SummaryMitochondria perform central functions in cellular bioenergetics, metabolism, and signaling, and their dysfunction has been linked to numerous diseases. The available studies cover only part of the mitochondrial proteome, and a separation of core mitochondrial proteins from associated fractions has not been achieved. We developed an integrative experimental approach to define the proteome of east mitochondria. We classified > 3,300 proteins of mitochondria and mitochondria-associated fractions and defined 901 high-confidence mitochondrial proteins, expanding the set of mitochondrial proteins by 82. Our analysis includes protein abundance under fermentable and nonfermentable growth, submitochondrial localization, single-protein experiments, and subcellular classification of mitochondria-associated fractions. We identified mitochondrial interactors of respiratory chain supercomplexes, ATP synthase, AAA proteases, the mitochondrial contact site and cristae organizing system (MICOS), and the coenzyme Q biosynthesis cluster, as well as mitochondrial proteins with dual cellular localization. The integrative proteome provides a high-confidence source for the characterization of physiological and pathophysiological functions of mitochondria and their integration into the cellular environment.

The mammalian circadian clock is built on a feedback loop in which PER and CRY proteins repress their own transcription. We found that in mouse liver nuclei all three PERs, both CRYs, and Casein Kinase-1δ (CK1δ) are present together in a ~1.9-MDa repressor assembly that quantitatively incorporates its CLOCK-BMAL1 transcription factor target. Prior to incorporation, CLOCK-BMAL1 exists in a 750-kDa complex. Single-particle electron microscopy (EM) revealed nuclear PER complexes purified from mouse liver to be quasi-spherical ~40-nm structures. In the cytoplasm, PERs, CRYs, and CK1δ were distributed into several complexes of ~0.9–1.1 MDa that appear to constitute an assembly pathway regulated by GAPVD1, a cytoplasmic trafficking factor. Single-particle EM of two purified cytoplasmic PER complexes revealed ~20-nm and ~25-nm structures, respectively, characterized by flexibly-tethered globular domains. Our results define the macromolecular assemblies comprising the circadian feedback loop and provide an initial structural view of endogenous eukaryotic clock machinery.

The MRS2/MGT gene family in Arabidopsis thaliana belongs to the superfamily of CorA-MRS2-ALR-type membrane proteins. Proteins of this type are characterized by a GMN tripeptide motif (Gly-Met-Asn) at the end of the first of two C-terminal transmembrane domains and have been characterized as magnesium transporters. Using the recently established mag-fura-2 system allowing direct measurement of Mg 2+ uptake into mitochondria of Saccharomyces cerevisiae, we find that all members of the Arabidopsis family complement the corresponding yeast mrs2 mutant. Highly different patterns of tissue-specific expression were observed for the MRS2/MGT family members in planta. Six of them are expressed in root tissues, indicating a possible involvement in plant magnesium supply and distribution after uptake from the soil substrate. Homozygous T-DNA insertion knockout lines were obtained for four members of the MRS2/MGT gene family. A strong, magnesium-dependent phenotype of growth retardation was found for mrs2-7 when Mg 2+ concentrations were lowered to 50 mM in hydroponic cultures. Ectopic overexpression of MRS2-7 from the cauliflower mosaic virus 35S promoter results in complementation and increased biomass accumulation. Green fluorescent protein reporter gene fusions indicate a location of MRS2-7 in the endomembrane system. Hence, contrary to what is frequently found in analyses of plant gene families, a single gene family member knockout results in a strong, environmentally dependent phenotype.

In bacteria, magnesium uptake is mainly mediated by the well-characterized CorA type of membrane proteins. In recent years, functional homologues have been characterized in the inner mitochondrial membrane of yeast and mammals (the MRS2/LPE10 type), in the plasma membrane of yeast (the ALR/MNR type) and, as an extended family of proteins, in the model plant Arabidopsis thaliana. Despite generally low sequence similarity, individual proteins can functionally complement each other over large phylogenetic distances. All these proteins are characterized by a universally conserved Gly-Met-Asn (GMN) motif at the end of the first of two conserved transmembrane domains near the C-terminus. Mutations of the GMN motif are known to abolish Mg(2+) transport, but the naturally occurring variants GVN and GIN may be associated with the transport of other divalent cations, such as zinc and cadmium, respectively. We refer to this whole class of proteins as the 2-TM-GxN type. The functional membrane channel is thought to be formed by oligomers containing four or five subunits. The wealth of sequence data now available allows us to explore the evolutionary diversification of the basic 2-TM-GxN model within the so-called metal ion transporter (MIT) superfamily. Here we report phylogenetic analyses on more than 360 homologous protein sequences derived from genomic sequences from representatives of all three domains of life. Independent gene duplications have occurred in fungi, plants and proteobacteria at different phylogenetic depths. Moreover, there is ample evidence for several instances of horizontal gene transfer of members of the 2-TM-GxN superfamily in Eubacteria and Archaea. Only single genes of the MRS2 type have been identified in vertebrate genomes. In contrast, 15 members are found in the model plant Arabidopsis thaliana, which appear to have arisen by at least four independent founder events before the diversification of flowering plants. Phylogenetic clade assignment seems to correlate with alterations in the highly conserved sequence around the GMN motif. This presumably forms an integral part of the pore surface, and changes in its structure may result in altered transport capacities for different divalent cations.

Spatial organization of signalling events of the phytohormone auxin is fundamental for maintaining a dynamic transition from plant stem cells to differentiated descendants. The cambium, the stem cell niche mediating wood formation, fundamentally depends on auxin signalling but its exact role and spatial organization is obscure. Here we show that, while auxin signalling levels increase in differentiating cambium descendants, a moderate level of signalling in cambial stem cells is essential for cambium activity. We identify the auxin-dependent transcription factor ARF5/MONOPTEROS to cell-autonomously restrict the number of stem cells by directly attenuating the activity of the stem cell-promoting WOX4 gene. In contrast, ARF3 and ARF4 function as cambium activators in a redundant fashion from outside of WOX4-expressing cells. Our results reveal an influence of auxin signalling on distinct cambium features by specific signalling components and allow the conceptual integration of plant stem cell systems with distinct anatomies.

Mgr2 is a new component of the mitochondrial presequence translocase required for efficient coupling of the TIM23 core complex to Tim21, respiratory chain complexes, and the translocase of the outer mitochondrial membrane.

The mitochondrial inner membrane harbors the complexes of the respiratory chain and translocase complexes for precursor proteins. We have identified a further subunit of the carrier translocase (TIM22 complex) that surprisingly is identical to subunit 3 of respiratory complex II, succinate dehydrogenase (Sdh3). The membrane-integral protein Sdh3 plays specific functions in electron transfer in complex II. We show by genetic and biochemical approaches that Sdh3 also plays specific functions in the TIM22 complex. Sdh3 forms a subcomplex with Tim18 and is involved in biogenesis and assembly of the membrane-integral subunits of the TIM22 complex. We conclude that the assembly of Sdh3 with different partner proteins, Sdh4 and Tim18, recruits it to two different mitochondrial membrane complexes with functions in bioenergetics and protein biogenesis, respectively.

Many mitochondrial proteins are synthesized as preproteins carrying amino-terminal presequences in the cytosol. The preproteins are imported by the translocase of the outer mitochondrial membrane (TOM) and the presequence translocase of the inner membrane (TIM). Tim50 and Tim23 transfer preproteins through the intermembrane space to the inner membrane. We report the crystal structure of the intermembrane space domain of yeast Tim50 to 1.83 Å resolution. A protruding β-hairpin of Tim50 is crucial for interaction with Tim23, providing a molecular basis for the cooperation of Tim50 and Tim23 in preprotein translocation to the protein-conducting channel of the mitochondrial inner membrane.