Biophysical Society Conference | Tahoe 2022

Molecular Biophysics of Membranes

Poster Abstracts

37-POS Board 10 INVESTIGATION INTO THE MECHANOCHEMISTRY OF NUMBER ASYMMETRY GENERATION IN MODEL MEMBRANES: A COMBINED MASS-ACTION AND MICROMECHANICAL STUDY Tyler R Reagle 1 ; Yuxin Xie 1 ; Tobias Baumgart 1 ; 1 University of Pennsylvania, Dept. of Chemistry, Philadlephia, PA, USA Nonvesicular lipid transport is a mechanism employed by lipid trafficking machinery to regulate compositional inhomogeneities of biomembranes. Globally, lipid transfer proteins (LTPs) catalyze nonvesicular lipid exchange between different organelles to efficiently pool the cellular lipidome. Locally, transmembrane flippases and scramblases catalyze the nonvesicular exchange of lipids between the two leaflets of a single bilayer. Flippases catalyze active lipid transport in which the energetically-costly process of membrane lipid asymmetry generation is coupled to ATP hydrolysis. Contrarily, scramblases catalyze passive transport between both leaflets in which transport proceeds downhill to relax gradients from membrane lipid asymmetry. Membrane lipid asymmetry refers to both 1) dissimilar leaflet-specific lipid compositions across the bilayer (I.e. compositional asymmetry) and 2) unequal numbers of lipid molecules composing both leaflets across the bilayer (I.e. number asymmetry). While it is recognized that a bilayer’s asymmetry is a result of the regulated activities of flippases/scramblases, whether lipid flux catalyzed by LTPs could also be influenced by the specific compositional/number asymmetries of the exchanging bilayers is an open question. To consider this, we have used the lipid chelator, methyl- β -cyclodextrin (mbCD), to probe the physical chemistry and micromechanics of number asymmetry relevant for lipid transport in vivo. We scrutinized the equilibrium binding model of mbCD-lipid complexation under conditions when unbound mbCD/mbCD-lipid inclusion complexes in the aqueous phase were in coexistence with membranes with nonzero number asymmetry. In doing so, we identified whether number asymmetry contributed detectably to lipid mass-action. We also performed micromechanical experiments upon giant unilamellar vesicles with number asymmetry using optical trapping/micropipette aspiration. We observed that, at some threshold mechanical tension, spontaneous lipid exchange between both leaflets occurred to relax initial number asymmetry within the membrane. Our results highlight our ongoing efforts to study the mechanochemistry of number asymmetry and propose an interplay between flippases and LTPs in lipid trafficking.

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