Special Issue on Liposomes, Exosomes, and Virosomes

This special issue of the Biophysical Journal hosts a collection of articles submitted by participants of the Biophysical Society Thematic Meeting “Liposomes, Exosomes, Virosomes: From Modeling Complex Membrane Processes to Medical Diagnostics and Drug Delivery”, which took place in September 2016 in Ascona, Switzerland. All articles deal with aspects of generating or characterizing membrane vesicles, proteins or lipids in them, and interactions between them. Not only have membrane vesicles become useful tools to study fundamental properties of biomembranes and cellular organelles and viruses, but they have also been frequently employed in diagnostics and drug or gene delivery and thus have become important tools for practical applications.The volume contains two Perspectives that review assisted and unassisted protein insertion into membranes by Kuhn et al. and the impact of monogangliosides on amyloid fibril formation by Cebecauer et al. The Letter by Ritzmann et al. introduces a clever new method to orient proton pumps in liposomes. Two intriguing articles by Cornell et al. and Burns et al. describe lipid order and miscibility transitions in membranes derived from mammalian and zebrafish cell lines and how they are influenced by alcohols and growth temperature, respectively. The targeting of herpes simplex virus to glycosaminoglycans is the topic of a paper by Peerboom et al. Three articles deal with membrane fusion: Zdanowicz et al. show that the synaptic regulatory protein complexin binds not only to the complete N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) complex as previously known, but also to a binary acceptor t-SNARE complex on target membranes where receptor binding is cooperative with membrane curvature-dependent lipid binding. Witkowska and Jahn developed a single particle fusion assay between proteoliposomes or chromaffine granules with giant unilamellar vesicles bearing cognate SNAREs. Flavier and Boxer studied single particle fusion between vesicles and supported lipid bilayers bearing hybridizing DNA and show that the length of the lipid anchor in the membrane determines the relative frequencies of full and hemi-fusion. In another exciting contribution, Larsen et al. examined how curvature and lipid composition synergize to recruit the small G signaling protein N-Ras to lipid bilayers.Protein folding and insertion into membranes has caught the attention of many investigators for more than two decades. Knowledge of the structural and dynamical properties of the unfolded state is important to fully understand membrane protein folding. Krainer et al. have characterized this state for the outer membrane phospholipase A. Many cell penetrating peptides fold and insert into membranes in a pH-dependent fashion as shown by Wolf et al. for the histidine-rich peptide LAH4-L1 using solid-state NMR. Virucidal peptides modeled after the membrane proximal and transmembrane regions of the HIV envelope glycoprotein gp41 insert into the lipid bilayer of the viral envelope and thereby inhibit infectivity as shown by Carravilla et al. Kruger et al. show that dual color cross-correlation spectroscopy is a powerful and highly sensitive method to measure protein binding to membranes. Sezgin et al. have developed polarity sensitive fluorescent dyes that are capable of probing lipid order in cell membranes with super-resolved stimulated emission depletion microscopy. Henrich et al. prepared supported membranes from nanodiscs harboring cell-free expressed Krokinobacter rhodopsin-2 and measured electrophysiologically the protein’s ion pumping activity while optically determining chromophoric transitions in the same sample after flash photolysis on modified gold electrodes.The generation and sorting of extracellular vesicles of different sizes by endosomal-sorting-complex-required-for-transport proteins and the AAA+ ATPase VPS4 in HEK293 cells is the topic of an exciting contribution by Jackson et al. The issue closes with a paper by Del Piccolo and Hristova, who employ a Forster resonance energy transfer-based method to analyze the interaction of epidermal growth factor receptor and the cytoplasmic adaptor protein growth factor receptor-bound protein 2 in live cells. In conclusion, the collection of highly diverse articles assembled in this Special Issue highlights the vitality of contemporary membrane biophysics and the broad topics that were lively discussed at the Liposomes, Exosomes, and Virosomes meeting in Ascona last fall.