Cholesterol-Enriched Domain Formation Induced by Viral-Encoded, Membrane-Active Amphipathic Peptide

Abstract

The α-helical (AH) domain of the hepatitis C virus nonstructural protein NS5A, anchored at the cytoplasmic leaflet of the endoplasmic reticulum, plays a role in viral replication. However, the peptides derived from this domain also exhibit remarkably broad-spectrum virocidal activity, raising questions about their modes of membrane association. Here, using giant lipid vesicles, we show that the AH peptide discriminates between membrane compositions. In cholesterol-containing membranes, peptide binding induces microdomain formation. By contrast, cholesterol-depleted membranes undergo global softening at elevated peptide concentrations. Furthermore, in mixed populations, the presence of ∼100 nm vesicles of viral dimensions suppresses these peptide-induced perturbations in giant unilamellar vesicles, suggesting size-dependent membrane association. These synergistic composition- and size-dependent interactions explain, in part, how the AH domain might on the one hand segregate molecules needed for viral assembly and on the other hand furnish peptides that exhibit broad-spectrum virocidal activity.

Figure 1

AH-peptide-induced lateral reorganization in raft-forming GUVs. (a and b) Fluorescence images of raft-forming GUVs consisting of equimolar concentrations of POPC, SM, and Ch doped with Rho-B DOPE upon incubation with 5 μM AH peptide. (a) (i) No phase-segregated domains at the optical length scale are visible before peptide addition. Domains are evident at (ii) 126 s, (iii) 196 s, (iv) 293 s, (v) 439 s, and (vi) 542 s after addition. Scale bar, 30 μm. (b) Selected images reveal domain growth at (i) 90 s, (ii) 129 s, (iii) 183 s, (iv) 299 s, (v) 473 s, (vi) 537 s, (vii) 714 s, (viii) 741 s, and (ix) 788 s after peptide introduction. Scale bar, 5 μm. (c) Fluorescence intensity profiles of a GUV perimeter, illustrating domain growth in (a) (left vesicle). (d) Growth of the average domain area as a function of time, estimated from images in (b). (e) Fluorescence images of raft GUVs were visualized using L_d-phase (Rho-B DOPE, red) and L_o-phase (DP-EG10-biotin) markers. The L_o-phase marker was fluorescently visualized by binding Oregon Green 488 NeutrAvidin (green) introduced in the ambient bath (1 μM). (i–iii) Confocal slices of the equatorial plane before AH peptide addition. Scale bar, 10 μm. (iv–vi) 3D reconstructions generated from a Z-stack in samples incubated with 2 μM AH peptide. Scale bar, 15 μm. (f) Fluorescence images reveal domain formation after incubation with a mixture of 1.5 μM 5-TAMRA-AH (red) and 1.5 μM AH (unlabeled) peptides in raft GUVs doped with NBD-PE (green). (i–iii) Confocal fluorescence images for green, red, and merge. Scale bar, 20 μm. (iv–vi) 3D reconstructions generated from a Z-stack for green, red, and merge channels. Scale bar, 10 μm. (g) Fluorescence intensity along the vesicle perimeter, measured here in terms of the angle relative to an arbitrary point designated as 0° for the red (5-TAMRA-AH) and green (NBD-PE) channels in (f) (i and ii), shows colocalization of 5-TAMRA-AH in NBD-PE-rich L_d-phase domains. See text for details. To see this figure in color, go online.

Figure 2

AH-peptide-induced softening of POPC GUVs. (a and b) Fluorescence images of GUVs doped with Rho-B DOPE incubated with 14 μM AH peptide. Scale bar, 20 μM. (a) Frame (i) was captured 13 min after peptide introduction. Subsequent frames in the panel (ii–ix) are consecutive, 20 s apart. (b) Frame (i) was captured 5.5 min after peptide introduction, and subsequent frames were acquired 30 s apart. (c) Shape factor plot (S = 4πA/P²) of a vesicle cross section, showing dynamic shape changes of the GUV surface in (b) (middle vesicle). (d) Plot demonstrating dynamic softening of POPC GUVs during incubation with AH peptide, characterized by changes in the membrane bending rigidity (see text for details). (e) Fluorescence images of GUVs doped with NBD-PE incubated with 5-TAMRA-labeled AH peptide. Initially unbound 5-TAMRA AH (i) adsorbs onto the GUV membranes (ii) and eventually decreases the membrane rigidity (iii). Frames were captured at (i) 30 s, (ii) 160 s, and (iii) 52 min after introduction of 5-TAMRA-labeled AH peptide. Scale bars, 10 μm. To see this figure in color, go online.

Figure 3

Leakage assays. Permeabilization of GUV membranes was measured by monitoring the relative fluorescence intensity of 2-NBD-labeled glucose (green) inside and outside the vesicle after incubation with the AH peptide. (a) Fluorescence image sequence of GUVs consisting of equimolar concentrations of POPC, Ch, and SM doped with Rho-B, upon incubation with 4 μM AH peptide. Images were taken at 1) 607 s, 2) 637 s, 3) 645 s, 4) 652 s, 5) 667 s, and 6) 682 s after peptide addition, corresponding to quantitative leakage values in (b). Scale bar, 10 μm. (b) Normalized fluorescence intensity profile for leakage of 2-NBD-labeled glucose into the raft GUV in (a). (c) Fluorescence image sequence of a POPC GUV doped with Rho-B after incubation with 10 μM AH peptide (at 20 s). Images were taken at 1) 70 s, 2) 100 s, 3) 107 s, 4) 146 s, 5) 166 s, and 6) 205 s, corresponding to quantitative leakage values in (d). Scale bar, 10 μm. (d) Normalized fluorescence intensity profile for leakage of 2-NBDG into a POPC GUV. To see this figure in color, go online.

Figure 4

POPC and raft GUV-LUV competition assays. GUVs and LUVs consisting of either raft-forming (equimolar POPC, egg-SM, and Ch) or POPC lipids incubated with the AH peptide are shown. The four permutations of vesicle size (GUV or LUV) and composition (raft or POPC) were tested. (a) Cartoon depicting GUV and LUV competition assays with AH peptide. (b–e) Selected fluorescence images of GUVs doped with NBD-PE, and LUVs doped with Rho-B DOPE, before and after incubation with AH peptide. (b) GUV/LUV: POPC/POPC, incubated at 500 nM and 15 μM AH peptide. (c) GUV/LUV: raft/POPC. Postincubation images were taken at 119 s and 209 s after addition of 1 μM AH peptide. (d) (i) GUV/LUV: raft/raft. Postincubation images were taken at 63 s and 10 min with 1 μM AH peptide. (ii) Maximum Z-projection constructed from a Z-stack after 5 min of incubation with 1 μM AH. (e) GUV/LUV: POPC/raft. Postincubation images were taken at 69 s and 201 s after addition 2 μM AH peptide. Scale bar, 10 μm. To see this figure in color, go online.

Figure 5

Characterization of membrane fluidity in competition assays. Measurements of the lateral diffusion constants of fluorescently labeled probe lipids preinserted into GUVs or delivered by interacting LUVs, using time-lapse images from FRAP experiments. (a) Selected fluorescence image sequences of NBD-labeled POPC GUVs (green) and material decorating the GUV surface delivered by Rho-B-labeled POPC LUVs (red) from the surrounding bulk. Fluorescence was monitored before and directly after bleaching, until complete recovery was achieved. (i–iii) GUV fluorescent probe (green) recovery in (i) the absence of LUVs or AH peptide (prebleach, 0 s, 1 s, 2 s, 100 s after bleaching; Movie S4), (ii) the presence of LUVs without incubation with AH peptide (prebleach, 0 s, 1 s, 2 s, 60 s; Movie S15), and (iii) the presence of LUVs and after incubation with 2 μM AH peptide (prebleach, 0 s, 2 s, 5 s, 50 s; Movie S16). (iv) LUV-derived material with Rho-B (red) decorating the GUV, showing recovery after addition of and incubation with 2 μM AH peptide (prebleach, 0 s, 2 s, 5 s, 60 s; Movie S17). Scale bars, 10 μm. (b–e) Normalized fluorescence recovery curves for FRAP experiments: (b) (e-i), D = 9.56 μm²/s; (c) (e-ii), D = 9.38 μm²/s; (d) (e-iii), D = 2.15 μm²/s; and (e) (e-iv), D = 1.83 μm²/s. To see this figure in color, go online.