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Time-resolved measurements of an ion channel conformational change driven by a membrane phase transition

Significance Membranes act as a host matrix to many different biomolecules, such as membrane proteins. As highly dynamic structures, membranes have many modes of fluctuation accessible under physiological conditions. By inducing a phase transition that thins a membrane with a temperature jump and monitoring the response of a model ion channel, we are able to track in a time-resolved manner how this protein responds to fast structural changes in the membrane. Using temperature-jump infrared spectroscopy, we are able to trigger a gel-to-fluid phase transition in lipid vesicles and monitor in real time how a membrane protein responds to structural changes in the membrane. The melting of lipid domains in 1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles is observed to occur in as fast as 50 ns, with a temperature dependence characteristic of critical slowing. Gramicidin D (gD) added to the membrane responds primarily to the change in thickness of the membrane on a timescale coincident with the membrane melting. Using structure-based spectral modeling, we assign the conformational changes to compression and rotation of a partially dissociated gD dimer. Free energy calculations indicate that the high rate is a result of near-barrierless diffusion on a protein energy landscape that is radically reshaped by membrane thinning. The structural changes associated with the phase transition are similar to the fluctuation modes of fluid phase membranes, highlighting the importance of understanding the dynamic nature of the membrane environment around proteins.