Self-association of Transmembrane α-Helices in Model Membranes: IMPORTANCE OF HELIX ORIENTATION AND ROLE OF HYDROPHOBIC MISMATCH

Emma Sparr,Walter L Ash,Petr V Nazarov,Dirk T.S Rijkers,Marcus A Hemminga,D Peter Tieleman,J Antoinette Killian

doi:10.1074/jbc.m502810200

Abstract

Interactions between transmembrane helices play a key role in almost all cellular processes involving membrane proteins. We have investigated helix-helix interactions in lipid bilayers with synthetic tryptophan-flanked peptides that mimic the membrane spanning parts of membrane proteins. The peptides were functionalized with pyrene to allow the self-association of the helices to be monitored by pyrene fluorescence and Trp-pyrene fluorescence resonance energy transfer (FRET). Specific labeling of peptides at either their N or C terminus has shown that helix-helix association occurs almost exclusively between antiparallel helices. Furthermore, computer modeling suggested that antiparallel association arises primarily from the electrostatic interactions between alpha-helix backbone atoms. We propose that such interactions may provide a force for the preferentially antiparallel association of helices in polytopic membrane proteins. Helix-helix association was also found to depend on the lipid environment. In bilayers of dioleoylphosphatidylcholine, in which the hydrophobic length of the peptides approximately matched the bilayer thickness, association between the helices was found to require peptide/lipid ratios exceeding 1/25. Self-association of the helices was promoted by either increasing or decreasing the bilayer thickness, and by adding cholesterol. These results indicate that helix-helix association in membrane proteins can be promoted by unfavorable protein-lipid interactions.

Highlights

Interactions between transmembrane helices play a key role in almost all cellular processes involving membrane proteins
We propose that such interactions may provide a force for the preferentially antiparallel association of helices in polytopic membrane proteins
It was previously shown with Lys-flanked TM peptides that hydrophobic mismatch does promote self-association, both when a helix-helix recognition motif is present [13], and in the absence of such a motif [14, 15]. It is still not clear whether or not helix-helix association can be considered a general response of TM helices to mismatch. This is because many other responses to hydrophobic mismatch can occur, such as ordering/disordering of the lipid acyl chains, alterations in helix tilt angle, adaptations of the peptide backbone, and because it has been shown that the type and extent of the responses that occur depend on the composition of the TM helix

Summary

Introduction

Interactions between transmembrane helices play a key role in almost all cellular processes involving membrane proteins. In the case of hydrophobic mismatch, helix-helix interactions may be promoted because of relatively unfavorable lipid-helix interactions It was previously shown with Lys-flanked TM peptides that hydrophobic mismatch does promote self-association, both when a helix-helix recognition motif is present [13], and in the absence of such a motif [14, 15]. It is still not clear whether or not helix-helix association can be considered a general response of TM helices to mismatch. Trp-flanked peptides, which were designed to mimic the membrane spanning parts of intrinsic membrane proteins, showed very different responses to hydrophobic mismatch than analogous Lysflanked peptides [14, 15]

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