Abstract
We characterized the size, distribution, and fluidity of microdomains in a lipid bilayer containing phosphatidylinositol (PI) and revealed their roles during the two-dimensional assembly of a membrane deformation protein (FBP17). The morphology of the supported lipid bilayer (SLB) consisting of PI and phosphatidylcholine (PC) on a mica substrate was observed with atomic force microscope (AFM). Single particle tracking (SPT) was performed for the PI+PC-SLB on the mica substrate by using the diagonal illumination setup. The AFM topography showed that PI-derived submicron domains existed in the PI+PC-SLB. The spatiotemporal dependence of the lateral lipid diffusion obtained by SPT showed that the microdomain had lower fluidity than the surrounding region and worked as the obstacles for the lipid diffusion. We observed the two-dimensional assembly of FBP17, which is one of F-BAR family proteins included in endocytosis processes and has the function generating lipid bilayer tubules in vitro. At the initial stage of the FBP17 assembly, the PI-derived microdomain worked as a scaffold for the FBP17 adsorption, and the fluid surrounding region supplied FBP17 to grow the FBP17 domain via the lateral molecular diffusion. This study demonstrated an example clearly revealing the roles of two lipid microregions during the protein reaction on a lipid bilayer.
Highlights
The lateral diffusion and assembly of lipids and proteins in and on lipid bilayer membranes are fundamental key factors of various reactions in plasma membranes, such as transportation of materials and signals into and out of cells
The results indicate that each of the F-BAR proteins behaves differently at the initial processes of assembly, e.g., adsorption, diffusion, and nucleation, on the lipid bilayer containing PI-enriched domains
The supported lipid bilayer was formed through the immersion of a freshly cleaved mica substrate into the vesicle suspension and the incubation at 45 ◦ C for 60 min following the previous protocols [30,31,32]
Summary
The lateral diffusion and assembly of lipids and proteins in and on lipid bilayer membranes are fundamental key factors of various reactions in plasma membranes, such as transportation of materials and signals into and out of cells. The relation between the properties and functions of the assembled structures of lipid bilayer membranes during the reactions with proteins are still elusive. Colocalization of specific lipids and proteins, e.g., sphingomyelin, cholesterol, and transporter proteins has been proved in previous studies, but the relation between the properties of the lipid domains, and the mechanism and roles of the colocalization are not always clear. In the previous studies of lipid localization, phase separation and domain formation in lipid bilayer membranes, intermolecular interaction at the hydrophobic core have been investigated in detail, for example, the phase transition of acyl chains between liquid crystalline and gel phases, and higher affinity of cholesterol to saturated acyl chains than to unsaturated ones [3]. Recent theoretical study shows the clustering of PI derivatives in PC membranes [6] They proposed fluid-fluid demixing induced by the hydrogen bond between the inositol rings of PI. It was proposed that the generation and the local accumulation of these specific lipids and proteins are precisely controlled at the moment of during the signaling reaction such as allergic reactions [11,12]
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