Plate Tectonics of Virus Shell Assembly and Reorganization in Phage Φ8, a Distant Relative of Mammalian Reoviruses

Summary

The hallmark of a virus is its capsid, which harbors the viral genome and is formed from protein subunits, which assemble following precise geometric rules. dsRNA viruses use an unusual protein multiplicity (120 copies) to form their closed capsids. We have determined the atomic structure of the capsid protein (P1) from the dsRNA cystovirus Φ8. In the crystal P1 forms pentamers, very similar in shape to facets of empty procapsids, suggesting an unexpected assembly pathway that proceeds via a pentameric intermediate. Unlike the elongated proteins used by dsRNA mammalian reoviruses, P1 has a compact trapezoid-like shape and a distinct arrangement in the shell, with two near-identical conformers in nonequivalent structural environments. Nevertheless, structural similarity with the analogous protein from the mammalian viruses suggests a common ancestor. The unusual shape of the molecule may facilitate dramatic capsid expansion during phage maturation, allowing P1 to switch interaction interfaces to provide capsid plasticity.

Graphical Abstract

Highlights

• •

  Crystal structure of the major capsid protein P1 of the Pseudomonas phage Φ8

• •

  Φ8 P1 shares a common ancestor with mammalian reoviruses

• •

  Φ8 P1’s trapezoidal shape may facilitate capsid expansion during maturation

• •

  The pentameric organization of Φ8 P1 suggests a revised assembly pathway

Abstract

El Omari et al. report a structure of the dsRNA bacteriophage ϕ8 capsid protein P1. P1 crystallizes as a pentamer, suggesting a new pathway for procapsid assembly. P1 displays a novel fold and a trapezoidal shape, distinct from that of other dsRNA virus, which may facilitate capsid expansion during maturation.