Granulovirus Pk-1 Kinase Activity Relies On A Side-To-Side Dimerization Mode Centered On The Regulatory Alpha C Helix

The life cycle of Baculoviridae family insect viruses depends on the viral protein kinase, PK-1, to phosphorylate the regulatory protein, p6.9, to induce baculoviral genome release. Here, we report the crystal structure of Cydia pomenella granulovirus PK-1, which, owing to its likely ancestral origin among host cell AGC kinases, exhibits a eukaryotic protein kinase fold. PK-1 occurs as a rigid dimer, where an antiparallel arrangement of the alpha C helices at the dimer core stabilizes PK-1 in a closed, active conformation. Dimerization is facilitated by C-lobe:C-lobe and N-lobe:N-lobe interactions between protomers, including the domain-swapping of an N-terminal helix that crowns a contiguous beta -sheet formed by the two N-lobes. PK-1 retains a dimeric conformation in solution, which is crucial for catalytic activity. Our studies raise the prospect that parallel, side-to-side dimeric arrangements that lock kinase domains in a catalytically-active conformation could function more broadly as a regulatory mechanism among eukaryotic protein kinases. The viral Protein Kinase-1 (PK-1) phosphorylates the regulatory protein p6.9, which facilitates baculoviral genome release. Here, the authors combine X-ray crystallography with biophysical and biochemical analyses as well as molecular dynamics simulations to characterize Cydia pomenella granulovirus PK-1, which forms a dimer with a parallel side-to-side arrangement of the kinase domains and furthermore, they provide insights into its catalytic mechanism and evolutionary relationships with other kinases.