Phosphoinositide Domains and Gradients in Asymmetric Lipid Bilayers: Physical Properties and Protein Recognition

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Lipids are a class of biomolecules that affect or even control virtually all processes that occur on cellular membranes. They are very important for cell physiology and human biology. Biological membranes are bilayers of lipids. The two parts of the bilayer have different overall lipid composition as well as different lateral composition. Each part contains domains that have distinct lipid compositions and physical properties. Different proteins are attracted to these domains. As a result, lipids and proteins form supramolecular structures that are platforms for a large number of important cellular functions, most notably cell signaling. With this award, the Chemistry of Life Processes Program in the Chemistry Division and the Molecular Biophysics Program in the Division of Molecular and Cellular Biosciences are funding Dr. Arne Gericke from Worcester Polytechnic Institute to investigate the domains in biological membrane mimics. More specifically, the Gericke research group investigates whether the domains situated in the two opposing bilayers line up ("register") with each other and if so, whether the two domains situated in register physically affect each other. Undergraduate, graduate and postdoctoral students participate in the research. The undergraduate students have the opportunity to participate in a 10-week summer research experience at the Technical University Dortmund, Germany. Dr. Gericke continues to develop and teach a summer camp for middle school students that introduces the students to the unique properties of water as the foundation for all life, to the properties of detergents and lipids, and to the unique and important properties of biological membranes. Phosphoinositide (PIPs) lipids affect an extraordinary variety of protein functions. The specificity of PIP-mediated signaling is rooted in the chemical properties of the lipid's headgroup, which mediates specific interactions with distinct protein motifs. Temporal control of signaling is provided by kinases and phosphatases that phosphorylate or dephosphorylate PIPs. The spatial control of PIP-mediated signaling stems from the non-uniform lateral distribution of PIPs in the cell membrane, which gives rise to PIP-enriched domains and gradients. Rafts, which are liquid-ordered domains enriched in sphingolipid and cholesterol, are found in the outer leaflet of the plasma membrane and affect signaling events on the inner leaflet. Many PIP-mediated signaling events have been associated with lipid rafts; however, given the chemical nature of PIPs, they are expected to partition into fluid membrane patches rather than rafts. Aim 1 addresses this long-standing conundrum by fabricating asymmetric lipid bilayers that exhibit a lipid-raft mixture on the outer leaflet and PIP-containing lipid mixtures on the inner leaflet. Dr. Gericke's group uses these asymmetric lipid bilayers to determine the conditions that result in domain registration. Aim 2 explores how three peripheral membrane proteins, MARCKS, PTEN, and Akt, interact with three types of PIP domains (fluid PIP/cholesterol, liquid-ordered PIP/cholesterol, and PIP/cholesterol in contact with a raft domain in the opposing leaflet). The MARCKS interactions with PIPs are non-specific, purely electrostatic, PTEN binds phosphatidylinositol-4,5-bisphosphate, and the Akt protein has a pocket to which phosphatidylinositol-4,5-bisphosphate binds. Lipid gradients are crucial for important cellular processes, including cytokinesis and directed cell movement. While cell-based studies have highlighted the importance of lipid gradients for cellular function, the physiochemical underpinnings of lipid gradients have not been addressed. Dr. Gericke's group has developed a microfluidic lipid gradient generator to systematically study how lipid gradients interact and/or protein gradients in the aqueous phase above the lipid bilayer create and maintain lipid gradients. In aim 3, Dr. Gericke investigates how opposing gradients of PI3K and PTEN create lipid gradients and the effect of pre-existing domains on gradient development.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.