Phosphorylation of eukaryotic translation initiation factor 2α (eIF2α) is a conserved eukaryotic mechanism to limit protein synthesis under stress conditions. Baculovirus PK2, which resembles the C-terminal half of a protein kinase domain, inhibits eIF2α family kinases in vivo, thereby increasing viral fitness in the face of host immunological and stress responses. The mechanism by which PK2 modulates eIF2α stress response signaling remains unknown. To address this issue, a combination of biochemical, biophysical and in vivo approaches were employed to probe the mechanism of PK2 inhibition on a prototypical human eIF2α kinase, the RNA-dependent protein kinase (PKR). We discovered that PK2 inhibits PKR catalytic activity by directly binding its kinase domain. This direct interaction requires both the kinase-like C-lobe fold of PK2 and a critical 22 residue N-terminal extension that precedes it. We further show that the PK2 N-terminal extension is required but not sufficient for the ability of PK2 function.

Phosphorylation of eukaryotic translation initiation factor 2α (eIF2α) is a conserved eukaryotic mechanism to limit protein synthesis under stress conditions. Baculovirus PK2, which resembles the C-terminal half of a protein kinase domain, inhibits eIF2α family kinases in vivo, thereby increasing viral fitness in the face of host immunological and stress responses. The mechanism by which PK2 modulates eIF2α stress response signaling remains unknown. To address this issue, a combination of biochemical, biophysical and in vivo approaches were employed to probe the mechanism of PK2 inhibition on a prototypical human eIF2α kinase, the RNA-dependent protein kinase (PKR). We discovered that PK2 inhibits PKR catalytic activity by directly binding its kinase domain. This direct interaction requires both the kinase-like C-lobe fold of PK2 and a critical 22 residue N-terminal extension that precedes it. We further show that the PK2 N-terminal extension is required but not sufficient for the ability of PK2 function.