Mapping the ligand binding pocket in the cellular retinaldehyde binding protein

Area of Science:

• Biochemistry
• Molecular Biology
• Vision Science

Background:

• Cellular retinaldehyde binding protein (CRALBP) is crucial for the visual cycle, facilitating retinoid transport and enzymatic reactions.
• Previous studies suggested involvement of methionine (Met) and tryptophan (Trp) residues in CRALBP's retinoid interactions.

Purpose of the Study:

• To investigate the role of specific Met and Trp residues in CRALBP's retinoid binding and substrate carrier functions.
• To elucidate the structural basis of CRALBP's ligand interactions using site-directed mutagenesis and biophysical analyses.

Main Methods:

• Production and purification of recombinant human CRALBP (rCRALBP) mutants (M208A, M222A, M225A, W165F, W244F).
• Evaluation of retinoid binding affinities and substrate carrier functions using UV-visible spectroscopy and kinetic assays.
• Analysis of ligand-dependent conformational changes using Heteronuclear single quantum correlation (HSQC) NMR spectroscopy.

Main Results:

• All mutants retained the ability to bind 11-cis- and 9-cis-retinal, indicating overall protein integrity.
• Mutations in Met and Trp residues altered UV-visible spectra and reduced retinoid binding affinities.
• Specific mutants (M222A, M225A, W244F) exhibited impaired substrate carrier function for 11-cis-retinol oxidation by RDH5.
• NMR studies confirmed localized structural changes and ligand-dependent conformational alterations involving Met-208, Met-222, Trp-165, and Trp-244.

Conclusions:

• Methionine residues 208, 222, 225, and Tryptophan residues 165, 244 are integral components of the CRALBP ligand binding cavity.
• These residues play critical roles in CRALBP's retinoid interactions, substrate binding, and overall function in the visual cycle.
• The findings provide a structural basis for understanding CRALBP's mechanism of action and potential therapeutic targeting.