In a screen designed to identify novel inducers of autophagy, we discovered that STAT3 inhibitors potently stimulate the autophagic flux. Accordingly, genetic inhibition of STAT3 stimulated autophagy in vitro and in vivo, while overexpression of STAT3 variants, encompassing wild-type, nonphosphorylatable, and extranuclear STAT3, inhibited starvation-induced autophagy. The SH2 domain of STAT3 was found to interact with the catalytic domain of the eIF2α kinase 2 EIF2AK2, best known as protein kinase R (PKR). Pharmacological and genetic inhibition of STAT3 stimulated the activating phosphorylation of PKR and consequent eIF2α hyperphosphorylation. Moreover, PKR depletion inhibited autophagy as initiated by chemical STAT3 inhibitors or free fatty acids like palmitate. STAT3-targeting chemicals and palmitate caused the disruption of inhibitory STAT3-PKR interactions, followed by PKR-dependent eIF2α phosphorylation, which facilitates autophagy induction. These results unravel an unsuspected mechanism of autophagy control that involves STAT3 and PKR as interacting partners.

The proteolytic turnover of mitochondrial proteins is poorly understood. Here, we used a combination of dynamic isotope labeling and mass spectrometry to gain a global overview of mitochondrial protein turnover in yeast cells. Intriguingly, we found an exceptionally high turnover of the NADH dehydrogenase, Nde1. This homolog of the mammalian apoptosis inducing factor, AIF, forms two distinct topomers in mitochondria, one residing in the intermembrane space while the other spans the outer membrane and is exposed to the cytosol. The surface-exposed topomer triggers cell death in response to pro-apoptotic stimuli. The surface-exposed topomer is degraded by the cytosolic proteasome/Cdc48 system and the mitochondrial protease Yme1; however, it is strongly enriched in respiratory-deficient cells. Our data suggest that in addition to their role in electron transfer, mitochondrial NADH dehydrogenases such as Nde1 or AIF integrate signals from energy metabolism and cytosolic proteostasis to eliminate compromised cells from growing populations.

Endonuclease G (EndoG) is located in mitochondria yet translocates into the nucleus of apoptotic cells during human degenerative diseases. Nonetheless, a direct involvement of EndoG in cell-death execution remains equivocal, and the mechanism for mitochondrio-nuclear translocation is not known. Here, we show that the yeast homolog of EndoG (Nuc1p) can efficiently trigger apoptotic cell death when excluded from mitochondria. Nuc1p induces apoptosis in yeast independently of metacaspase or of apoptosis inducing factor. Instead, the permeability transition pore, karyopherin Kap123p, and histone H2B interact with Nuc1p and are required for cell death upon Nuc1p overexpression, suggesting a pathway in which mitochondrial pore opening, nuclear import, and chromatin association are successively involved in EndoG-mediated death. Deletion of NUC1 diminishes apoptotic death when mitochondrial respiration is increased but enhances necrotic death when oxidative phosphorylation is repressed, pointing to dual--lethal and vital--roles for EndoG.