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

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.

Human herpesvirus portal proteins: Structure, function, and antiviral prospects

“…Viral genome encapsidation is an essential part of dsDNA virus assembly and maturation. Studies with bacteriophages and herpesviruses have identified an oligomeric protein structure associated with 1 of the capsid vertices called a portal . DNA is delivered via the portal into a preformed, empty capsid.…”

“…Portal, though often described as a channel or pore for simplicity, is not a passive entry point for DNA. Rather, structurally conserved domains interact with the viral genome, the major capsid protein, viral scaffold protein, and terminase subunits during packaging into the empty capsid . The assembly of portal monomers likely triggers the co‐polymerization of capsid and scaffolding proteins to form empty procapsids .…”

“…Structural features of phage portal oligomers, monomers, and the conserved “core.” A, Portal protein structures T4 (3JA7), P22 (5JJ3), and SPP1 (2JES) were obtained from the Research Collaboratory for Structural Bioinformatics Protein Data Bank ( www.rcsb.org) and created with Protein Workshop . The portal oligomer is shown beside a complete portal monomer and a truncated monomer showing the structurally conserved core (alpha helix: red; beta strand: blue; for simplicity, the P22 structures are depicted without the extended C‐terminal helical barrel).…”

Nano-channel of viral DNA packaging motor as single pore to differentiate peptides with single amino acid difference

“…[30–36] In dsDNA animal or bacterial viruses such as Phi29, SPP1, T3, T4, T5, and T7, their genome enters and exits the bacteriophage capsid during replication and infection, respectively, through a portal protein channel called the connector. [37–43] Bacteriophage T7 belongs to the Podoviridae family, which infects Escherichia coli ( E.coli ). Data from cryo-electron microscopy reveals that the T7 connector is composed of twelve subunits encoded by gene 8 (gp8) with a molecular weight of 59 k Da.…”

High-resolution structure of podovirus tail adaptor suggests repositioning of an octad motif that mediates the sequential tail assembly

“…Subsequently, the ∼40-kb viral genome is translocated into the procapsid through the central channel of the portal by DNA-packaging motor, resulting in densely packed DNA (11). The high pressure built by DNA packing in the capsid lattice triggers conformational change in portal protein, which subsequently releases the motor, terminating the DNA packaging (12)(13)(14). A set of tail proteins attach to the portal vertex to prevent the leakage of DNA.…”