2.A.90.1.3
STRA6 of 667 aas and 9 - 12 putative TMSs (Zhong et al. 2013).  Mechanistic features have been discussed (Kawaguchi et al. 2015), and it was concluded that energy-independent facilitated diffusion, from external holo-retinol binding protein (RBP) to internal CRBP-1 or the corresponding CRABP (for retinoic acid) after release of the substrate into the iipid environment of the membrane or an enzyme that forms retinol esters (LRAT) provides the most probable mechanism of transport.  STRA6 binds RBP with a 1:1 stoichiometry (Breen et al. 2015). Zhong et al. 2020 purified and identified STRA6-associated proteins and found that the major STRA6-associated protein is calmodulin, consistent with the cryo-EM study of zebrafish STRA6 associated with calmodulin. They showed that increased calcium/calmodulin promotes cellular vitamin A efflux and suppresses vitamin A influx through STRA6.  Also, calmodulin enhances the binding of apo-RBP to STRA6, and this enhancement is much more pronounced for apo-RBP than holo-RBP. Thus, calmodulin seems to regulate STRA6's vitamin A influx vs efflux activities by modulating its preferential interaction with apo-RBP or holo-RBP (Zhong et al. 2020).