Figure 7.

Constrained necks can join virions and liposomes at pH 5.5. (A) Serial 5.3 nm-thick sections through a liposome–virion complex; liposome marked with ‘L' and virus with ‘F' in slice 1. A neck with 10–15 nm-wide mouth is seen in slices 4–6. Liposomal membrane and viral envelope density appear continuous, although the strong liposomal density exhibits a pronounced boundary at the base of the neck. Sample acidified for 8 min. Microscope defocus 5 μm. Scalebar 50 nm. (B) In the 10.6 nm-thick section through another complex, an ∼15 nm-wide neck connects a virion and liposome. Sample acidified for 5 min. Microscope defocus 3 μm. Scalebar 50 nm. (C) A density plot along the channel axis (path indicated by black arrowheads in (B)) indicates that three ridges of density traverse the central section of the neck. The density may be due to an intact matrix layer or liposome and virus membrane leaflets. (D) Low-dose cryo-EM image of influenza X31 virions at pH 5.0 in the absence of liposomes suggests that the viral envelopes are more labile at pH 5.0 and give rise to envelope-derived lipidic vesicles (marked ‘V'); no comparable vesicles were observed in identical samples that were kept at pH 7.5. Two virions (marked ‘F') seen fusing with a vesicle in the lower left quadrant appear to be undergoing expansive pore dilation; an alternative explanation for this feature is that the virions may be caught in the process of shedding or blebbing their membranes. Sample acidified at pH 5.0 for 10 min. Microscope defocus 3 μm. Scalebar 100 nm.