Evolution in Action: N and C Termini of Subunits in Related T = 4 Viruses Exchange Roles as Molecular Switches

SummaryThe T = 4 tetravirus and T = 3 nodavirus capsid proteins undergo closely similar autoproteolysis to produce the N-terminal β and C-terminal, lipophilic γ polypeptides. The γ peptides and the N termini of β also act as molecular switches that determine their quasi equivalent capsid structures. The crystal structure of Providence virus (PrV), only the second of a tetravirus (the first was NωV), reveals conserved folds and cleavage sites, but the protein termini have completely different structures and the opposite functions of those in NωV. N termini of β form the molecular switch in PrV, whereas γ peptides play this role in NωV. PrV γ peptides instead interact with packaged RNA at the particle two-folds by using a repeating sequence pattern found in only four other RNA- or membrane-binding proteins. The disposition of peptide termini in PrV is closely related to those in nodaviruses, suggesting that PrV may be closer to the primordial T = 4 particle than NωV