Cholesterol mediates membrane curvature during fusion events

Abstract

Biomembranes undergo extensive shape changes as they perform vital cellular functions. The mechanisms by which lipids and proteins control membrane curvature remain unclear. We use x-ray reflectivity, grazing incidence x-ray diffraction, and epifluorescence microscopy to study binding of HIV-1 glycoprotein gp41's membrane-bending domain to DPPC/cholesterol monolayers of various compositions at the air-liquid interface. The results offer a new insight into how membrane curvature could be regulated by cholesterol during fusion of the viral lipid envelope and the host cell membranes.

FIG. 1

(a) Increase in the mean molecular area in the pure DPPC film and DPPC/cholesterol monolayers with χ_CHOL 0.13, 0.25, and 0.46 after injection of FP₂₃. (b) EFM images of DPPC before FP₂₃ injection and (c) 15 minutes after. Liquid-disordered (bright) phase of DPPC covers ~50% of the area before and ~65%–70% after FP₂₃ injection. This suggests that FP₂₃ interacts mainly with the liquid-disordered phase of DPPC.

FIG. 2

The electron density distributions in the DPPC film before injection of FP₂₃ and in the DPPC and DPPC/cholesterol monolayers after FP₂₃ injection perpendicular to the aqueous interface. Insert is XR data (symbols) and corresponding fits (lines) normalized by Fresnel reflectivity plotted against scattering vector q_Z. For clarity the reflectivity curves are shifter vertically.

FIG. 3

Results of the XR data analysis from left to right: cartoon schematics of the out-of-plane molecular arrangement of DPPC, cholesterol, and FP₂₃ and of the slab association; thicknesses of the corresponding slabs, L_i; The contribution of lipids to the ED of a slab; Experimental ED in a slab. FP₂₃ is depicted as an α-helix in the DPPC film and as a β-turn in DPPC/cholesterol monolayers based on the previous reports [12,13].