The C-terminal part of the human cytomegalovirus terminase subunit pUL51 is central for terminase complex assembly

The cleavage and packaging of the human cytomegalovirus (HCMV) genome is accomplished by the viral terminase, comprising pUL56 and pUL89, and the recently identified pUL51 subunit. Since knowledge about pUL51 is scarce, we aimed at identifying pUL51 domains that are important for terminase assembly. In silico analysis suggested that the N-terminal half of pUL51 is intrinsically disordered, and that α-helices are present in the C-terminal part. Linker-scanning mutagenesis of pUL51 in the context of the viral genome revealed that amino acid insertions into the predicted α-helices are not compatible with viral growth, whereas upon mutagenesis of the putatively disordered parts interaction with pUL56 and pUL89 was retained and viral progeny was produced. Replacement of pUL51 with the closely related M51 protein of mouse cytomegalovirus did not lead to viable virus, indicating that M51 cannot substitute for pUL51, and swapping the M51 and UL51 N- and C-termini demonstrated the critical role of the pUL51 C-terminal part in building the terminase complex. Notably, the pUL51 C-terminus alone turned out to be sufficient to enable terminase assembly, its nuclear localization and plaque formation. Using HCMV mutants expressing differently tagged pUL51 versions, we did not detect oligomerization of pUL51, as has been proposed for the pUL51 orthologues of other herpesviruses. These data provide an insight into the interaction of pUL51 with the other two terminase components, and provide the basis for unravelling the mode of action of novel antiviral drugs targeting the HCMV terminase.