Viral Ejection Proteins: Mosaically Conserved, Conformational Gymnasts

Abstract

Bacterial viruses (or bacteriophages) have developed formidable ways to deliver their genetic information inside bacteria, overcoming the complexity of the bacterial-cell envelope. In short-tailed phages of the Podoviridae superfamily, genome ejection is mediated by a set of mysterious internal virion proteins, also called ejection or pilot proteins, which are required for infectivity. The ejection proteins are challenging to study due to their plastic structures and transient assembly and have remained less characterized than classical components such as the phage coat protein or terminase subunit. However, a spate of recent cryo-EM structures has elucidated key features underscoring these proteins’ assembly and conformational gymnastics that accompany their expulsion from the virion head through the portal protein channel into the host. In this review, we will use a phage-T7-centric approach to critically review the status of the literature on ejection proteins, decipher the conformational changes of T7 ejection proteins in the pre- and post-ejection conformation, and predict the conservation of these proteins in other Podoviridae. The challenge is to relate the structure of the ejection proteins to the mechanisms of genome ejection, which are exceedingly complex and use the host’s machinery.

High-resolution cryo-EM structure of the Shigella virus Sf6 genome delivery tail machine

“…A hinge between the Sf6 tailspike N terminus and core makes this spike very flexible, unlike P22 ( 9 ). Because there is a density that we ascribe to protein in the bottom of the tail channel, we propose that once the tail channel is fully assembled and sealed, some of the ejection proteins encapsidated in the procapsid may move into the tail channel, ready to be ejected into the host cell envelope upon attachment ( 32 ). The end of the Sf6 genome is visible in the upper part of the portal, possibly bound or threaded through other ejection proteins clustered in the proximity of the portal barrel ( 36 ).…”

“…Genome ejection, the mirror process of genome packaging, is triggered by the interaction of an Sf6 virion with primary (LPS) and secondary (outer membrane proteins OmpA and OmpC) receptors on the Shigella surface ( 13 ). This initial recognition event is followed by the ejection of internal virion proteins (or ejection proteins) that extend the tail ( 32 , 33 ). Only one Sf6 ejection protein, gp12, similar to T7 gp15 ( 33 , 34 ), has been characterized ( 35 ).…”

“…Only one Sf6 ejection protein, gp12, similar to T7 gp15 ( 33 , 34 ), has been characterized ( 35 ). On the basis of genomic location and sequence conservation, Sf6 ejection proteins are similar to P22 ( 32 ) and may exist in two conformations: loosely bound to the portal barrel inside the virion ( 36 ) and assembled into a transenvelope channel in the host cell envelope after ejection ( 37 ). In vitro, genome ejection can be mediated by LPS and OmpA and OmpC, albeit this ejection is slower than observed during natural infection ( 13 ).…”

Journey to the center of the phage; revealing the ejectosome of Pectobacterium bacteriophage ΦM1

“…Podophages belonging to the family Autographiviridae, otherwise known as the T7-like supergroup, possess non-contractile tails shorter than the width of the bacterial cell envelope (7)(8)(9). Some podophages translocate their genome into the host cytoplasm via a set of internal proteins known 3 to form a protective channel that spans the entire width of the Gram-negative cell wall (10)(11)(12).…”

“…The T7 tetrameric ejection protein, gp16, encodes an LTase domain known to locally degrade peptidoglycan during this process, allowing complete penetration of the cell wall (8, 9). After genome delivery into the cytoplasm, the ejectosome rapidly disassembles and the cell wall is mended, preventing premature cell lysis (8, 14, 15).…”