Membrane Deformation Induced by Cation Binding to Negatively Charged Phospholipids

Abstract

The divalent cation Ca2+ plays a critical role in many cell signaling and membrane trafficking pathways, including exocytosis and membrane fusion. Ca2+ signal transduction is commonly thought to occur through interaction with protein partners; however, it may also interact with negatively charged phospholipids within the plasma membrane leading to changes in lipid behavior.Here we show that an asymmetric concentration of Ca2+ ions across the membrane can generate tubules or protrusions from membranes containing phosphatidylserine (PS) or phosphatidylinositol-4,5-bisphosphate (PIP2). We transferred single vesicles with controlled membrane tension into Ca2+ solution and observed the formation of inward tubules, indicating negative curvature generation by Ca2+. The Ca2+-induced membrane deformation is inhibited by high tension or low cation binding, in a similar manner as we have previously observed for BAR domain proteins. We propose that the membrane deformation is caused by Ca2+-induced clustering of PS and PIP2 creating negative curvature. In confirmation of this we find that Mg2+, which is known to only weakly cluster PS and PIP2, only generates curvature at relatively high concentrations and low tension. Additionally, for PS vesicles the membrane deformation requires high Ca2+ concentration (a few millimolar), while PIP2 vesicles, which are more sensitive to Ca2+-induced clustering, are deformed at lower Ca2+ concentrations. The presence of salt ions (Na+) also influences Ca2+-induced membrane curvature. High salt content can reduce Ca2+-induced curvature by shielding Ca2+ ions from the membrane. An asymmetric salt concentration can influence membrane structure and either enhance or oppose Ca2+-induced membrane curvature. The observed Ca2+-induced membrane deformation represents a new mechanism for Ca2+ signaling events. This mechanism is likely to be especially important for the role of Ca2+ in exocytosis and membrane fusion events.