The complex process of cellular vitamin A uptake: first insights by NMR and other biophysical techniques

Vitamin A is essential for diverse aspects of life, ranging from embryogenesis to the proper functioning of most adult organs. It circulates in blood bound to serum retinol binding protein (RBP) and is transported into cells by a membrane receptor termed Stimulated by Retinoic Acid 6 (STRA6) [1]. The mechanism of the STRA6-mediated translocation of retinol from holo-RBP into target cells appears to be unique [2]. There are also evidences that a specific binding site for the intracellular carriers (CRBPs) might exist on the cytoplasmic side of the membrane [3] and that CRBP-I plays a key role other than simply to sustain retinol transport [4]. However, until now it is not clear whether the apo-CRBPs may bind STRA6 or they interact with the membrane for retinol uptake. To gain first insights into this complex process, we have investigated the interactions of CRBP-I and CRBP-II with biomembrane mimetic systems. NMR experiments were performed at different protein:vesicles molar ratios and by varying the composition of the phospholipid liposomes and the ionic strength. Chemical shifts perturbations and line shape analysis provided insights into the interacting residues and proteins conformational dynamics. As the signals were broadened beyond detection at latest steps of the titration, the NMR data have been complemented by other biophysical measurements. The results revealed striking differences between CRBP-I and CRBP-II, despite they exhibit the same fold (a beta-barrel with two short alpha-helices) and identical retinol-binding motifs. Moreover, the interactions of the two homologs with the lipid bilayers depend upon the phospholipid composition and ionic strength. These new evidences complement the lessons learned from our former studies which had suggested that the two primary cellular retinol carriers exhibit different mechanisms of ligand uptake [5, 6]. These differences may account for their distinct functional roles in the modulation of intracellular retinoid metabolism. References [1] R. Kawaguchi, J. Yu, J. Honda, J. Hu, J. Whitelegge, P. Ping, P. Wiita, D. Bok, and H. Sun Science 315, 820-825 (2007). [2] R. Kawaguchi, J. Yu, M. Ter-Stepanian, M. Zhong, G. Cheng, Q. Yuan, M. Jin, G.H. Travis, D. Ong, and H. Sun ACS Chemical Biology 6, 1041-1051 (2011). [3] C. Redondo, M. Vouropoulou, J. Evans, and J.B.C. Findlay The FASEB J. 22, 1043-1054 (2008). [4] D.C. Berry, S.M. O'Byrne, A.C. Vreeland, W.S. Blaner, and N. Noy Mol. Cell. Biol. (2012) in press. [5] T. Mittag, L. Franzoni, D. Cavazzini, B. Schaffhausen, G.L. Rossi, and U.L. Günther J. Am. Chem. Soc. 128, 9844-9848 (2006). [6] L. Franzoni, D. Cavazzini, G.L. Rossi, and C. Lücke J. Lipid Res., 51, 1332-1343 (2010).