Structure-functional relationship of cellular retinoic acid-binding proteins I and II interacting with natural and synthetic ligands.

Charles W E Tomlinson,Ehmke Pohl,Andrew Whiting,Katy A S Cornish

doi:10.1107/s2059798320015247

Charles W E Tomlinson, Ehmke Pohl + Show 2 more

Open Access

https://doi.org/10.1107/s2059798320015247

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Abstract

A detailed understanding of the interactions between small-molecule ligands and their proposed binding targets is of the utmost importance for modern drug-development programs. Cellular retinoic acid-binding proteins I and II (CRABPI and CRABPII) facilitate a number of vital retinoid signalling pathways in mammalian cells and offer a gateway to manipulation of signalling that could potentially reduce phenotypes in serious diseases, including cancer and neurodegeneration. Although structurally very similar, the two proteins possess distinctly different biological functions, with their signalling influence being exerted through both genomic and nongenomic pathways. In this article, crystal structures are presented of the L29C mutant of Homo sapiens CRABPI in complex with naturally occurring fatty acids (1.64 Å resolution) and with the synthetic retinoid DC645 (2.41 Å resolution), and of CRABPII in complex with the ligands DC479 (1.80 Å resolution) and DC645 (1.71 Å resolution). DC645 and DC479 are two potential drug compounds identified in a recent synthetic retinoid development program. In particular, DC645 has recently been shown to have disease-modifying capabilities in neurodegenerative disease models by activating both genomic and nongenomic signalling pathways. These co-crystal structures demonstrate a canonical binding behaviour akin to that exhibited with all-trans-retinoic acid and help to explain how the compounds are able to exert an influence on part of the retinoid signalling cascade.

Highlights

The cellular retinoic acid-binding proteins CRABPI and CRABPII act as gatekeepers and facilitators for the passage of all-trans-retinoic acid through the cytoplasm to access retinoic acid receptors (RARs) in the nucleus
As part of crystal-screening efforts, we identified that an L29C mutation in Homo sapiens CRABPI allowed vastly improved crystallization and diffraction to atomic resolution, which was subsequently determined to be due to partial dimerization at the mutated surface residue
It was determined that this density was most likely to represent fatty-acid byproducts from expression that were incorporated into the binding site based on their hydrophobic structure and carboxylic acid head group, and it was fitted as the 14-carbon myristic acid (MYR) and the 13-carbon tridecanoic acid (TDA) in the two monomers

Summary

Introduction

The cellular retinoic acid-binding proteins CRABPI and CRABPII act as gatekeepers and facilitators for the passage of all-trans-retinoic acid (and related derivatives) through the cytoplasm to access retinoic acid receptors (RARs) in the nucleus. This journey is of eminent importance, as retinoid signalling has a huge impact on the growth and development of mammalian organisms throughout their lifetimes, with a particular focus on embryonic development. CRABPI shares high sequence identity (77%) with CRABPII but, unlike its sister protein, does not contain the appropriate residues to form a nuclear localization sequence (NLS).

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