Membranes: Shaping biological matter.

173 news & views t he space occupied by living organisms is determined by membranes. Each cell is delimited by a membrane, and additional membranes form the functional hierarchy of compartments packed within it. The intracellular compartments, for example vesicles and organelles, define the spatial organization of metabolic pathways, thus providing the topological framework for cellular life. Recent advances in structural techniques and super-resolution microscopy now allow imaging of intracellular membrane structures with unprecedented resolution. Paul Wiggins and co-workers 1 are, perhaps, first to notice that the wealth of membrane structures obtained awaits detailed elastic modelling. The core of each membrane is the lipid bilayer, the bimolecular film consisting of two monolayers of amphiphilic molecules, the lipids. This bilayer is primarily responsible for the barrier function of membranes. However, the lipids are also deeply involved in regulation of membrane shape and dynamics: both depend on the material properties of the lipid bilayer 2,3. One property is the elasticity, the resistance to stretching and compression that accompanies most of the deformations of lipid bilayers imposed by the specialized proteins that determine the shape of cellular membranes. The lipid bilayer can be approximated as a continuous elastic surface whose behaviour is characterized by a limited set of bulk material parameters 4. Despite its relative simplicity, the elastic approximation has been successfully used to explain a wide variety of membrane phenomena, from the multitude of shapes of red blood cells 5 to the structure of metastable intermediates of membrane fission and fusion 6. How can continuum elastic modelling help us to understand the action of specialized proteins creating cellular shape (and thus how the shape is encoded) at small, almost molecular scales? Proteins can be simply considered as a new bulk component effectively altering averaged elastic parameters of the lipid bilayer. Different algorithms linking protein embedment into a membrane to elastic deformations have recently been developed 2,7,8 , yielding phenomenological descriptions of membrane morphogenesis in cells. The Wiggins group take a different approach. They assume that proteins act externally, by applying forces required to support a membrane shape from its boundary. The force distribution can be calculated by analysing the membrane shape within this boundary, by the established algorithms of shape parameterization and localized force balancing 5,7,9. The researchers tested this approach on a simple experimental system in which a thin membrane cylinder (tether) was pulled from a giant vesicle (Fig. 1a). The …