摘要 : The interaction of calmodulin (CaM) with the receptor for retinol uptake, STRA6, involves an alpha-helix termed BP2 that is located on the intracellular side of this homodimeric transporter (Chen et al., 2016 [1]). In the absence ... 展开 The interaction of calmodulin (CaM) with the receptor for retinol uptake, STRA6, involves an alpha-helix termed BP2 that is located on the intracellular side of this homodimeric transporter (Chen et al., 2016 [1]). In the absence of Ca2+, NMR data showed that a peptide derived from BP2 bound to the C-terminal lobe (C-lobe) of Mg2+-bound CaM ((CaM)-Ca-Mg). Upon titration of Ca2+ into (CaM)-Ca-Mg-BP2, NMR chemical shift perturbations (CSPs) were observed for residues in the C-lobe, including those in the EF-hand Ca2+-binding domains, EF3 and EF4 (K-Ca(D) = 60 +/- 7 nM). As higher concentrations of free Ca2+ were achieved, CSPs occurred for residues in the N-terminal lobe (N-lobe) including those in EF1 and EF2 (K-Ca(D) = 1000 +/- 16 0 nM). Thermodynamic and kinetic Ca2+ binding studies showed that BP2 addition increased the Ca2+-binding affinity of CaM and slowed its Ca2+ dissociation rates (k(off)) in both the C- and N-lobe EF-hand domains, respectively. These data are consistent with BP2 binding to the C-lobe of CaM at low free Ca2+ concentrations (<100 nM) like those found at resting intracellular levels. As free Ca2+ levels approach 1000 nM, which is typical inside a cell upon an intracellular Ca2+-signaling event, BP2 is shown here to interact with both the N- and C-lobes of Ca2+-loaded M-Ca ((CaM)-Ca-Ca-BP2). Because this structural rearrangement observed for the (CaM)-Ca-Ca-BP2 complex occurs as intracellular free Ca2+ concentrations approach those typical of a Ca2+-signaling event (K-Ca(D) = 1000 +/- 160 nM), this conformational change could be relevant to vitamin A transport by full-length (CaM)-Ca-Ca-STRA6. (C) 2021 The Authors. Published by Elsevier Ltd. 收起