A tight honeycomb lattice (shown in 3D) covers the growing poxvirus membrane.

The spherical membrane surrounding the poxvirus genome begins to form as an open, cup-like structure. Some have argued that the edge of the cup must be composed of two closely apposed membranes (imagine a collapsed vesicle); otherwise, the hydrophobic edges of a single membrane would be exposed to the cytoplasm. But on page 269, John Heuser confirms that poxvirus has only a single membrane bilayer. He finds that the growing membrane is stabilized by a proteinaceous lattice.Heuser imaged freeze-fractured cells infected with poxvirus. Freeze-fracturing tends to split membranes randomly along bilayer planes, thus revealing all the bilayers that are present. As only one viral membrane plane was revealed, and no known membrane is resistant to this technique, Heuser concluded that the virus has only one bilayer.

The EM images also revealed a honeycomb-shaped lattice on the outer surface of the growing viral membrane. In older studies, the lattice appeared only as spikes on the membrane. Using deep-etch EM, Heuser was able to preserve and view the entire face of the lattice.

Like clathrin, the lattice curves the membrane it surrounds. It may also stabilize the hydrophobic edges of the incomplete membrane within the cytoplasm. Indeed, others have found that preventing the spikes from associating with the nacent viral membrane causes it to collapse. This is the mechanism of action of rifampicin—an antibiotic that aggregates the lattice protein and blocks poxvirus DNA encapsulation.