Functions as retinol transporter. 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. 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. Important for the homeostasis of vitamin A and its derivatives, such as retinoic acid. STRA6-mediated transport is particularly important in the eye, and under conditions of dietary vitamin A deficiency. Does not transport retinoic acid.

> 85% as determined by SDS-PAGE.