Interactions of cytoplasmic retinol-binding proteins with phospholipid vesicles: insights into the physiological functions

Vitamin A plays a key role in vision, cell growth and differentiation. In the cell, retinol has several fates: (a) it can be stored as retinyl ester of long chain fatty acids through the action of lecithin-retinol acyl transferase (LRAT) [1, 2]; (b) most non-esterified retinol is probably bound to cellular retinol-binding protein I (CRBP-I); (c) retinol can enter the oxidative pathway for the synthesis of retinoic acid, through the action of retinol dehydrogenases (RDH) [3]. CRBP-I is ubiquitous, whereas the homologous CRBP-II is expressed solely in the enterocytes. Our current understanding of these processes remains largely incomplete, but there is evidence that while LRAT is able to access CRBP-bound retinol, the membrane-bound RDH are inactive towards the protein/ligand complex, suggesting that the membrane microenvironment may trigger retinol transfer from the holo protein to the oxidative enzymes. To address this hypothesis we have performed a suite of NMR experiments with retinol-bound CRBP-I and CRBP-II in the presence of model membranes composed of either anionic or zwitterionic phospholipids, at varying protein:lipid molar ratios and ionic strength. Besides NMR, other biophysical techniques were employed to achieve a better understanding of the ongoing processes. The results in the presence of phospholipid bilayers will be discussed, in comparison with our previous data collected in buffer [4, 5]. All these studies may help to understand certain aspects of the distinct physiological functions of CRBPs. References [1] J. Amengual et al. J. Biol. Chem. 287, 24216-24227 (2012). [2] W. Jiang and J.L. Napoli Biochim. Biophys. Acta 1820, 859-869 (2012). [3] S. Portè et al. Chemico-Biological Interactions 202, 186-194 (2013). [4] T. Mittag, L. Franzoni et al. J. Am. Chem. Soc. 128, 9844-9848 (2006). [5] L. Franzoni et al. J. Lipid Res. 51, 1332-1343 (2010).