Cell membranes are composed of both bilayer and nonbilayer lipids, whose molecular shape and interactions support or oppose their packing into flat bilayer sheets. While the role of the former in maintaining membrane structural integrity is clear, a growing body of data highlights the importance of nonbilayer lipids in membrane function, including fission and fusion processes in trafficking, and the dynamics of membrane-associated proteins. In this review, we provide a comprehensive overview of nonbilayer lipids and their roles in membrane biology, disease, and drug delivery. First, we introduce the lipid chemistry that gives rise to intrinsic curvature and curvature elastic stress (CES), and we survey the chemical diversity of nonbilayer lipids across organelles and cell types. Second, we discuss relevant methodological advances for characterizing the physicochemical properties of nonbilayer lipids, including X-ray scattering, lipidomics, and molecular dynamics. We then examine how nonbilayer lipids contribute to membrane fusion and fission and how the CES they generate modulates membrane protein localization and function. Next, we consider how the biophysical principles governing nonbilayer lipid behavior can inform the rational design of lipid nanoparticle formulations for therapeutic delivery. Finally, we explore how cells regulate lipid intrinsic curvature as a distinct homeostatic parameter and discuss the implications of its dysregulation in liver disease, neurodegeneration, and mitochondrial dysfunction. We highlight developments in methods spanning the interface between chemistry and biology that can help investigate the function and regulation of nonbilayer lipid chemistry, with far-reaching potential to answer questions in human health, disease, and drug delivery.
