The UbiB family member Cqd1 forms a novel membrane contact site in mitochondria.

Mitochondria are essential organelles of eukaryotic cells and are characterized by their unique and complex membrane system. They are confined from the cytosol by an envelope consisting of two membranes. Signals, metabolites, proteins and lipids have to be transferred across these membranes via proteinaceous contact sites to keep mitochondria functional. In the present study, we identified a novel mitochondrial contact site in Saccharomyces cerevisiae that is formed by the inner membrane protein Cqd1 and the outer membrane proteins Por1 and Om14. Similar to what is found for the mitochondrial porin Por1, Cqd1 is highly conserved, suggesting that this complex is conserved in form and function from yeast to human. Cqd1 is a member of the UbiB protein kinase-like family (also called aarF domain-containing kinases). It was recently shown that Cqd1, in cooperation with Cqd2, controls the cellular distribution of coenzyme Q by a yet unknown mechanism. Our data suggest that Cqd1 is additionally involved in phospholipid homeostasis. Moreover, overexpression of CQD1 and CQD2 causes tethering of mitochondria to the endoplasmic reticulum, which might explain the ability of Cqd2 to rescue ERMES deletion phenotypes.

The dsRBP Staufen2 governs RNP assembly of neuronal Argonaute proteins.

Mature microRNAs are bound by a member of the Argonaute (Ago1-4) protein family, forming the core of the RNA-induced silencing complex (RISC). Association of RISC with target mRNAs results in ribonucleoprotein (RNP) assembly involved in translational silencing or RNA degradation. Yet, the dynamics of RNP assembly and its underlying functional implications are unknown. Here, we have characterized the role of the RNA-binding protein Staufen2, a candidate Ago interactor, in RNP assembly. Staufen2 depletion resulted in the upregulation of Ago1/2 and the RISC effector proteins Ddx6 and Dcp1a. This upregulation was accompanied by the displacement of Ago1/2 from processing bodies, large RNPs implicated in RNA storage, and subsequent association of Ago2 with polysomes. In parallel, Staufen2 deficiency decreased global translation and increased dendritic branching. As the observed phenotypes can be rescued by Ago1/2 knockdown, we propose a working model in which both Staufen2 and Ago proteins depend on each other and contribute to neuronal homeostasis.

Evidence of a role for cAMP in mitochondrial regulation in ovarian granulosa cells.

In the ovary, proliferation and differentiation of granulosa cells (GCs) drive follicular growth. Our immunohistochemical study in a non-human primate, the Rhesus monkey, showed that the mitochondrial activity marker protein cytochrome c oxidase subunit 4 (COX4) increases in GCs in parallel to follicle size, and furthermore, its intracellular localization changes. This suggested that there is mitochondrial biogenesis and trafficking, and implicates the actions of gonadotropins, which regulate follicular growth and ovulation. Human KGN cells, i.e. granulosa tumour cells, were therefore used to study these possibilities. To robustly elevate cAMP, and thereby mimic the actions of gonadotropins, we used forskolin (FSK). FSK increased the cell size and the amount of mitochondrial DNA of KGN cells within 24 h. As revealed by MitoTracker™ experiments and ultrastructural 3D reconstruction, FSK treatment induced the formation of elaborate mitochondrial networks. H89, a protein kinase A (PKA) inhibitor, reduced the network formation. A proteomic analysis indicated that FSK elevated the levels of regulators of the cytoskeleton, among others (data available via ProteomeXchange with identifier PXD032160). The steroidogenic enzyme CYP11A1 (Cytochrome P450 Family 11 Subfamily A Member 1), located in mitochondria, was more than 3-fold increased by FSK, implying that the cAMP/PKA-associated structural changes occur in parallel with the acquisition of steroidogenic competence of mitochondria in KGN cells. In summary, the observations show increases in mitochondria and suggest intracellular trafficking of mitochondria in GCs during follicular growth, and indicate that they may partially be under the control of gonadotropins and cAMP. In line with this, increased cAMP in KGN cells profoundly affected mitochondrial dynamics in a PKA-dependent manner and implicated cytoskeletal changes.

Pumilio2 Promotes Growth of Mature Neurons.

RNA-binding proteins (RBPs) are essential regulators controlling both the cellular transcriptome and translatome. These processes enable cellular plasticity, an important prerequisite for growth. Cellular growth is a complex, tightly controlled process. Using cancer cells as model, we looked for RBPs displaying strong expression in published transcriptome datasets. Interestingly, we found the Pumilio (Pum) protein family to be highly expressed in all these cells. Moreover, we observed that Pum2 is regulated by basic fibroblast growth factor (bFGF). bFGF selectively enhances protein levels of Pum2 and the eukaryotic initiation factor 4E (eIF4E). Exploiting atomic force microscopy and in vitro pulldown assays, we show that Pum2 selects for eIF4E mRNA binding. Loss of Pum2 reduces eIF4E translation. Accordingly, depletion of Pum2 led to decreased soma size and dendritic branching of mature neurons, which was accompanied by a reduction in essential growth factors. In conclusion, we identify Pum2 as an important growth factor for mature neurons. Consequently, it is tempting to speculate that Pum2 may promote cancer growth.

RGS4 RNA Secondary Structure Mediates Staufen2 RNP Assembly in Neurons.

RNA-binding proteins (RBPs) act as posttranscriptional regulators controlling the fate of target mRNAs. Unraveling how RNAs are recognized by RBPs and in turn are assembled into neuronal RNA granules is therefore key to understanding the underlying mechanism. While RNA sequence elements have been extensively characterized, the functional impact of RNA secondary structures is only recently being explored. Here, we show that Staufen2 binds complex, long-ranged RNA hairpins in the 3'-untranslated region (UTR) of its targets. These structures are involved in the assembly of Staufen2 into RNA granules. Furthermore, we provide direct evidence that a defined Rgs4 RNA duplex regulates Staufen2-dependent RNA localization to distal dendrites. Importantly, disrupting the RNA hairpin impairs the observed effects. Finally, we show that these secondary structures differently affect protein expression in neurons. In conclusion, our data reveal the importance of RNA secondary structure in regulating RNA granule assembly, localization and eventually translation. It is therefore tempting to speculate that secondary structures represent an important code for cells to control the intracellular fate of their mRNAs.

The MICOS complex, a structural element of mitochondria with versatile functions.

Mitochondria perform a plethora of functions in various cells of different tissues. Their architecture differs remarkably, for instance in neurons versus steroidogenic cells. Furthermore, aberrant mitochondrial architecture results in mitochondrial dysfunction. This indicates strongly that mitochondrial architecture and function are intimately linked. Therefore, a deep knowledge about the determinants of mitochondrial architecture and their function on a molecular level is of utmost importance. In the past decades, various proteins and protein complexes essential for formation of mitochondrial architecture have been identified. Here we will review the current knowledge of the MICOS complex, one of the major structural elements of mitochondria. MICOS is a multi-subunit complex present in the inner mitochondrial membrane. Multiple interaction partners in the inner and outer mitochondrial membrane point to participation in a multitude of important processes, such as generation of mitochondrial architecture, lipid metabolism, and protein import into mitochondria. Since the MICOS complex is highly conserved in form and function throughout evolution, we will highlight the importance of MICOS for mammals. We will emphasize in particular the current knowledge of the association of MICOS with severe human diseases, including Charcot-Marie-Tooth disease type 2, Alzheimer's disease, Parkinson's disease, Frontotemporal Dementia and Amyotrophic Lateral Sclerosis.

Assembly of intermediates for rapid membrane fusion.

Membrane fusion is essential for intracellular protein sorting, cell growth, hormone secretion, and neurotransmission. Rapid membrane fusion requires tethering and Sec1-Munc18 (SM) function to catalyze R-, Qa-, Qb-, and Qc-SNARE complex assembly in trans, as well as SNARE engagement by the SNARE-binding chaperone Sec17/αSNAP. The hexameric vacuolar HOPS (homotypic fusion and vacuole protein sorting) complex in the yeast Saccharomyces cerevisiae tethers membranes through its affinities for the membrane Rab GTPase Ypt7. HOPS also has specific affinities for the vacuolar SNAREs and catalyzes SNARE complex assembly, but the order of their assembly into a 4-SNARE complex is unclear. We now report defined assembly intermediates on the path to membrane fusion. We found that a prefusion intermediate will assemble with HOPS and the R, Qa, and Qc SNAREs, and that this assembly undergoes rapid fusion upon addition of Qb and Sec17. HOPS-tethered membranes and all four vacuolar SNAREs formed a complex that underwent an even more dramatic burst of fusion upon Sec17p addition. These findings provide initial insights into an ordered fusion pathway consisting of the following intermediates and events: 1) Rab- and HOPS-tethered membranes, 2) a HOPS:R:Qa:Qc trans-complex, 3) a HOPS:4-SNARE trans-complex, 4) an engagement with Sec17, and 5) the rapid lipid rearrangements during fusion. In conclusion, our results indicate that the R:Qa:Qc complex forms in the context of membrane, Ypt7, HOPS, and trans-SNARE assembly and serves as a functional intermediate for rapid fusion after addition of the Qb-SNARE and Sec17 proteins.

Lateral release of proteins from the TOM complex into the outer membrane of mitochondria.

The TOM complex of the outer membrane of mitochondria is the entry gate for the vast majority of precursor proteins that are imported into the mitochondria. It is made up by receptors and a protein conducting channel. Although precursor proteins of all subcompartments of mitochondria use the TOM complex, it is not known whether its channel can only mediate passage across the outer membrane or also lateral release into the outer membrane. To study this, we have generated fusion proteins of GFP and Tim23 which are inserted into the inner membrane and, at the same time, are spanning either the TOM complex or are integrated into the outer membrane. Our results demonstrate that the TOM complex, depending on sequence determinants in the precursors, can act both as a protein conducting pore and as an insertase mediating lateral release into the outer membrane.

The mitochondrial contact site complex, a determinant of mitochondrial architecture.

Mitochondria are organelles with a complex architecture. They are bounded by an envelope consisting of the outer membrane and the inner boundary membrane (IBM). Narrow crista junctions (CJs) link the IBM to the cristae. OMs and IBMs are firmly connected by contact sites (CS). The molecular nature of the CS remained unknown. Using quantitative high-resolution mass spectrometry we identified a novel complex, the mitochondrial contact site (MICOS) complex, formed by a set of mitochondrial membrane proteins that is essential for the formation of CS. MICOS is preferentially located at the CJs. Upon loss of one of the MICOS subunits, CJs disappear completely or are impaired, showing that CJs require the presence of CS to form a superstructure that links the IBM to the cristae. Loss of MICOS subunits results in loss of respiratory competence and altered inheritance of mitochondrial DNA.

An Improved Method to Isolate Mitochondrial Contact Sites.

Mitochondria are present in virtually all eukaryotic cells and perform essential functions that go far beyond energy production, for instance, the synthesis of iron-sulfur clusters, lipids, or proteins, Ca2+ buffering, and the induction of apoptosis. Likewise, mitochondrial dysfunction results in severe human diseases such as cancer, diabetes, and neurodegeneration. In order to perform these functions, mitochondria have to communicate with the rest of the cell across their envelope, which consists of two membranes. Therefore, these two membranes have to interact constantly. Proteinaceous contact sites between the mitochondrial inner and outer membranes are essential in this respect. So far, several contact sites have been identified. In the method described here, Saccharomyces cerevisiae mitochondria are used to isolate contact sites and, thus, identify candidates that qualify for contact site proteins. We used this method to identify the mitochondrial contact site and cristae organizing system (MICOS) complex, one of the major contact site-forming complexes in the mitochondrial inner membrane, which is conserved from yeast to humans. Recently, we further improved this method to identify a novel contact site consisting of Cqd1 and the Por1-Om14 complex.

RNA supply drives physiological granule assembly in neurons.

Membraneless cytoplasmic condensates of mRNAs and proteins, known as RNA granules, play pivotal roles in the regulation of mRNA fate. Their maintenance fine-tunes time and location of protein expression, affecting many cellular processes, which require complex protein distribution. Here, we report that RNA granules-monitored by DEAD-Box helicase 6 (DDX6)-disassemble during neuronal maturation both in cell culture and in vivo. This process requires neuronal function, as synaptic inhibition results in reversible granule assembly. Importantly, granule assembly is dependent on the RNA-binding protein Staufen2, known for its role in RNA localization. Altering the levels of free cytoplasmic mRNA reveals that RNA availability facilitates DDX6 granule formation. Specifically depleting RNA from DDX6 granules confirms RNA as an important driver of granule formation. Moreover, RNA is required for DDX6 granule assembly upon synaptic inhibition. Together, this data demonstrates how RNA supply favors RNA granule assembly, which not only impacts subcellular RNA localization but also translation-dependent synaptic plasticity, learning, and memory.

Pumilio2 and Staufen2 selectively balance the synaptic proteome.

Neurons have the capacity to adapt to environmental stimuli, a phenomenon termed cellular plasticity. The underlying processes are controlled by a network of RNA-binding proteins (RBPs). Their precise impact, however, is largely unknown. To address this important question, we chose Pumilio2 (Pum2) and Staufen2 (Stau2), which both regulate synaptic transmission. Surprisingly, even though both RBPs dynamically interact with each other in neurons, their respective impact on the transcriptome and proteome is highly selective. Although Pum2 deficiency leads to reduced translation and protein expression, Stau2 depletion preferentially impacts RNA levels and increases protein abundance. Furthermore, we show that Pum2 activates expression of key GABAergic synaptic components, e.g., the GABAA receptor scaffold protein Gephyrin. Consequently, Pum2 depletion selectively reduced the amplitude of miniature inhibitory postsynaptic currents. Together, our data argue for an important role of RBPs to maintain proteostasis in order to control distinct aspects of synaptic transmission.

HOPS recognizes each SNARE, assembling ternary trans-complexes for rapid fusion upon engagement with the 4th SNARE.

Yeast vacuole fusion requires R-SNARE, Q-SNAREs, and HOPS. A HOPS SM-family subunit binds the R- and Qa-SNAREs. We now report that HOPS binds each of the four SNAREs. HOPS catalyzes fusion when the Q-SNAREs are not pre-assembled, ushering them into a functional complex. Co-incubation of HOPS, proteoliposomes bearing R-SNARE, and proteoliposomes with any two Q-SNAREs yields a rapid-fusion complex with 3 SNAREs in a trans-assembly. The missing Q-SNARE then induces sudden fusion. HOPS can 'template' SNARE complex assembly through SM recognition of R- and Qa-SNAREs. Though the Qa-SNARE is essential for spontaneous SNARE assembly, HOPS also assembles a rapid-fusion complex between R- and QbQc-SNARE proteoliposomes in the absence of Qa-SNARE, awaiting Qa for fusion. HOPS-dependent fusion is saturable at low concentrations of each Q-SNARE, showing binding site functionality. HOPS thus tethers membranes and recognizes each SNARE, assembling R+Qa or R+QbQc rapid fusion intermediates.

Isolation of Contact Sites Between Inner and Outer Mitochondrial Membranes.

Mitochondria are essential organelles of all eukaryotic cells. They perform a plethora of important metabolic functions and have a highly complex architecture that differs drastically between different cells and tissues. Mitochondria are delimited from the cytosol by the mitochondrial envelope that consists of the outer membrane and the inner membrane. The inner membrane is subdivided into the inner boundary membrane that runs parallel to the outer membrane and the crista membrane. Both sections of the inner membrane are linked by crista junctions. A further important architectural element of mitochondria are the contact sites between outer membrane and inner membrane. These sites were observed a long time ago by classical electron microscopy, but their molecular structure was identified only recently when it was recognized that proteins of crista junctions and proteins of the outer membrane are responsible for these strong contacts. Mitochondrial function is severely affected when contact sites are disturbed. This underlines the notion that mitochondrial architecture and function are intimately connected. In the following a method is described to generate and to isolate membrane vesicles from isolated yeast mitochondria that contain these contact sites.

Analysis of Yeast Mitochondria by Electron Microscopy.

Budding yeast Saccharomyces cerevisiae represents a widely used model organism for the study of mitochondrial biogenesis and architecture. Electron microscopy is an essential tool in the analysis of cellular ultrastructure and the precise localization of proteins to organellar subcompartments. We provide here detailed protocols for the analysis of yeast mitochondria by transmission electron microscopy: (1) chemical fixation and Epon embedding of yeast cells and isolated mitochondria, and (2) cryosectioning and immunolabeling of yeast cells and isolated mitochondria according to the Tokuyasu method.

An evidence based hypothesis on the existence of two pathways of mitochondrial crista formation.

Metabolic function and architecture of mitochondria are intimately linked. More than 60 years ago, cristae were discovered as characteristic elements of mitochondria that harbor the protein complexes of oxidative phosphorylation, but how cristae are formed, remained an open question. Here we present experimental results obtained with yeast that support a novel hypothesis on the existence of two molecular pathways that lead to the generation of lamellar and tubular cristae. Formation of lamellar cristae depends on the mitochondrial fusion machinery through a pathway that is required also for homeostasis of mitochondria and mitochondrial DNA. Tubular cristae are formed via invaginations of the inner boundary membrane by a pathway independent of the fusion machinery. Dimerization of the F1FO-ATP synthase and the presence of the MICOS complex are necessary for both pathways. The proposed hypothesis is suggested to apply also to higher eukaryotes, since the key components are conserved in structure and function throughout evolution.

Aim24 and MICOS modulate respiratory function, tafazzin-related cardiolipin modification and mitochondrial architecture.

Structure and function of mitochondria are intimately linked. In a search for components that participate in building the elaborate architecture of this complex organelle we have identified Aim24, an inner membrane protein. Aim24 interacts with the MICOS complex that is required for the formation of crista junctions and contact sites between inner and outer membranes. Aim24 is necessary for the integrity of the MICOS complex, for normal respiratory growth and mitochondrial ultrastructure. Modification of MICOS subunits Mic12 or Mic26 by His-tags in the absence of Aim24 leads to complete loss of cristae and respiratory complexes. In addition, the level of tafazzin, a cardiolipin transacylase, is drastically reduced and the composition of cardiolipin is modified like in mutants lacking tafazzin. In conclusion, Aim24 by interacting with the MICOS complex plays a key role in mitochondrial architecture, composition and function. DOI: http://dx.doi.org/10.7554/eLife.01684.001.

The C-terminal domain of Fcj1 is required for formation of crista junctions and interacts with the TOB/SAM complex in mitochondria.

Crista junctions (CJs) are tubular invaginations of the inner membrane of mitochondria that connect the inner boundary with the cristae membrane. These architectural elements are critical for mitochondrial function. The yeast inner membrane protein Fcj1, called mitofilin in mammals, was reported to be preferentially located at CJs and crucial for their formation. Here we investigate the functional roles of individual domains of Fcj1. The most conserved part of Fcj1, the C-terminal domain, is essential for Fcj1 function. In its absence, formation of CJ is strongly impaired and irregular, and stacked cristae are present. This domain interacts with full-length Fcj1, suggesting a role in oligomer formation. It also interacts with Tob55 of the translocase of outer membrane ß-barrel proteins (TOB)/sorting and assembly machinery (SAM) complex, which is required for the insertion of ß-barrel proteins into the outer membrane. The association of the TOB/SAM complex with contact sites depends on the presence of Fcj1. The biogenesis of ß-barrel proteins is not significantly affected in the absence of Fcj1. However, down-regulation of the TOB/SAM complex leads to altered cristae morphology and a moderate reduction in the number of CJs. We propose that the C-terminal domain of Fcj1 is critical for the interaction of Fcj1 with the TOB/SAM complex and thereby for stabilizing CJs in close proximity to the outer membrane. These results assign novel functions to both the C-terminal domain of Fcj1 and the TOB/SAM complex.

Allosteric coupling of two different functional active sites in monomeric Plasmodium falciparum glyoxalase I.

Glyoxalase I (GloI) catalyzes the glutathione-dependent conversion of 2-oxoaldehydes to S-2-hydroxyacylglutathione derivatives. Studies on GloI from diverse organisms such as man, bacteria, yeast, and different parasites show striking differences among these potentially isofunctional enzymes as far as metal content and the number of active sites per subunit are concerned. So far, it is not known whether this structural variability is linked to catalytic or regulatory features in vivo. Here we show that recombinant GloI from the malaria parasite Plasmodium falciparum has a high- and a low-affinity binding site for the diastereomeric hemithioacetals formed by addition of glutathione to methylglyoxal. Both active sites of the monomeric enzyme are functional and have similar k(cat)(app) values. Proteolytic susceptibility studies and detailed analyses of the steady-state kinetics of active-site mutants suggest that both reaction centers can adopt two discrete conformations and are allosterically coupled. As a result of the positive homotropic allosteric coupling, P. falciparum GloI has an increased affinity at low substrate concentrations and an increased activity at higher substrate concentrations. This could also be the case for GloI from yeast and other organisms. Potential physiologically relevant differences between monomeric GloI and homodimeric GloI are discussed. Our results provide a strong basis for drug development strategies and significantly enhance our understanding of GloI kinetics and structure-function relationships. Furthermore, they extend the current knowledge on allosteric regulation of monomeric proteins in general.