Juxtaposition between endoplasmic reticulum (ER) and mitochondria is a common structural feature, providing the physical basis for intercommunication during Ca(2+) signalling; yet, the molecular mechanisms controlling this interaction are unknown. Here we show that mitofusin 2, a mitochondrial dynamin-related protein mutated in the inherited motor neuropathy Charcot-Marie-Tooth type IIa, is enriched at the ER-mitochondria interface. Ablation or silencing of mitofusin 2 in mouse embryonic fibroblasts and HeLa cells disrupts ER morphology and loosens ER-mitochondria interactions, thereby reducing the efficiency of mitochondrial Ca(2+) uptake in response to stimuli that generate inositol-1,4,5-trisphosphate. An in vitro assay as well as genetic and biochemical evidences support a model in which mitofusin 2 on the ER bridges the two organelles by engaging in homotypic and heterotypic complexes with mitofusin 1 or 2 on the surface of mitochondria. Thus, mitofusin 2 tethers ER to mitochondria, a juxtaposition required for efficient mitochondrial Ca(2+) uptake.

Mitofusin 2 tethers endoplasmic reticulum to mitochondria

This study shows that autophagosomes form at sites of contact between the endoplasmic reticulum and mitochondria, and that formation requires the SNARE protein syntaxin 17.

/pdf/autophagosomes-form-at-er-mitochondria-contact-sites-8xp82068qm.pdf

Autophagosomes form at ER–mitochondria contact sites

During the past two decades calcium (Ca(2+)) accumulation in energized mitochondria has emerged as a biological process of utmost physiological relevance. Mitochondrial Ca(2+) uptake was shown to control intracellular Ca(2+) signalling, cell metabolism, cell survival and other cell-type specific functions by buffering cytosolic Ca(2+) levels and regulating mitochondrial effectors. Recently, the identity of mitochondrial Ca(2+) transporters has been revealed, opening new perspectives for investigation and molecular intervention.

Mitochondria as sensors and regulators of calcium signalling

The ADP-ribosylation factor (ARF) small GTPases regulate vesicular traffic and organelle structure by recruiting coat proteins, regulating phospholipid metabolism and modulating the structure of actin at membrane surfaces. Recent advances in our understanding of the signalling pathways that are regulated by ARF1 and ARF6, two of the best characterized ARF proteins, provide a molecular context for ARF protein function in fundamental biological processes, such as secretion, endocytosis, phagocytosis, cytokinesis, cell adhesion and tumour-cell invasion.

ARF proteins: roles in membrane traffic and beyond.

The role of mitochondria in cell metabolism and survival is controlled by calcium signals that are commonly transmitted at the close associations between mitochondria and endoplasmic reticulum (ER). However, the physical linkage of the ER–mitochondria interface and its relevance for cell function remains elusive. We show by electron tomography that ER and mitochondria are adjoined by tethers that are ∼10 nm at the smooth ER and ∼25 nm at the rough ER. Limited proteolysis separates ER from mitochondria, whereas expression of a short “synthetic linker” (<5 nm) leads to tightening of the associations. Although normal connections are necessary and sufficient for proper propagation of ER-derived calcium signals to the mitochondria, tightened connections, synthetic or naturally observed under apoptosis-inducing conditions, make mitochondria prone to Ca2+ overloading and ensuing permeability transition. These results reveal an unexpected dependence of cell function and survival on the maintenance of proper spacing between the ER and mitochondria.

https://rupress.org/jcb/article-pdf/174/7/915/1326212/915.pdf

Structural and functional features and significance of the physical linkage between ER and mitochondria

The endoplasmic reticulum (ER) and mitochondria form contacts that support communication between these two organelles, including synthesis and transfer of lipids, and the exchange of calcium, which regulates ER chaperones, mitochondrial ATP production, and apoptosis. Despite the fundamental roles for ER–mitochondria contacts, little is known about the molecules that regulate them. Here we report the identification of a multifunctional sorting protein, PACS-2, that integrates ER–mitochondria communication, ER homeostasis, and apoptosis. PACS-2 controls the apposition of mitochondria with the ER, as depletion of PACS-2 causes BAP31-dependent mitochondria fragmentation and uncoupling from the ER. PACS-2 also controls formation of ER lipid-synthesizing centers found on mitochondria-associated membranes and ER homeostasis. However, in response to apoptotic inducers, PACS-2 translocates Bid to mitochondria, which initiates a sequence of events including the formation of mitochondrial truncated Bid, the release of cytochrome c, and the activation of caspase-3, thereby causing cell death. Together, our results identify PACS-2 as a novel sorting protein that links the ER–mitochondria axis to ER homeostasis and the control of cell fate, and provide new insights into Bid action.

/pdf/pacs-2-controls-endoplasmic-reticulum-mitochondria-1iqbo09hal.pdf

PACS-2 controls endoplasmic reticulum-mitochondria communication and Bid-mediated apoptosis.

https://www.cell.com/cell/pdf/S0092-8674(02)01162-5.pdf

HIV-1 Nef Downregulates MHC-I by a PACS-1- and PI3K-Regulated ARF6 Endocytic Pathway

Calnexin is an endoplasmic reticulum (ER) lectin that mediates protein folding on the rough ER. Calnexin also interacts with ER calcium pumps that localize to the mitochondria-associated membrane (MAM). Depending on ER homeostasis, varying amounts of calnexin target to the plasma membrane. However, no regulated sorting mechanism is so far known for calnexin. Our results now describe how the interaction of calnexin with the cytosolic sorting protein PACS-2 distributes calnexin between the rough ER, the MAM, and the plasma membrane. Under control conditions, more than 80% of calnexin localizes to the ER, with the majority on the MAM. PACS-2 knockdown disrupts the calnexin distribution within the ER and increases its levels on the cell surface. Phosphorylation by protein kinase CK2 of two calnexin cytosolic serines (Ser554/564) reduces calnexin binding to PACS-2. Consistent with this, a Ser554/564 Asp phosphomimic mutation partially reproduces PACS-2 knockdown by increasing the calnexin signal on the cell surface and reducing it on the MAM. PACS-2 knockdown does not reduce retention of other ER markers. Therefore, our results suggest that the phosphorylation state of the calnexin cytosolic domain and its interaction with PACS-2 sort this chaperone between domains of the ER and the plasma membrane.

/pdf/the-subcellular-distribution-of-calnexin-is-mediated-by-pacs-4gireth91t.pdf

The Subcellular Distribution of Calnexin Is Mediated by PACS-2

Erratum: PACS-2 controls endoplasmic reticulum-mitochondria communication and Bid-mediated apoptosis (EMBO Journal (2005) 24 (717-729) doi:10.1038/sj.emboj.7600559)

Anastassia D. Blagoveshchenskaya

Papers

PACS-2 controls endoplasmic reticulum-mitochondria communication and Bid-mediated apoptosis.

HIV-1 Nef Downregulates MHC-I by a PACS-1- and PI3K-Regulated ARF6 Endocytic Pathway

The Subcellular Distribution of Calnexin Is Mediated by PACS-2

Erratum: PACS-2 controls endoplasmic reticulum-mitochondria communication and Bid-mediated apoptosis (EMBO Journal (2005) 24 (717-729) doi:10.1038/sj.emboj.7600559)