The outer membrane of eukaryotic cells, termed the plasma membrane, is a lipid bilayer that contains hundreds of different lipids and even more types of membrane proteins. The outer half of the bilayer faces the world outside the cell, while the inner half faces the cytosol. Both the protein and the lipid composition of the plasma membrane are different in each half of the bilayer, giving rise to the jargon term that these membranes are "asymmetric," meaning that the two halves of the bilayer are chemically distinct.
The very real complexity of the living cell plasma membrane can be reduced in so-called "model bilayers" that have a small number of lipids, and few or no membrane proteins. Only a handful of researchers attempt to understand the complexity created by the asymmetry of the two halves of the bilayer, mostly because asymmetric model bilayers are extremely challenging to prepare. In this research, we describe a new method for creating model asymmetric bilayers and studying how one of the halves, here termed leaflet, influences the properties of the other half. The extent of influence can potentially affect many aspects of cellular function.
The cover image for the September 17 issue of Biophysical Journal is an artistic rendering of a hemifusion event generating an asymmetric vesicle. An initially symmetric vesicle is represented by a bilayer of lipids colored in green (head groups) and dark yellow (chains). The symmetric vesicle hemifuses with a supported lipid bilayer, in which lipids are colored in blue and dark yellow (bottom and backplane of the image) to represent a different lipid composition than that of the symmetric vesicle. When hemifusion occurs, the external leaflets of the symmetric vesicle and the supported lipid bilayer are connected and can exchange their lipids. Hemifusion ends when the vesicle detaches from the supported bilayer with a new external leaflet. The central image depicts the newly-formed asymmetric vesicle, where the inner half of the bilayer contains the green-head-group lipids and the external half contains the blue-head-group lipids. The expression “kiss-and-run asymmetric vesicles,” coined by Heerklotz and London in the New and Notable commentary that accompanies our article, describes the method. Our findings add to the existing methods and discussions related to plasma membrane asymmetry.
The image was created using the software Blender (https://www.blender.org/) that supports three-dimensional modeling and animations. The supported lipid bilayers were created from a double regular lattice of lipids in a plane geometry and vesicles were created from a regular lattice of lipids on the top of concentric spherical shells. The image was created by Thais Azevedo Enoki.
Readers interested in learning more about the authors’ research can visit the websites listed below:
https://www.enokita.com
http://feigensonlab.mbg.cornell.edu/
- Thais A. Enoki and Gerald W. Feigenson