Biophysical Society Thematic Meeting | Ascona, Switzerland

Liposomes, Exosomes, and Virosomes: From Modeling Complex Membrane Processes to Medical Diagnostics and Drug Delivery

Tuesday Speaker Abstracts

Using Liposomes as a Model System for Probing the Biochemical Mechanisms of Intracellular Membrane Tethering Christopher Stroupe . University of Virginia School of Medicine, Charlottesville, VA, USA. Membrane tethering is a physical association of two intracellular membranes prior to fusion of their lipid bilayers. Many proteins and protein complexes have been proposed to act as membrane tethering factors, but the biochemical mechanisms by which these factors mediate inter-membrane associations remain murky. Here, we have used large and small unilamellar liposomes as models to investigate membrane tethering mediated by the conserved HOPS/Class C Vps complex, an effector for the yeast vacuolar Rab GTPase Ypt7p. To assay tethering, we quantified co-localization of red- and green-labeled liposomes in a confocal fluorescence microscope. We found that for HOPS to tether large liposomes (diameter ~120 nm), Ypt7p is required on both apposed membranes. In contrast, HOPS can tether Ypt7p-free small liposomes (diameter ~55 nm) via a direct interaction between these highly-curved membranes and a curvature-sensing ALPS (amphipathic lipid packing sensor) motif on the Vps41p HOPS subunit. Finally, we found that HOPS can interact directly with the autophagosomal protein Atg8p (the yeast homolog of mammalian LC3B) to tether membranes bearing Atg8p to membranes bearing Ypt7p. Thus, we have shown here, for the first time, how a Rab effector engages in protein- protein and protein-lipid interactions to tether intracellular membranes. This study therefore demonstrates the power and flexibility of using liposomes as chemically-defined models of intracellular membranes in order to address hitherto unexplored questions in cell biology. Furthermore, HOPS is required for cellular entry of the Ebola virus, while autophagosome fusion is involved in diverse pathological states, including cancer, ischemia-reperfusion injury, and protein misfolding diseases. Thus, this study shows how liposomes can be used as platforms for understanding conditions of critical importance in biomedical research.

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