A complex unfolding pathway of α-helical membrane proteins in SDS-containing micelles

Abstract

The thermodynamic and kinetic principles guiding folding of transmembrane (TM) proteins are still not very well understood, at least when compared with soluble proteins. However, there is a considerable interest in better understanding these processes, not just since membrane proteins are important drug targets. Membrane proteins constitute ∼25–30% of total proteins encoded in any organism, and many diseases are caused by alterations in TM protein structure and function ( 1 Wallin E. von Heijne G. Genome-wide analysis of integral membrane proteins from eubacterial, archaean, and eukaryotic organisms. Protein Sci. 1998; 7: 1029-1038 Crossref PubMed Scopus (1229) Google Scholar , 2 Marinko J.T. Huang H. Sanders C.R. et al. Folding and misfolding of human membrane proteins in health and disease: from single molecules to cellular proteostasis. Chem. Rev. 2019; 119: 5537-5606 Crossref PubMed Scopus (97) Google Scholar ). The thermodynamic stability of soluble proteins is classically studied via protein denaturation induced by heat or chaotropic salts, typically urea or guanidine hydrochloride, followed by refolding. Yet, TM proteins typically aggregate at increasing temperatures, preventing refolding, and as for in vitro studies, isolated TM proteins are analyzed when incorporated into a lipid bilayer or a detergent micelle, where individual TM helices are shielded from the aqueous environment. Consequently, whereas urea or GdnHCl can unfold extramembranous domains to disrupt TM contacts, the properties of the intimate solvent (here, the membrane or detergent micelles) have to be modified. Cys-labeling kinetics of membrane protein GlpG: a role for specific SDS binding and micelle changes?Otzen et al.Biophysical JournalAugust 6, 2021In BriefEmpirically, α-helical membrane protein folding stability in surfactant micelles can be tuned by varying the mole fraction MFSDS of anionic (sodium dodecyl sulfate (SDS)) relative to nonionic (e.g., dodecyl maltoside (DDM)) surfactant, but we lack a satisfying physical explanation of this phenomenon. Cysteine labeling (CL) has thus far only been used to study the topology of membrane proteins, not their stability or folding behavior. Here, we use CL to investigate membrane protein folding in mixed DDM-SDS micelles. Full-Text PDF