Solution NMR: A powerful tool for structural and functional studies of membrane proteins in reconstituted environments

Robbins Puthenveetil,Olga Vinogradova

doi:10.1074/jbc.rev119.009178

Abstract

A third of the genes in prokaryotic and eukaryotic genomes encode membrane proteins that are either essential for signal transduction and solute transport or function as scaffold structures. Unlike many of their soluble counterparts, the overall structural and functional organization of membrane proteins is sparingly understood. Recent advances in X-ray crystallography, cryo-EM, and nuclear magnetic resonance (NMR) are closing this gap by enabling an in-depth view of these ever-elusive proteins at atomic resolution. Despite substantial technological advancements, however, the overall proportion of membrane protein entries in the Protein Data Bank (PDB) remains <4%. This paucity is mainly attributed to difficulties associated with their expression and purification, propensity to form large multisubunit complexes, and challenges pertinent to identification of an ideal detergent, lipid, or detergent/lipid mixture that closely mimic their native environment. NMR is a powerful technique to obtain atomic-resolution and dynamic details of a protein in solution. This is accomplished through an assortment of isotopic labeling schemes designed to acquire multiple spectra that facilitate deduction of the final protein structure. In this review, we discuss current approaches and technological developments in the determination of membrane protein structures by solution NMR and highlight recent structural and mechanistic insights gained with this technique. We also discuss strategies for overcoming size limitations in NMR applications, and we explore a plethora of membrane mimetics available for the structural and mechanistic understanding of these essential cellular proteins.

Highlights

A third of the genes in prokaryotic and eukaryotic genomes encode membrane proteins that are either essential for signal transduction and solute transport or function as scaffold structures
We find that identifying the oligomeric state of a ␤-barrel porin in a nanodisc may be important before pursual of nuclear magnetic resonance (NMR) studies
saposin A (SapA) discs are compatible with a variety of lipid molecules and are flexible enough to accommodate large membrane protein complexes with variable molecular weights, as demonstrated by cryo-EM structures of archaeal mechanosensitive channel T2 (32.9 kDa), with four predicted transmembrane helices forming as a putative homopentamer, and bacterial peptide transporter PepTSo2 (56 kDa), with 14 transmembrane helices forming a homotetramer [101]

Summary

Introduction

A third of the genes in prokaryotic and eukaryotic genomes encode membrane proteins that are either essential for signal transduction and solute transport or function as scaffold structures. SapA discs are compatible with a variety of lipid molecules and are flexible enough to accommodate large membrane protein complexes with variable molecular weights, as demonstrated by cryo-EM structures of archaeal mechanosensitive channel T2 (32.9 kDa), with four predicted transmembrane helices forming as a putative homopentamer, and bacterial peptide transporter PepTSo2 (56 kDa), with 14 transmembrane helices forming a homotetramer [101].

Results

Conclusion