A photoswitchable helical peptide with light-controllable interface / transmembrane topology in lipidic membranes

Abstract

According to the three-step model, the spontaneous insertion and folding of helical transmembrane (TM) polypeptides into lipid bilayers is driven by three sequential equilibria: solution-to-membrane interface (MI) partition, unstructured-to-helical folding, and MI-to-TM helix insertion. However, understanding these three steps with molecular detail has been challenged by the lack of suitable experimental approaches to rapidly and reversibly perturb membrane-bound hydrophobic polypeptides out of equilibrium. Here, we report on a 24-residues-long hydrophobic α-helical polypeptide, covalently coupled to an azobenzene photoswitch (KCALP-azo), which displays a light-controllable TM/MI equilibrium in hydrated lipid bilayers. FTIR spectroscopy shows that dark-adapted KCALP-azo (trans azobenzene) folds as a TM α-helix, with its central TM region displaying an average tilt of 36 ± 4° with the membrane normal (TM topology). After trans-to-cis photoisomerization of the azobenzene moiety with UV light (reversed with blue light), spectral changes by FTIR spectroscopy indicate that the helical structure of KCALP-azo is maintained but the peptide experiences a more polar environment. Interestingly, pH changes induced similar spectral alterations in the helical peptide LAH4, with a well-characterized pH-dependent TM/MI equilibrium. Polarized experiments confirmed that the membrane topology of KCALP-azo is altered by light, with its helix tilt changing reversibly from 32 ± 5° (TM topology, blue light) to 79 ± 8° (MI topology, UV light). Further analysis indicates that, while the trans isomer of KCALP-azo is ≈100% TM, the cis isomer exists in a ≈90% TM and ≈10% MI mixture. Strategies to further increase the perturbation of the TM/MI equilibrium with the light are briefly discussed.