# Cryo-EM structures of bacteriophage T4 portal-neck connector complexes reveal a viral genome retention mechanism involving a host component

## TL;DR

The study reveals how bacteriophage T4 prevents DNA leakage during assembly using a two-part gate mechanism involving a host protein.

## Contribution

Discovery of a novel genome retention mechanism involving a host-derived Hfq protein in phage T4 assembly.

## Key findings

- A double 'genome-gate' in the portal-neck complex seals the pressurized viral head.
- The host protein Hfq stabilizes the neck and aids in its attachment to the viral head.
- The gp13/gp14/Hfq complex waits for genome packaging completion before sealing the head.

## Abstract

Bacteriophage T4 is a model for tailed viruses, the most abundant biological entities on Earth. During virion assembly, the DNA genome is tightly packed inside the head, which then attaches to tail via a portal-neck connector. Keeping this pressurized head leak-proof during these transactions is a challenge because even a few basepairs leakage could disrupt assembly, but the mechanisms are not understood. Here, we solved the structures of in vitro-assembled portal-neck complexes and found a double “genome-gate” in the neck that plugs the packaged head. The first gate is formed by gp14 hexamer, while the second, unexpectedly, consists of a host component, Hfq hexamer. Furthermore, Hfq stabilizes the neck and facilitate its correct docking onto the head. Thus, a pre-assembled, gp13/gp14/Hfq neck complex awaits genome packaging completion, triggers portal conformational changes and packaging motor ejection, which exposes binding sites for the neck to securely seal the pressurized head.