Tailed, double-stranded DNA bacteriophages provide a well-characterized model system for the study of viral assembly, especially for herpesviruses and adenoviruses. A wealth of genetic, structural, and biochemical work has allowed for the development of assembly models and an understanding of the DNA packaging process. The portal complex is an essential player in all aspects of bacteriophage and herpesvirus assembly. Despite having low sequence similarity, portal structures across bacteriophages share the portal fold and maintain a conserved function. Due to their dynamic role, portal proteins are surprisingly plastic, and their conformations change for each stage of assembly. Because the maturation process is dependent on the portal protein, researchers have been working to validate this protein as a potential antiviral drug target. Here we review recent work on the role of portal complexes in capsid assembly, including DNA packaging, as well as portal ring assembly and incorporation and analysis of portal structures.

Decoration proteins are viral accessory gene products that adorn the surfaces of some phages and viral capsids, particularly tailed dsDNA phages. These proteins often play a “cementing” role, reinforcing capsids against accumulating internal pressure due to genome packaging, or environmental insults such as extremes of temperature or pH. Many decoration proteins serve alternative functions, including target cell recognition, participation in viral assembly, capsid size determination, or modulation of host gene expression. Examples that currently have structures characterized to high-resolution fall into five main folding motifs: β-tulip, β-tadpole, OB-fold, Ig-like, and a rare knotted α-helical fold. Most of these folding motifs have structure homologs in virus and target cell proteins, suggesting horizontal gene transfer was important in their evolution. Oligomerization states of decoration proteins range from monomers to trimers, with the latter most typical. Decoration proteins bind to a variety of loci on capsids that include icosahedral 2-, 3-, and 5-fold symmetry axes, as well as pseudo-symmetry sites. These binding sites often correspond to “weak points” on the capsid lattice. Because of their unique abilities to bind virus surfaces noncovalently, decoration proteins are increasingly exploited for technology, with uses including phage display, viral functionalization, vaccination, and improved nanoparticle design for imaging and drug delivery. These applications will undoubtedly benefit from further advances in our understanding of these versatile augmenters of viral functions.

The oligomerization and incorporation of the bacteriophage P22 portal protein complex into procapsids (PCs) depends upon an interaction with scaffolding protein, but the region of the portal protein that interacts with scaffolding protein has not been defined. In herpes simplex virus 1 (HSV-1), conserved tryptophan residues located in the wing domain are required for portal-scaffolding protein interactions. In this study, tryptophan residues (W) present at positions 41, 44, 207 and 211 within the wing domain of the bacteriophage P22 portal protein were mutated to both conserved and non-conserved amino acids. Substitutions at each of these positions were shown to impair portal function in vivo, resulting in a lethal phenotype by complementation. The alanine substitutions caused the most severe defects and were thus further characterized. An analysis of infected cell lysates for the W to A mutants revealed that all the portal protein variants except W211A, which has a temperature-sensitive incorporation defect, were successfully recruited into procapsids. By charge detection mass spectrometry, all W to A mutant portal proteins were shown to form stable dodecameric rings except the variant W41A, which dissociated readily to monomers. Together, these results suggest that for P22 conserved tryptophan, residues in the wing domain of the portal protein play key roles in portal protein oligomerization and incorporation into procapsids, ultimately affecting the functionality of the portal protein at specific stages of virus assembly.

Scaffolding proteins are essential for the assembly of most tailed, double-stranded DNA bacteriophages as well as herpesviruses. These proteins interact specifically with the coat proteins to efficiently assemble procapsids with the correct morphology. A helix-turn-helix (HTH) domain of bacteriophage P22 scaffolding protein is essential for coat binding, but the presence of additional coat protein binding sites has been predicted. An alanine substitution at scaffolding protein residue L245 causes a strong cold-sensitive phenotype. Bothin vivoandin vitroassembly with L245A scaffolding protein yields aberrant and petite particles at non-permissive temperatures. The L245A scaffolding protein is destabilized as determined by thermal melts monitored by circular dichroism. Through crosslinking studies, residue L245 interacts with the coat protein A-domain residue D198, which has been predicted previously to contain a scaffolding protein binding site. L245 also binds R101 in the coat protein P-domain as well as E18 in the N-arm. These results demonstrate the presence of secondary coat binding sites that may function in conjunction with the HTH domain to promote the assembly of procapsids with the correct curvature.ImportanceMany dsDNA viruses, including tailed bacteriophages and Herpesviruses, assemble precursor capsids, or procapsids, using an essential catalytic scaffolding protein. How scaffolding proteins induce proper assembly of their major capsid proteins remains unclear. The scaffolding protein of bacteriophage P22 has a C-terminal helix-turn-helix domain that interacts with the N-arm of the coat protein to activate it for assembly. Here, a second potential coat protein interaction site is identified in scaffolding protein at residue L245. Residue L245 may be involved in stabilizing a small helical domain based the effect of the substitution scaffolding protein stability and a predicted model of the scaffolding protein fold, thereby indicating additional complexity in the interaction between coat and scaffolding proteins.