1HN, 13C, and 15N resonance assignments of Ca2+-calmodulin bound to a BP2 peptide derived from the sheep STRA6 vitamin A transporter (residues 625–656)

Vitamin A is essential for vision and many other biological processes required for human health and survival. Extracellular retinol binding protein (RBP) delivers vitamin A into the cell upon binding to the vitamin A transporter, STRA6. However, when retinol free RBP binds to STRA6, it induces vitamin A transport out of the cell. The bi-directionality of vitamin A transport is thought to be regulated further by an intracellular protein-protein interaction (PPI) between STRA6 and the EF-hand Ca²⁺-binding protein, calmodulin (CaM). Insights regarding how CaM regulates vitamin A transport were originally provided at atomic resolution by a cryoEM structure of the zebrafish STRA6-CaM complex. This cryoEM structure, together with NMR studies, confirmed that three STRA6 helices (i.e., BP0, BP1, and BP2) comprised the CaM-STRA6 binding interface, with BP2 providing the major set of interactions. NMR and other biophysical methods demonstrated that zebrafish BP2 peptide (zfBP2) binding to CaM involved a Ca²⁺-dependent type 2 binding and functional folding mechanism of action, which could influence structural, dynamic, and allosteric functions of STRA6. To expand our understanding of vitamin A transport to a mammalian STRA6 transporter, the backbone and sidechain ¹H^N, ¹³C, and ¹⁵N resonances were assigned here for ^CaCaM (148 residues) when bound to a sheep BP2 peptide (32 residues) (shBP2). Interestingly, the NMR data showed ^CaCaM resonances were affected differently upon binding shBP2 versus zfBP2. Such differences may be useful for distinguishing important features regarding ^CaCaM complexes with mammalian versus zebrafish STRA6.