The PERK Arm of the Unfolded Protein Response Regulates Mitochondrial Morphology during Acute Endoplasmic Reticulum Stress.

Endoplasmic reticulum (ER) stress is transmitted to mitochondria and is associated with pathologic mitochondrial dysfunction in diverse diseases. The PERK arm of the unfolded protein response (UPR) protects mitochondria during ER stress through the transcriptional and translational remodeling of mitochondrial molecular quality control pathways. Here, we show that ER stress also induces dynamic remodeling of mitochondrial morphology by promoting protective stress-induced mitochondrial hyperfusion (SIMH). ER-stress-associated SIMH is regulated by the PERK arm of the UPR and activated by eIF2α phosphorylation-dependent translation attenuation. We show that PERK-regulated SIMH is a protective mechanism to prevent pathologic mitochondrial fragmentation and promote mitochondrial metabolism in response to ER stress. These results identify PERK-dependent SIMH as a protective stress-responsive mechanism that regulates mitochondrial morphology during ER stress. Furthermore, our results show that PERK integrates transcriptional and translational signaling to coordinate mitochondrial molecular and organellar quality control in response to pathologic ER insults.

YME1L degradation reduces mitochondrial proteolytic capacity during oxidative stress.

Mitochondrial proteostasis is maintained by a network of ATP-dependent quality control proteases including the inner membrane protease YME1L. Here, we show that YME1L is a stress-sensitive mitochondrial protease that is rapidly degraded in response to acute oxidative stress. This degradation requires reductions in cellular ATP and involves the activity of the ATP-independent protease OMA1. Oxidative stress-dependent reductions in YME1L inhibit protective YME1L-dependent functions and increase cellular sensitivity to oxidative insult. Collectively, our results identify stress-induced YME1L degradation as a biologic process that attenuates protective regulation of mitochondrial proteostasis and promotes cellular death in response to oxidative stress.

Mitochondrial ADCK3 employs an atypical protein kinase-like fold to enable coenzyme Q biosynthesis.

The ancient UbiB protein kinase-like family is involved in isoprenoid lipid biosynthesis and is implicated in human diseases, but demonstration of UbiB kinase activity has remained elusive for unknown reasons. Here, we quantitatively define UbiB-specific sequence motifs and reveal their positions within the crystal structure of a UbiB protein, ADCK3. We find that multiple UbiB-specific features are poised to inhibit protein kinase activity, including an N-terminal domain that occupies the typical substrate binding pocket and a unique A-rich loop that limits ATP binding by establishing an unusual selectivity for ADP. A single alanine-to-glycine mutation of this loop flips this coenzyme selectivity and enables autophosphorylation but inhibits coenzyme Q biosynthesis in vivo, demonstrating functional relevance for this unique feature. Our work provides mechanistic insight into UbiB enzyme activity and establishes a molecular foundation for further investigation of how UbiB family proteins affect diseases and diverse biological pathways.

Stress-responsive regulation of mitochondria through the ER unfolded protein response.

The endoplasmic reticulum (ER) and mitochondria form physical interactions involved in the regulation of biologic functions including mitochondrial bioenergetics and apoptotic signaling. To coordinate these functions during stress, cells must coregulate ER and mitochondria through stress-responsive signaling pathways such as the ER unfolded protein response (UPR). Although the UPR is traditionally viewed as a signaling pathway responsible for regulating ER proteostasis, it is becoming increasingly clear that the protein kinase RNA (PKR)-like endoplasmic reticulum kinase (PERK) signaling pathway within the UPR can also regulate mitochondria proteostasis and function in response to pathologic insults that induce ER stress. Here, we discuss the contributions of PERK in coordinating ER-mitochondrial activities and describe the mechanisms by which PERK adapts mitochondrial proteostasis and function in response to ER stress.