CryoEM structure of the outer membrane secretin channel pIV from the f1 filamentous bacteriophage

Abstract

The Ff family of filamentous bacteriophages infect gram-negative bacteria, but do not cause lysis of their host cell. Instead, new virions are extruded via the phage-encoded pIV protein, which has homology with bacterial secretins. Here, we have determined the structure of pIV from the f1 filamentous bacteriophage at 2.7 A resolution by cryo-electron microscopy, the first near-atomic structure of a phage secretin. Fifteen f1 pIV monomers assemble to form a gated channel in the bacterial outer membrane, with associated soluble domains projecting into the periplasm. We model channel opening and propose a mechanism for phage-mediated gate movement. By single-cell microfluidics experiments, we demonstrate the potential for secretins such as pIV to be used as adjuvants to increase the uptake and efficacy of antibiotics in bacteria. Finally, we compare the f1 pIV structure to its homologues to reveal similarities and differences between phage and bacterial secretins.

Structural lessons on bacterial secretins

“…While their structures are similar, the internal electrostatic characteristics of bacterial secretins vary significantly from one secretin type to another but also within the same type (figure 2) [27,28,30]. The overall amount of charged amino acids is around 20 % depending on the secretin's type, but their distribution on the quaternary structure is not homogeneous along the channel of each secretin.…”

“…This structural feature separates the periplasmic chamber from the top cavity and is therefore named central gate (CG) (Figure 1). The gating function of CG is supported by the fact that most of the previously identified mutations in the FPE F1 phage secretin pIV, leading to gain of function growth on maltopentaose, bile salt permeability or antibiotic sensibility [34], mapped to the CG [27]. CGs, most often captured under closed conformation in isolated particles, must adopt an alternative open conformation to allow the passage of the different structures secreted or assembled by secretins.…”

Cryo-electron microscopy of the f1 filamentous phage reveals a new paradigm in viral infection and assembly

“…(13) The L4 linker closes around the body of the phage at the H1 βhairpin loop (B) and the C domain of pIII folds down over the hydrophobic helices of pVI to secede the phage from the cytoplasmic membrane. ( 14) Phage passes through the pIV secretin complex 19 in the outer membrane and is released.…”

“…1A). We recently determined the structure of pIV by cryo-electron microscopy (cryoEM), revealing how the gates in the channel would need to open to allow phage to egress 19 .…”

Genomic evidence for inter-class host transition between abundant streamlined heterotrophs by a novel and ubiquitous marine Methylophage

“…PilQ within the type IV secretion system in Vibrio cholerae) or extrusion of filamentous phages during chronic infection (e.g. pIV in bacteriophages Ff) (67). Like CsgGF in an inverted conformation, PilQ comprises a β-barrel, with a secondary pore below, attached by two hinged α-helices.…”