The Viscosity of Lipid and Protein Membranes

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Non-technical: This award by the Biomaterials program in the Division of Materials Research to University of Oregon is to apply recently developed methods for measuring membrane fluidity to several basic issues involving the nature of two-dimensional flows in membranes, the relationships between viscosity and membrane tension, and the dependence of viscosity on lipid structure. Membranes are critical components of all living cells that provide a flexible framework in which lipids and proteins build structures, perform chemical reactions, and spatially reorganize. These tasks are made possible by the physical properties of the lipid bilayer, the two-molecule-thick structure that forms the basis of all cellular membranes. The two-dimensional fluidity of lipid bilayers is essential to their function, as it enables the mobility of membrane molecules. Characterization of bilayer viscosity, however, remains minimal, which limits our ability to develop quantitative, predictive models of important processes such as intracellular cargo trafficking and signal transduction. The proposed experiments will also enhance educational and outreach activities. For example, discussions of membrane behavior and biophysical modes of inquiry will illuminate a course for non-science majors developed by the PI that explores the physical properties of organisms and biological materials, with the broad aim of promoting scientific literacy. Regarding outreach, the PI initiated and co-runs a week long Physics Day Camp for high school students from low socioeconomic backgrounds that provides an exposure to science as well as, more broadly, a greater familiarity with the nature of higher education. The PI and his group lead activities related to biomaterials and microscopy. These studies also provide opportunities for training undergraduates and graduate students in methods of membrane biophysics, optics, and computational image analysis. Technical: The two-dimensional fluidity of cellular membranes is essential to their function, as it enables the mobility of constituent lipids and proteins as well as the spatial reorganization of lipid domains, protein complexes, and other larger-scale structures. Though the existence and importance of membrane fluidity are well established, characterizations of the material properties underlying it remain crude. Specifically, the viscosity of lipid bilayers has proven challenging to measure due to membranes' fragility and small size, and also due to the intrinsic complexity of two-dimensional hydrodynamics. Our incomplete understanding of membrane viscosity frustrates models of membrane dynamics. In the proposed experiments, the investigator will build on techniques that have recently developed in making use of both single- and two-point rheological approaches to address various fundamental questions of membrane biophysics. The researcher will examine whether standard hydrodynamic models are applicable at short length scales, where they have not been rigorously tested. Relationships between viscosity and membrane tension, and between viscosity and out-of-plane fluctuations will also will be studies with this project. The dependence of lipid bilayer viscosity on acyl chain length, temperature, the presence of proteins that insert into the bilayer, and the density of membrane inclusions, all of which will provide a rich picture of membrane material properties will be characterized with this research study. The proposed studies will impact educational work, most notably related to a general education biophysics course for non-science majors, outreach work, related especially to a day camp for high school students from low socioeconomic backgrounds, and training of students in a range of methods related to membrane biophysics, optics, and computational image analysis.