Bilayer thickness determines the alignment of model polyproline helices in lipid membranes.

Vladimir Kubyshkin,Stephan L Grage,Nediljko Budisa,Anne S Ulrich

doi:10.1039/c9cp02996f

Abstract

Our understanding of protein folds relies fundamentally on the set of secondary structures found in the proteomes. Yet, there also exist intriguing structures and motifs that are underrepresented in natural biopolymeric systems. One example is the polyproline II helix, which is usually considered to have a polar character and therefore does not form membrane spanning sections of membrane proteins. In our work, we have introduced specially designed polyproline II helices into the hydrophobic membrane milieu and used 19F NMR to monitor the helix alignment in oriented lipid bilayers. Our results show that these artificial hydrophobic peptides can adopt several different alignment states. If the helix is shorter than the thickness of the hydrophobic core of the membrane, it is submerged into the bilayer with its long axis parallel to the membrane plane. The polyproline helix adopts a transmembrane alignment when its length exceeds the bilayer thickness. If the peptide length roughly matches the lipid thickness, a coexistence of both states is observed. We thus show that the lipid thickness plays a determining role in the occurrence of a transmembrane polyproline II helix. We also found that the adaptation of polyproline II helices to hydrophobic mismatch is in some notable aspects different from α-helices. Finally, our results prove that the polyproline II helix is a competent structure for the construction of transmembrane peptide segments, despite the fact that no such motif has ever been reported in natural systems.

Highlights

When considering the secondary structure elements that carry the side-chain functions, this natural versatility vanishes
The peptide sequence was based on an oligomeric octahydroindole-2-carboxylic acid (Oic) structure to enable the desired selectivity for a hydrophobic environment (Fig. 2A)
Sufficient hydrophobicity of the core peptides can be inferred from the fact that oligo-Oic peptides with 6 or more residues are not soluble in water, but they are soluble in hexane as well as other organic solvents, such as dichloromethane or chloroform, which are typical solvents for lipids.[18]

Summary

Introduction

When considering the secondary structure elements that carry the side-chain functions, this natural versatility vanishes. The vast majority of integral TM proteins are constructed from a-helices that are often regarded as ‘‘TM helices’’; another class of membrane proteins consists of b-barrel structures (only occur in special types of membranes).[9] In the aqueous medium, nature operates with a much larger repertoire of secondary structures, but only a few are present in membrane embedded proteins Among these structures excluded from the membrane, the most widespread is the polyproline-II (PII) helix, regarded as a ‘‘semi-extended’’ helix (Fig. 1A).[10,11] Interestingly, in the history of life evolution, the PII helix was present at the very beginning of protein biosynthesis, before the a-helix became the dominant protein fold.[12] It is quite surprising that this 22396 | Phys.

Results

Discussion

Conclusion