There are four known eIF2α kinases that might be responsible for the overwhelming phosphorylation we observed in early ischemia:
GCN2: A yeast kinase that doesn't hang out in human neurons. We had to let him go.
HRI: The Heme-Regulated-Inhibitor is responsible for phsosphorylating eIF2α and inhibiting protein synthesis when there's not enough heme around to make hemoglobin. Not found in brain. No charges filed.
PKR: This guy sure looked dirty. PKR, aka Protein Kinase R, is found in neurons, and phosphorylates eiF2α in response to viral attack. By sensing viral infection and phosphorylating eIF2α, PKR can shut down the translation machinery before viruses can hijack it. But here's the deal: PKR has also been shown to be activated by Ca2+. So as you might imagine, PKR was our prime suspect for a while. We took him to trial, and the major witness against him was a "knockout" mouse that lacks the gene for PKR. If PKR was the guilty kinase, the mouse should not be able to mount the huge eIF2α phosphorylation we saw in ischemia. We subjected this animal to brain reperfusion--and it phosphorylated eIF2α. Not Guilty.
PERK: This one lawyered up right away. Assaying his activity was difficult, and required challenging immunoprecipitation techniques with hard-to get antibodies, and correct interpretation of subtle mobility shifts on Western blot analysis. In short, he impeded our investigation in a way that just made him look guilty. And guilty he was. Eventually Dr. Rita Kumar and others in our lab nailed him for it. The kinase was PERK.