29 / 120
Liposomes, Exosomes, and Virosomes: From Modeling Complex
Membrane Processes to Medical Diagnostics and Drug Delivery
Tuesday Speaker Abstracts
24
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.