The discovery of the multiple roles of mitochondria-endoplasmic reticulum (ER) juxtaposition in cell biology often relied upon the exploitation of Mitofusin (Mfn) 2 as an ER-mitochondria tether. However, this established Mfn2 function was recently questioned, calling for a critical re-evaluation of Mfn2's role in ER-mitochondria cross-talk. Electron microscopy and fluorescence-based probes of organelle proximity confirmed that ER-mitochondria juxtaposition was reduced by constitutive or acute Mfn2 deletion. Functionally, mitochondrial uptake of Ca 2+ released from the ER was reduced following acute Mfn2 ablation, as well as in Mfn2−/− cells overexpressing the mitochondrial calcium uniporter. Mitochondrial Ca 2+ uptake rate and extent were normal in isolated Mfn2 −/− liver mitochondria, consistent with the finding that acute or chronic Mfn2 ablation or overexpression did not alter mitochondrial calcium uniporter complex component levels. Hence, Mfn2 stands as a bona fide ER-mitochondria tether whose ablation decreases interorganellar juxtaposition and communication.he endoplasmic reticulum (ER) and mitochondria are physically coupled to control mitochondrial Ca 2+ uptake, lipid transfer, autophagosome formation, ER stress, and apoptosis (1-6). Juxtaposition is mediated by protein structures that can be visualized in electron microscopy (EM) and electron tomography (ET) studies. These physical tethers span 6-15 nm when connecting smooth, and 19-30 nm when connecting rough ER to mitochondria (7). Operationally, an ER-mitochondria tether should fulfill at least these minimal criteria: (i) it is retrieved on the outer mitochondrial membrane (OMM); (ii) it is retrieved in mitochondria-associated ER membranes, the ER subdomain involved in interaction with mitochondria; (iii) it interacts in trans with a homo-or heterotypic interactor on the opposing membrane; (iv) its deletion increases the distance between the ER and mitochondria; or (v) its deletion reduces exchange of Ca 2+ and lipids between the ER and mitochondria. The molecular nature of ER-mitochondria tethers remained elusive for many years. The scaffold protein PACS2 (phosphofurin acidic cluster sorting protein 2) modulates their extent (8), and they include the heterotypic association between the inositol triphosphate (IP3) receptor on the ER and the OMM voltagedependent anion channel (9). Another protein that fulfils the operational criteria to be defined as a tether is Mitofusin 2 (Mfn2). This OMM profusion protein is also retrieved in mitochondria-associated ER membranes and ER Mfn2 interacts in trans with Mfn1 or Mfn2 on the mitochondria to physically tether the organelles. Mfn2 ablation increases the distance between the ER and mitochondria and decreases agonist-evoked Ca 2+ transfer from the ER to mitochondria (10) that depends on the generation of high Ca 2+ microdomains at their interface (11,12). The role of Mfn2 as a tether was confirmed independently in the heart (13), in pro-opiomelanocortin neurons (14), and in the liver (15).Mfn2-dependent tethe...

The mitochondria-associated membrane (MAM) is a specialized subdomain of the endoplasmic reticulum (ER) which acts as an intracellular signaling hub. MAM dysfunction has been related to liver disease. We report a high-throughput mass spectrometry-based proteomics characterization of MAMs from mouse liver, which portrays them as an extremely complex compartment involved in different metabolic processes, including steroid metabolism. Interestingly, we identified caveolin-1 (CAV1) as an integral component of hepatic MAMs, which determine the relative cholesterol content of these ER subdomains. Finally, a detailed comparative proteomics analysis between MAMs from wild type and CAV1-deficient mice suggests that functional CAV1 contributes to the recruitment and regulation of intracellular steroid and lipoprotein metabolism-related processes accrued at MAMs. The potential impact of these novel aspects of CAV1 biology on global cell homeostasis and disease is discussed.