The holin of bacteriophage lambda forms rings with large diameter

Summary Holins control the length of the infection cycle of tailed phages (the Caudovirales) by oligomerizing to form lethal holes in the cytoplasmic membrane at a time dictated by their primary structure. Nothing is currently known about the physical basis of their oligomerization or the structure of the oligomers formed by any known holin. Here we use electron microscopy and single-particle analysis to character- ize structures formed by the bacteriophage l holin (S105) in vitro. In non-ionic or mild zwitterionic detergents, purified S105, but not the lysis-defective variant S105A52V, forms rings of at least two size classes, the most common having inner and outer diameters of 8.5 and 23 nm respectively, and contain- ing approximately 72 S105 monomers. The height of these rings, 4 nm, closely matches the thickness of the lipid bilayer. The central channel is of unprec- edented size for channels formed by integral mem- brane proteins, consistent with the non-specific nature of holin-mediated membrane permeabiliza- tion. S105 present in detergent-solubilized rings and in inverted membrane vesicles showed similar sen- sitivities to proteolysis and cysteine-specific modifi- cation, suggesting that the rings are representative of the lethal holes formed by S105 to terminate the infection cycle and initiate lysis.