Retinol transporter. Accepts retinol from the extracellular retinol-binding protein rbp4, mediates retinol transport across the cell membrane, and then transmits retinol to the cytoplasmic retinol-binding protein rbp1 (PubMed:27563101). Required for normal vitamin A homeostasis (PubMed:18316031). Homodimer. Interacts (via C-terminus) with calmodulin. Detected in embryos (at protein level). Detected in the yolk syncytium and in mesendodermal cells in the head and trunk region during early somitogenesis. Detected in the developing eyes, the anterior midbrain, the pineal gland and in anterior somites at 24 hpf. Detected only in the eyes and the pineal gland at 3 and 4 dpf. Detected in retinal pigment epithelium at 4 dpf. Contrary to predictions, contains nine transmembrane helices, with an extracellular N-terminus and a cytoplasmic C-terminus. Besides, contains one long helix that dips into the membrane and then runs more or less parallel to the membrane surface. Morpholino knockdown causes severe defects in embryonic development, with microphthalmia, a curved body axis and heart edema. The morphant hearts do not loop normally, heart atria are strongly dilated and blood circulation is severely impaired. Morphants have an altered morphology of the craniofacial skeleton, with malformation of the first and second arches and absence of the branchial arches. Embryonic heads at 4 dpf display reduced total retinyl ester levels and reduced levels of the visual pigment 11-cis-retinal. Morpholino knockdown of both stra6 and rbp4 alleviates the developmental impairment that is observed in stra6 morphants, suggesting the phenotype is due to impaired vitamin A homeostasis and excessive accumulation of retinoic acid.