Lipid membranes are highly dynamic assemblies that exhibit a rich hierarchy of dynamic modes, which range from individual molecular motions to collective membrane fluctuations. Among these dynamic modes are bilayer thickness fluctuations, which have been implicated in protein-membrane association but have only recently become experimentally accessible through neutron spin echo (NSE) spectroscopy. Here, we use the pH-low insertion peptide (pHLIP) as a tunable model protein to understand how the association state of membrane proteins, i.e., surface-associated (SA) or transmembrane (TM), influences membrane dynamics. pHLIP is highly sensitive to changes in pH, transitioning from a SA to a TM configuration below a critical pH value. This property allows both classes of membrane proteins to be studied in a single system. NSE measurements on DOPC-cholesterol membranes revealed that membrane thickness fluctuations are not noticeably impacted by SA pHLIP, but experience strong dampening in the presence of TM pHLIP. Combined with structural small-angle neutron / X-ray scattering (SANS / SAXS) measurements which show no changes in membrane thickness in either state of pHLIP, the NSE results suggest that the suppression of thickness fluctuations by TM pHLIP is driven by pHLIP-induced changes in the membrane viscosity. These conclusions are consistent with MD simulations and solid-state NMR data that show that pHLIP promotes distortion in the conformation of the lipid acyl chains in the vicinity of pHLIP, resulting in increased membrane viscosity and reduced lateral lipid diffusion. This observation implies that the ability of a TM helix to dampen thickness fluctuations may serve as an indirect means to modulate biological membrane functions. For instance, dampened fluctuations may hinder protein domain motions, prevent transient hydrophobic mismatch, and possibly limit the occurrence of pore formation in biological membranes.

The transmembrane helix of pHLIP slows down membrane thickness fluctuations and translational diffusion

ACS Spring Meeting 2021

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