SEMINAR INFORMATION

Title: Robust molecular tweezers reveal the speed limit of membrane protein folding

Speaker: Duyoung Min*

*Department of Chemistry, UNIST

Single-molecule tweezers, such as magnetic tweezers, are powerful tools for probing nm-scale structural changes in single membrane proteins under force. However, the weak molecular tethers used for the membrane protein studies have limited long-time, repetitive molecular transitions due to force-induced bond breakage. In this talk, we present a robust single-molecule tweezer method that uses dibenzocyclooctyne (DBCO) cycloaddition and traptavidin binding, enabling the estimation of the folding ‘speed limit’ of helical membrane proteins. By using this method, we were able to observe numerous structural transitions of a designer single-chained transmembrane (TM) homodimer for 9 h at 12 pN, and reveal its entire folding pathway, including the hidden dynamics of helix-coil transitions. We characterized the energy barrier heights and folding times for the transitions using a model-independent deconvolution method and the hidden Markov modeling (HMM) analysis, respectively. The Kramers rate framework yields a considerably low speed limit of 21 ms for a helical hairpin formation in lipid bilayers, compared to μs-level soluble protein folding. This large discrepancy is likely due to the highly viscous nature of lipid membranes, retarding the helix-helix interactions. Our results offer a more valid guideline for relating the kinetics and free energies of membrane protein folding.