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Coupling mechanisms between membrane leaflets (FWF)

Project Leader: Univ.-Prof. Dr. Peter Pohl

Project Start 1 August 2015

Summary:

Biological membranes contain ordered and disordered domains. Small cholesterol containing ordered domains are called rafts. Ordered domains from the two monolayers are always in register as has been shown using the somewhat larger-sized liquid-ordered domains in artificial lipid bilayers. Even though the driving force behind such alignment has thus far remained enigmatic, it is fundamental for the formation of signaling platforms. Among the manifold forces implicated are interactions at the membrane midplane, the mutual attraction of stiffed regions in the opposing leaflets, and the minimization of line tension between the somehow thicker ordered domains and the thinner disordered domains. The goal of the present application is to dissect the predictions of these hypotheses and thus to elucidate the molecular mechanism for domain registration. We start by measuring the interaction energy between both leaflets. Therefore small ordered domains will be induced by polymer adsorption to one leaflet of free-standing planar bilayers and their slippage against the other leaflet will be measured by fluorescence correlation spectroscopy as a function of temperature. Both by fluorescence correlation spectroscopy and fluorescence imaging, the slippage of larger domains of raft forming lipids in one leaflet against non-raft lipids in both leaflets will be monitored and the dependency of the friction on different lipid species will be established. This enables us to separate whether, acting via overhang at the midplane, ordered domains in one leaflet may induce lipids in the other leaflet to adopt an ordered state or whether the macroscopic appearance of domains may be explained by line tension driven merger of invisibly small domains into larger ones. In addition, we will exploit both electrostriction and hydrostatic pressure to test the theoretical dependence of domain size on surface tension. To clarify the role of line tension we will use polymers to induce domains of different sizes in the two leaflets and we will measure how their interaction energy depends on the mismatch in their sizes. Last but not least we will test the calculated energies for the midplane interaction of (i) ordered with disordered and (i) ordered with ordered domains by carrying out similar experiments with bipolar lipids where configurations exits without midplane interaction. Throughout the whole project experimental and theoretical work will go hand in hand to ensure a proper mathematical description of membrane mechanics. We expect a molecular picture to emerge in which well characterized driving forces explain how domain registration may occur in biological membranes.

Publications

Undulations Drive Domain Registration from the Two Membrane Leaflets
Timur R. Galimzyanov, Peter I. Kuzmin, Peter Pohl unf Sergey A. Akimov.
Biophysical Journal 2017. Volume 112, pp. 339-345. January 24 2017.

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Comment on "Elastic Membrane Deformations Govern Interleaflet Coupling of Lipid-Ordered Domains" Reply
Timur R. Galimzyanov, Rodion J. Molotkovsky, Fredric S. Cohen, Peter Pohl and Sergey A. Akimov
PRL 116, 079802 (2016). Published week ending 19 February 2016

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Elastic deformations of bolalipid membranes
Timur R. Galimzyanov, Peter I. Kuzmin, Peter Pohl and Sergey A. Akimov
Soft Matter, 12, 2357-2364. Published January 2016.

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Elastic Membrane Deformations Govern Interleaflet Coupling of Lipid-Ordered Domains
Timur R. Galimzyanov, Rodion J. Molotkovsky, Marine E. Bozdaganyan, Fredric S. Cohen, Peter Pohl, and Sergey A. Akimov
Physical Review Letters 115, 088101. Published 18 August 2015.

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