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.