in the yolk sac, and in the chorioallantoic placenta (PubMed:9203140, PubMed:23839944). Detected in white adipose tissue and skeletal muscle, but not in liver (at protein level) (PubMed:21368206). Widely expressed in adult, with high expression levels in the eye (PubMed:24852372). Detected in brain, cerebellum, testis, pituitary, pancreas, kidney, spleen, and female genital tract; and at very low levels in heart and lung (PubMed:9203140, PubMed:24852372). Not detected in liver (PubMed:9203140). During embryogenesis, strongly expressed in the periocular mesenchyme, in the developing eyes, in respiratory mesenchymes, and in respiratory and bronchial epithelium, as well as in the developing central nervous system and in different embryonic gut derivatives (the epithelium of the pharyngeal pouches, mesenchyme of the esophagus, stomach, intestine, and rectum). In Sertoli cells expression depends on the stage of the spermatogenic cycle. During early placentation, is expressed in the yolk sac membrane and the chorionic plate. Expression was no longer detected in the yolk sac membrane during mid-late gestation, but was found in the labyrinthine zone of the chorioallantoic placenta. Expressed during early dorsoventral limb patterning and during endochondral ossification. By retinoic acid. Synergistically up-regulated by Wnt1 and retinoids in mammary epithelial cells. Contrary to predictions, contains nine transmembrane helices, with an extracellular N-terminus and a cytoplasmic C-terminus (By similarity). Besides, contains one long helix that dips into the membrane and then runs more or less parallel to the membrane surface (By similarity). Phosphorylated on tyrosine residues in response to RBP4 binding (PubMed:21368206). Phosphorylation requires the presence of LRAT, suggesting it may be triggered by the uptake of retinol that is then metabolized within the cell to retinoids that function as signaling molecules (By similarity). No visible phenotype at birth (PubMed:22467576, PubMed:23839944, PubMed:24852372). Measurement of the eye diameter indicates a tendency towards slightly smaller eye diameter 6 weeks after birth, and this difference persists in 5 and 10 month old animals (PubMed:22467576). The retina inner and outer segments are shorter in mutant mice (PubMed:22467576, PubMed:24852372). The number of cone cells in the retina is reduced, giving rise to a reduced cone response to light stimulation (PubMed:22467576). The retinal pigment epithelium displays discolored regions, where cells have large vacuoles and a reduced melanin content (PubMed:24852372). Retina and retinal pigment epithelium from dark-adapted mutant mice display strongly reduced levels of all-trans-retinol, all-trans-retinyl palmitate, 11-cis-retinal, 11-cis-retinyl palmitate and all-trans-retinal (PubMed:22467576, PubMed:24852372). Electroretinograms (ERG) of dark-adapted retinas from mutant mice display reduced a- and b-wave amplitude in response to light (PubMed:22467576). In another knockout model, the electroretinogram shows a complete lack of response to light (PubMed:24852372). Dietary vitamin A supplements can alleviate the ocular retinoid deficiency, and can prevent the altered retinal responses to light observed in mutant mice (PubMed:24852372). Other tissues and organs (brain, heart, kidney, liver, lungs, muscle, pancreas, spleen, testis and white adipose tissue) display grossly normal retinoid levels in 12 week old mice kept on normal chow (PubMed:23839944). Embryonic development appears grossly normal, also when females are fed a vitamin A-deficient diet (PubMed:23839944). Embryonic eye development is altered, leading to persistent hyperplastic primary vitreous that forms a conical mass of cells between the optic nerve exit point and the lens in the eyes of mutant mice (PubMed:22467576). In contrast, persistent hyperplastic primary vitreous was not observed in another knockout experiment (PubMed:24852372). The retinoic acid-induced activation is impaired in retinoic acid receptor gamma-null F9 cells.