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Proc Natl Acad Sci U S A. 2015 Jun 30;112(26):7978-83. doi: 10.1073/pnas.1424751112. Epub 2015 Jun 8.

Cooperative folding of a polytopic α-helical membrane protein involves a compact N-terminal nucleus and nonnative loops.

Author information

1
Interdisciplinary Nanoscience Center, Department of Molecular Biology and Genetics, Center for Insoluble Protein Structures, Aarhus University, DK-8000 Aarhus C, Denmark;
2
Howard Hughes Medical Institute, Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
3
Interdisciplinary Nanoscience Center, Department of Molecular Biology and Genetics, Center for Insoluble Protein Structures, Aarhus University, DK-8000 Aarhus C, Denmark; dao@inano.au.dk.

Abstract

Despite the ubiquity of helical membrane proteins in nature and their pharmacological importance, the mechanisms guiding their folding remain unclear. We performed kinetic folding and unfolding experiments on 69 mutants (engineered every 2-3 residues throughout the 178-residue transmembrane domain) of GlpG, a membrane-embedded rhomboid protease from Escherichia coli. The only clustering of significantly positive ϕ-values occurs at the cytosolic termini of transmembrane helices 1 and 2, which we identify as a compact nucleus. The three loops flanking these helices show a preponderance of negative ϕ-values, which are sometimes taken to be indicative of nonnative interactions in the transition state. Mutations in transmembrane helices 3-6 yielded predominantly ϕ-values near zero, indicating that this part of the protein has denatured-state-level structure in the transition state. We propose that loops 1-3 undergo conformational rearrangements to position the folding nucleus correctly, which then drives folding of the rest of the domain. A compact N-terminal nucleus is consistent with the vectorial nature of cotranslational membrane insertion found in vivo. The origin of the interactions in the transition state that lead to a large number of negative ϕ-values remains to be elucidated.

KEYWORDS:

GlpG; folding; kinetics; membrane protein; rhomboid

PMID:
26056273
PMCID:
PMC4491732
DOI:
10.1073/pnas.1424751112
[Indexed for MEDLINE]
Free PMC Article

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