Specificity of Baculovirus P6.9 Basic DNA-Binding Proteins and Critical Role of the C Terminus in Virion Formation

Abstract

The majority of double-stranded DNA (dsDNA) viruses infecting eukaryotic organisms use host- or virus-expressed histones or protamine-like proteins to condense their genomes. In contrast, members of the Baculoviridae family use a protamine-like protein named P6.9. The dephosphorylated form of P6.9 binds to DNA in a non-sequence-specific manner. By using a p6.9 -null mutant of Autographa californica multiple nucleopolyhedrovirus ( Ac MNPV), we demonstrate that P6.9 is not required for viral DNA replication but is essential for the production of infectious virus. Virion production was rescued by P6.9 homologs from a number of Alpha baculovirus species and one Gammabaculovirus species but not from the genus Betabaculovirus , comprising the granuloviruses, or by the P6.9 homolog VP15 from the unrelated white spot syndrome virus of shrimp. Mutational analyses demonstrated that Ac MNPV P6.9 with a conserved 11-residue deletion of the C terminus was not capable of rescuing p6.9 -null Ac MNPV, while a chimeric Betabaculovirus P6.9 containing the P6.9 C-terminal region of an Alphabaculovirus strain was able to do so. This implies that the C terminus of baculovirus P6.9 contains sequence elements essential for virion formation. Such elements may possibly interact with species- or genus-specific domains of other nucleocapsid proteins during virus assembly.