Bacterial viruses fuse membranes

To infect an Escherichia coli bacterium, a λ virus must get its DNA through the cell’s outer membrane, through the mesh of cross-linked polymers that forms the cell wall, and then through the cell’s inner membrane. How the viruses’ progeny escape through those same three layers is the focus of a new paper by Ry Young of Texas A&M University and his collaborators. Previous research had elucidated two of the steps—opening a hole in the inner membrane and dissolving the polymer mesh. It had also implicated a pair of viral proteins, Rz and Rz1, in breaching the outer membrane. The longer molecule, Rz, binds to the inner membrane and has the potential to fold over onto itself; Rz1 binds to the outer membrane. Before the polymer mesh is dissolved, Rz and Rz1 bond to form spanin, a molecular complex that threads through the mesh to span the inner and outer membranes. From a set of biophysical and biochemical experiments, the researchers deduced spanin’s role in viral escape. When the polymer mesh is dissolved, the spanins are freed to clump together. Clumping releases energy that enables the spanins to bend almost double about their Rz hinges, so that the inner and outer membranes are pulled together and fused. That change in topology creates holes through which the cell’s contents, including the viral progeny, explosively debouch. (M. Rajaure et al., Proc. Natl. Acad. Sci. USA 112, 5497, 2015, doi:10.1073/pnas.1420588112 .)