The VDAC2-BAK axis regulates peroxisomal membrane permeability

Predicting the targeting of tail-anchored proteins to subcellular compartments in mammalian cells

Tail-anchored (TA) proteins contain a single transmembrane domain (TMD) at the C-terminus that anchors them to the membranes of organelles where they mediate critical cellular processes. Accordingly, mutations in genes encoding TA proteins have been identified in a number of severe inherited disorders. Despite the importance of correctly targeting a TA protein to its appropriate membrane, the mechanisms and signals involved are not fully understood. In this study, we identify additional peroxisomal TA proteins, discover more proteins that are present on multiple organelles, and reveal that a combination of TMD hydrophobicity and tail charge determines targeting to distinct organelle locations in mammals. Specifically, an increase in tail charge can override a hydrophobic TMD signal and re-direct a protein from the ER to peroxisomes or mitochondria and vice versa. We show that subtle changes in those parameters can shift TA proteins between organelles, explaining why peroxisomes and mitochondria have many of the same TA proteins. This enabled us to associate characteristic physicochemical parameters in TA proteins with particular organelle groups. Using this classification allowed successful prediction of the location of uncharacterized TA proteins for the first time.

Connecting mitochondrial dynamics and life-or-death events via Bcl-2 family proteins

The morphology of a population of mitochondria is the result of several interacting dynamical phenomena, including fission, fusion, movement, elimination and biogenesis. Each of these phenomena is controlled by underlying molecular machinery, and when defective can cause disease. New understanding of the relationships between form and function of mitochondria in health and disease is beginning to be unraveled on several fronts. Studies in mammals and model organisms have revealed that mitochondrial morphology, dynamics and function appear to be subject to regulation by the same proteins that regulate apoptotic cell death. One protein family that influences mitochondrial dynamics in both healthy and dying cells is the Bcl-2 protein family. Connecting mitochondrial dynamics with life-death pathway forks may arise from the intersection of Bcl-2 family proteins with the proteins and lipids that determine mitochondrial shape and function. Bcl-2 family proteins also have multifaceted influences on cells and mitochondria, including calcium handling, autophagy and energetics, as well as the subcellular localization of mitochondrial organelles to neuronal synapses. The remarkable range of physical or functional interactions by Bcl-2 family proteins is challenging to assimilate into a cohesive understanding. Most of their effects may be distinct from their direct roles in apoptotic cell death and are particularly apparent in the nervous system. Dual roles in mitochondrial dynamics and cell death extend beyond BCL-2 family proteins. In this review, we discuss many processes that govern mitochondrial structure and function in health and disease, and how Bcl-2 family proteins integrate into some of these processes.

BAK regulates catalase release from peroxisomes

Loss of voltage-dependent anion channel 2 (VDAC2) leads to impaired peroxisome biogenesis in mammalian cells. Knockdown of BAK restores peroxisomal biogenesis in VDAC2-deficient cells, where BAK localization shifts from mitochondria to peroxisomes. Moreover, overexpression of BAK activators in wild-type cells permeabilizes peroxisomes in a BAK-dependent manner. Together, BAK most likely regulates peroxisomal membrane permeability.

The peroxisomes strike BAK: Regulation of peroxisome integrity by the Bcl-2 family

Chipuk and Luna-Vargas discuss Hosoi et al.’s study revealing conserved regulation of mitochondrial and peroxisome membrane permeabilization by the Bcl2 family.

Within the mitochondrial pathway of apoptosis, VDAC2 controls both the localization and proapoptotic activity of BAK. In this issue, Hosoi et al. (2017. J. Cell Biol.https://doi.org/10.1083/jcb.201605002) find that loss of VDAC2 diverts BAK into peroxisome membranes, revealing the ability of BAK to control peroxisome membrane integrity and the release of soluble peroxisomal matrix proteins.