Calcium-Induced Sculpturing of the Plasma Membrane: Lipid Microclusters Cast by Anionic Lipids

The plasma membrane of the cell is a complex, tightly-controlled heterogeneous environment. While anionic lipids constitute a small part of all phospholipids in the plasma membrane, they play key functional roles in regulation of vital cellular processes such as blood clotting, signaling, vesicle fusion, and apoptosis. Lipid composition and distribution in the plasma membrane are highly dynamic, with anionic phosphatidylserine (PS) lipids typically found on the interior leaflet. In turn, the concentration of divalent Ca2+ ions, an important cellular messenger, is three orders of magnitude greater outside of the cell, spatially separating the ions from anionic lipids. PS lipids can become externalized to the outer leaflet of the plasma membrane in the case of cell injury, disease, or apoptotic changes allowing them to interact with Ca2+, resulting in changes in the plasma membrane structure and initiation of signaling cascades. Nonetheless, the molecular details of lipid-Ca2+ interactions are not completely understood. We performed molecular dynamics simulations of binary mixtures of PS and phosphatidylcholine (PC) lipids using highly mobile membrane mimetic model (HMMM) that enabled extended sampling. Simulations revealed that lipid microclusters are formed by higher order (three or more) lipid oligomers coordinated by Ca2+, with two distinct conformations of the PS headgroup serving as building blocks. In parallel, 13C-dephased 31P-observed solid-state nuclear magnetic resonance (SSNMR) rotational-echo double-resonance (REDOR) experiments were performed with spin-diluted Nanodisc samples to report on 31P-13C distances. Experimental data are consistent with both intra- and inter-molecular lipid headgroup characteristic distances of approximately 4.8 +/- 0.5 A that match well with all-atom MD results. To our knowledge, this is one of the first joint MD and high-resolution SSNMR studies to characterize how lipid microclusters are formed at the atomic level and how they reshape the plasma membrane.