Antibody reveals conformational latch regulating dimerization in β- and γ-herpesvirus proteases

Human herpesvirus (HHV) replication depends on the HHV protease (HHV Pr), an enzyme essential for capsid maturation. Because HHV Pr must transition from an inactive monomer to an active dimer, disrupting dimerization is a promising antiviral strategy. We isolate Fab5, a conformationally selective antibody from a naïve Fab-phage library that recognizes monomeric human cytomegalovirus protease (HCMV) Pr. A 2.6 Å cryo-EM structure reveals Fab5 binds a "latch loop" distal to the active site and dimer interface that secures the C-terminal tail in dimers. Structure-guided mutagenesis in both HCMV Pr and Kaposi's Sarcoma-associated herpesvirus (KSHV) Pr confirms the functional importance of a 3-residue motif present in β- and γ-HHV Pr latch loops, validating the mechanistic role of the latch loop in dimerization and activity. Because the latch loop is structurally conserved in all HHV Prs, the cryptic sites they form present an avenue for allosteric inhibitor development.