Portal protein functions akin to a DNA-sensor that couples genome-packaging to icosahedral capsid maturation

Ravi K Lokareddy,Gino Cingolani,Kristin N Parent,Pavel V Afonine,Rajeshwer S Sankhala,Ankoor Roy,Tina Motwani,Carolyn M Teschke

doi:10.1038/ncomms14310

Abstract

Tailed bacteriophages and herpesviruses assemble infectious particles via an empty precursor capsid (or ‘procapsid’) built by multiple copies of coat and scaffolding protein and by one dodecameric portal protein. Genome packaging triggers rearrangement of the coat protein and release of scaffolding protein, resulting in dramatic procapsid lattice expansion. Here, we provide structural evidence that the portal protein of the bacteriophage P22 exists in two distinct dodecameric conformations: an asymmetric assembly in the procapsid (PC-portal) that is competent for high affinity binding to the large terminase packaging protein, and a symmetric ring in the mature virion (MV-portal) that has negligible affinity for the packaging motor. Modelling studies indicate the structure of PC-portal is incompatible with DNA coaxially spooled around the portal vertex, suggesting that newly packaged DNA triggers the switch from PC- to MV-conformation. Thus, we propose the signal for termination of ‘Headful Packaging’ is a DNA-dependent symmetrization of portal protein.

Highlights

Tailed bacteriophages and herpesviruses assemble infectious particles via an empty precursor capsid built by multiple copies of coat and scaffolding protein and by one dodecameric portal protein
Nature’s most abundant viruses, and herpesviruses assemble empty precursor capsids known as procapsids that are subsequently filled with viral DNA by a powerful, virus-encoded genome-packaging motor[1,2]
Docking inside the density map obtained from a cryo-electron microscopic asymmetric reconstruction of P22 mature virion[8] revealed these X-ray models faithfully describe the conformation of portal protein in mature virion, when the capsid is filled with DNA and the portal is bound to gp[4]

Summary

Introduction

Tailed bacteriophages and herpesviruses assemble infectious particles via an empty precursor capsid (or ‘procapsid’) built by multiple copies of coat and scaffolding protein and by one dodecameric portal protein. Nature’s most abundant viruses, and herpesviruses assemble empty precursor capsids known as procapsids that are subsequently filled with viral DNA by a powerful, virus-encoded genome-packaging motor[1,2]. In the procapsid of bacteriophage P22, the portal protein makes intimate contacts with coat and scaffolding proteins[5,6], while in the mature virion, the portal assembly interacts intimately with viral genome and tail components[7,8]. The atomic structure of this pre-packaging intermediate together with the structure of P22 portal in mature virion[13] shed light on a dramatic and unexpected structural switch in the portal vertex that accompanies viral genome-packaging

Methods

Results

Conclusion