Physiologically Relevant Free Ca 2+ Ion Concentrations Regulate STRA6-Calmodulin Complex Formation via the BP2 Region of STRA6

J Mol Biol. 2021 Nov 5;433(22):167272. doi: 10.1016/j.jmb.2021.167272. Epub 2021 Sep 27.


The interaction of calmodulin (CaM) with the receptor for retinol uptake, STRA6, involves an α-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 (MgCaM). Upon titration of Ca2+ into MgCaM-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 (CaKD = 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 (CaKD = 1000 ± 160 nM). Thermodynamic and kinetic Ca2+ binding studies showed that BP2 addition increased the Ca2+-binding affinity of CaM and slowed its Ca2+ dissociation rates (koff) 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 CaM (CaCaM-BP2). Because this structural rearrangement observed for the CaCaM-BP2 complex occurs as intracellular free Ca2+ concentrations approach those typical of a Ca2+-signaling event (CaKD = 1000 ± 160 nM), this conformational change could be relevant to vitamin A transport by full-length CaCaM-STRA6.

Keywords: STRA6; calmodulin; retinol; retinol-binding protein; vitamin A receptor.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Calcium / metabolism*
  • Calmodulin / chemistry
  • Calmodulin / genetics
  • Calmodulin / metabolism*
  • EF Hand Motifs
  • Humans
  • Magnetic Resonance Spectroscopy
  • Membrane Transport Proteins / chemistry*
  • Membrane Transport Proteins / genetics
  • Membrane Transport Proteins / metabolism*
  • Multiprotein Complexes / metabolism
  • Peptide Fragments / chemistry
  • Peptide Fragments / metabolism
  • Protein Conformation
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Spectrometry, Fluorescence
  • Thermodynamics
  • Vitamin A / metabolism
  • Zebrafish Proteins / chemistry*
  • Zebrafish Proteins / genetics
  • Zebrafish Proteins / metabolism*


  • Calmodulin
  • Membrane Transport Proteins
  • Multiprotein Complexes
  • Peptide Fragments
  • Recombinant Proteins
  • STRA6 protein, zebrafish
  • Zebrafish Proteins
  • Vitamin A
  • Calcium