Milka Doktorova1, Ilya Levental1 and Frederick A. Heberle2 1Department of Molecular Physiology and Pharmacology, University of Virginia, Center for Membrane and Cell Physiology, Charlottesville, Virginia 22908, USA 2Department of Chemistry, University of Tennessee Knoxville, Knoxville, Tennessee 37996, USA Correspondence: md4xmvirginia.edu; il2syvirginia.edu; fheberleutk.edu

Almost all biomembranes are constructed as lipid bilayers and, in almost all of these, the two opposing monolayers (leaflets) have distinct lipid compositions. This lipid asymmetry arises through the concerted action of a suite of energy-dependent enzymes that maintain living bilayers in a far-from-equilibrium steady-state. Recent discoveries reveal that lipid compositional asymmetry imparts biophysical asymmetries and that this dualistic organization may have major consequences for cellular physiology. Importantly, while transbilayer asymmetry appears to be an essential, near-ubiquitous characteristic of biological membranes, it has been challenging to reproduce in reconstituted or synthetic systems. Although recent methodological developments have overcome some critical challenges, it remains difficult to extrapolate results from available models to biological systems. Concurrently, there are few experimental approaches for targeted, controlled manipulation of lipid asymmetry in living cells. Thus, the biophysical and functional consequences of membrane asymmetry remain almost wholly unexplored. This perspective summarizes the current state of knowledge and highlights emerging themes that are beginning to make inroads into the fundamental question of why life tends toward asymmetry in its bilayers.