Acid-Stable Capsid Structure of Helicobacter Pylori Bacteriophage KHP30 by Single-Particle Cryo-Electron Microscopy

The acid-stable capsid structure of Helicobacter pylori phage KHP30 was solved at 2.7 A resolution by cryo-electron microscopy. The capsid has icosahedral T=9 symmetry and consists of each 540 copies of two structural proteins, a major capsid protein and a cement protein. The major capsid proteins form 12 pentagonal capsomeres occupying icosahedral vertexes and 80 hexagonal capsomeres located at icosahedral faces and edges. The major capsid protein has a novel protruding loop extending to the neighboring subunit that stabilizes hexagonal capsomeres. Furthermore, the capsid is decorated with trimeric cement proteins with a jelly roll motif. The cement protein trimer sits on the quasi-three-fold axis formed by three major capsid proteincapsomeres, thereby enhancing the particle stability by connecting these capsomeres. Sequence and structure comparisons between the related Helicobacter pylori phages suggest a possible mechanism of phage adaptation to the human gastric environment.