A yeast mitochondrial outer membrane protein essential for protein import and cell viability

The gene encoding ISP42, an integral outermembrane protein located at the yeast mitochondrial protein import site was cloned, sequenced and modified. Yeast cells depleted of ISP42 accumulate uncleaved mitochondrial precursor proteins and then die. ISP42 is the first mitochondrial membrane protein shown to be indispensable for protein import and cell viability.     

Identification of a receptor for protein import into mitochondria

Anti-idiotypic antibodies, prepared using a chemically synthesized signal peptide of a mitochondrial precursor protein, recognized a mitochondrial integral membrane protein (p32). Fab fragments derived from both anti-idiotypic antibodies and monospecific antibodies against purified p32 inhibited protein import into mitochondria. Moreover, anti-p32 antibodies specifically immunoprecipitated a precursor-p32 complex after detergent solubilization of mitochondria. Immunoelectron microscopy and subfractionation of mitochondria indicate that p32 is located in contact sites between the outer and inner mitochondrial membranes.     

DNA-protein conjugates can enter mitochondria via the protein import pathway

Mitochondria import most of their proteins and small molecules from the cytoplasm. There is some tentative evidence that they import some of their RNAs, but it is not known how nucleic acids could enter mitochondria. Here, we show that isolated yeast mitochondria can import a single-stranded or double-stranded 24-base pair piece of DNA whose 5' end is covalently linked to the C-terminus of a mitochondrial precursor protein.     

Isolation and characterization of the gene for a yeast mitochondrial import receptor

We have previously identified an integral membrane protein (p32) from Saccharomyces cerevisiae as a receptor for protein import into mitochondria, and have localized it to the mitochondrial outer membrane at contact sites. Here we report isolation of the corresponding mitochondrial import receptor gene, termed MIR1. The deduced amino-acid sequence of p32 shows roughly 40% identity with proteins of bovine heart and rat liver that have been suggested to be mitochondrial phosphate carriers. Haploid cells carrying a disrupted MIR1 allele were unable to grow on a non-fermentable carbon source but grew in media containing glucose, indicating that the MIR1 protein is essential for mitochondrial function. Compared with wild type, amounts of some mitochondrial proteins were markedly reduced in cells containing a disrupted MIR1 allele, whereas levels of others were unchanged. This indicates that yeast contains more than one pathway for protein import into mitochondria.     

线粒体蛋白转运系统的序列相似性分析

通过在27个不同进化层次物种的基因组和蛋白组中搜索酵母线粒体蛋白转运系统亚基的同源序列, 并进一步分析了同源亚基序列相似性与其所在线粒体位置的关系. 结果表明, 位于线粒体相同位置的模块有类似的序列相似性曲线, 相似性曲线在模块内部一般有波峰和波谷. 从线粒体外膜到基质, 序列相似性整体升高. 线粒体蛋白转运系统亚基与一些功能不相关的蛋白也表现出序列相似关系, 且这些亚基多集中在线粒体的内膜和外膜.     

Rapid regulation of steroidogenesis by mitochondrial protein import

Most mitochondrial proteins are synthesized on cytoplasmic ribosomes and imported into mitochondria. The imported proteins are directed to one of four submitochondrial compartments--the outer mitochondrial membrane, the inner mitochondrial membrane, the intramembraneous space, or the matrix--where the protein then functions. Here we show that the steroidogenic acute regulatory protein (StAR), a mitochondrial protein required for stress responses, reproduction, and sexual differentiation of male fetuses, exerts its activity transiently at the outer mitochondrial membrane rather than at its final resting place in the matrix. We also show that its residence time at this outer membrane and its activity are regulated by its speed of mitochondrial import. This may be the first example of a mitochondrial protein exerting its biological activity in a compartment other than that to which it is finally targeted. This system enables steroidogenic cells to initiate and terminate massive levels of steroidogenesis within a few minutes, permitting the rapid regulation of serum steroid hormone concentrations.     

Binding of a specific ligand inhibits import of a purified precursor protein into mitochondria

Methotrexate, a folate antagonist, blocks import into mitochondria of mouse dihydrofolate reductase fused to a mitochondrial presequence. Methotrexate does not mask the presequence, but stabilizes the dihydrofolate reductase moiety. It does not inhibit import of the authentic precursor from which the presequence is derived. This suggests that dihydrofolate reductase must at least partly unfold in order to be transported across mitochondrial membranes.     

Mapping of the protein import machinery in the mitochondrial outer membrane by crosslinking of translocation intermediates.

Mitochondria contain a complex machinery for the import of nuclear-encoded proteins. Receptor proteins exposed on the outer membrane surface are required for the specific binding of precursor proteins to mitochondria, either by binding of cytosolic signal recognition factors or by direct recognition of the precursor polypeptides. Subsequently, the precursors are inserted into the outer membrane at the general insertion site GIP (general insertion protein). Here we report the analysis of receptors and GIP by crosslinking of translocation intermediates and by coimmunoprecipitation. Surface-accumulated precursors were crosslinked to the receptors MOM19 and MOM72, suggesting a direct interaction of preproteins with surface receptors. We identified three novel mitochondrial outer membrane proteins, MOM7, MOM8, and MOM30 that, together with the previously identified MOM38, seem to form the GIP site and are present in the mitochondrial receptor complex.     

Mitochondrial proteins essential for viability mediate protein import into yeast mitochondria

Only five mitochondrial proteins are known to be essential for viability of the yeast Saccharomyces cerevisiae; all of them are key components of the mitochondrial protein import system. Other components of this system are not essential for life; they include functionally redundant import receptors on the mitochondrial surface and enzymes acting upon only a few precursor proteins.     

RNA editing in wheat mitochondria results in the conservation of protein sequences

RNA editing is a process that results in the production of a messenger RNA with nucleotide sequences that differ from those of the template DNA, and provides another mechanism for modulating gene expression. The phenomenon was initially described in the mitochondria of protozoa. Here we report that RNA editing is also required for the correct expression of plant mitochondrial genes. It has previously been proposed that in plant mitochondria there is a departure from the universal genetic code, with CGG specifying tryptophan instead of arginine. This was because CGG codons are often found in plant mitochondrial genes at positions corresponding to those encoding conserved tryptophans in other organisms. We have now found, however, wheat mitochondrial gene sequences containing C residues that are edited to U residues in the corresponding mRNA sequences. In this way, CGG codons can be changed to UGG codons in the mRNA so that tryptophan may be encoded according to the universal genetic code. Furthermore, for each codon modification resulting from a C----U conversion that we studied, we found a corresponding change in the amino acid that was encoded. RNA editing in wheat mitochondria can thus maintain genetic information at the RNA level and as a result contribute to the conservation of mitochondrial protein sequences among plants.     

Targeting of bacterial chloramphenicol acetyltransferase to mitochondria in transgenic plants

Most mitochondrial proteins are encoded by nuclear genes and are synthesized as precursors containing a presequence at the N terminus. In yeast and in mammalian cells, the function of the presequence in mitochondrial targeting has been revealed by chimaeric gene studies. Fusion of a mitochondrial presequence to a foreign protein coding sequence enables the protein to be imported into mitochondria in vitro as well as in vivo. Whether plant mitochondrial presequences function in the same way has been unknown. We have previously isolated and characterized a nuclear gene (atp2-1) from Nicotiana plumbaginifolia that encodes the beta-subunit of the mitochondrial ATP synthase. We have constructed a chimaeric gene comprising a putative atp2-1 presequence fused to the bacterial chloramphenicol acetyltransferase (CAT) coding sequence and introduced it into the tobacco genome. We report here that a segment of 90 amino acids of the N terminus of the beta-subunit precursor is sufficient for the specific targeting of the CAT protein to mitochondria in transgenic plants. Our results demonstrate a high specificity for organelle targeting in plant cells.     

A cytosolic protein contains a cryptic mitochondrial targeting signal

Cytosolic dihydrofolate reductase from mouse contains a cryptic mitochondrial targeting sequence. If this sequence is attached to the amino terminus of 'passenger' proteins which by themselves cannot enter mitochondria, the resulting fusion proteins are transported into yeast mitochondria.     

Mitochondrial remnant organelles of Giardia function in iron-sulphur protein maturation

Giardia intestinalis (syn. lamblia) is one of the most widespread intestinal protozoan pathogens worldwide, causing hundreds of thousands of cases of diarrhoea each year. Giardia is a member of the diplomonads, often described as an ancient protist group whose primitive nature is suggested by the lack of typical eukaryotic organelles (for example, mitochondria, peroxisomes), the presence of a poorly developed endomembrane system and by their early branching in a number of gene phylogenies. The discovery of nuclear genes of putative mitochondrial ancestry in Giardia and the recent identification of mitochondrial remnant organelles in amitochondrial protists such as Entamoeba histolytica and Trachipleistophora hominis suggest that the eukaryotic amitochondrial state is not a primitive condition but is rather the result of reductive evolution. Using an in vitro protein reconstitution assay and specific antibodies against IscS and IscU--two mitochondrial marker proteins involved in iron-sulphur cluster biosynthesis--here we demonstrate that Giardia contains mitochondrial remnant organelles (mitosomes) bounded by double membranes that function in iron-sulphur protein maturation. Our results indicate that Giardia is not primitively amitochondrial and that it has retained a functional organelle derived from the original mitochondrial endosymbiont.     

Tom22 is a multifunctional organizer of the mitochondrial preprotein translocase.

Mitochondrial preproteins are imported by a multisubunit translocase of the outer membrane (TOM), including receptor proteins and a general import pore. The central receptor Tom22 binds preproteins through both its cytosolic domain and its intermembrane space domain and is stably associated with the channel protein Tom40 (refs 11-13). Here we report the unexpected observation that a yeast strain can survive without Tom22, although it is strongly reduced in growth and the import of mitochondrial proteins. Tom22 is a multifunctional protein that is required for the higher-level organization of the TOM machinery. In the absence of Tom22, the translocase dissociates into core complexes, representing the basic import units, but lacks a tight control of channel gating. The single membrane anchor of Tom22 is required for a stable interaction between the core complexes, whereas its cytosolic domain serves as docking point for the peripheral receptors Tom20 and Tom70. Thus a preprotein translocase can combine receptor functions with distinct organizing roles in a multidomain protein.     

Bcl-2 family proteins regulate the release of apoptogenic cytochrome c by the mitochondrial channel VDAC.

During transduction of an apoptotic (death) signal into the cell, there is an alteration in the permeability of the membranes of the cell's mitochondria, which causes the translocation of the apoptogenic protein cytochrome c into the cytoplasm, which in turn activates death-driving proteolytic proteins known as caspases. The Bcl-2 family of proteins, whose members may be anti-apoptotic or pro-apoptotic, regulates cell death by controlling this mitochondrial membrane permeability during apoptosis, but how that is achieved is unclear. Here we create liposomes that carry the mitochondrial porin channel (also called the voltage-dependent anion channel, or VDAC) to show that the recombinant pro-apoptotic proteins Bax and Bak accelerate the opening of VDAC, whereas the anti-apoptotic protein Bcl-x(L) closes VDAC by binding to it directly. Bax and Bak allow cytochrome c to pass through VDAC out of liposomes, but passage is prevented by Bcl-x(L). In agreement with this, VDAC1-deficient mitochondria from a mutant yeast did not exhibit a Bax/Bak-induced loss in membrane potential and cytochrome c release, both of which were inhibited by Bcl-x(L). Our results indicate that the Bcl-2 family of proteins bind to the VDAC in order to regulate the mitochondrial membrane potential and the release of cytochrome c during apoptosis.     

Identification of a receptor for protein import into chloroplasts and its localization to envelope contact zones

An anti-idiotypic antibody approach was used to identify an integral membrane protein of the chloroplast envelope as a receptor for protein import into the chloroplast stroma. The import receptor is found in contact sites between the outer and inner membrane of the chloroplast envelope.     

Evolutionary conservation of biogenesis of beta-barrel membrane proteins

The outer membranes of mitochondria and chloroplasts are distinguished by the presence of beta-barrel membrane proteins. The outer membrane of Gram-negative bacteria also harbours beta-barrel proteins. In mitochondria these proteins fulfil a variety of functions such as transport of small molecules (porin/VDAC), translocation of proteins (Tom40) and regulation of mitochondrial morphology (Mdm10). These proteins are encoded by the nucleus, synthesized in the cytosol, targeted to mitochondria as chaperone-bound species, recognized by the translocase of the outer membrane, and then inserted into the outer membrane where they assemble into functional oligomers. Whereas some knowledge has been accumulated on the pathways of insertion of proteins that span cellular membranes with alpha-helical segments, very little is known about how beta-barrel proteins are integrated into lipid bilayers and assembled into oligomeric structures. Here we describe a protein complex that is essential for the topogenesis of mitochondrial outer membrane beta-barrel proteins (TOB). We present evidence that important elements of the topogenesis of beta-barrel membrane proteins have been conserved during the evolution of mitochondria from endosymbiotic bacterial ancestors.     

Humanin peptide suppresses apoptosis by interfering with Bax activation

Bax (Bcl2-associated X protein) is an apoptosis-inducing protein that participates in cell death during normal development and in various diseases. Bax resides in an inactive state in the cytosol of many cells. In response to death stimuli, Bax protein undergoes conformational changes that expose membrane-targeting domains, resulting in its translocation to mitochondrial membranes, where Bax inserts and causes release of cytochrome c and other apoptogenic proteins. It is unknown what controls conversion of Bax from the inactive to active conformation. Here we show that Bax interacts with humanin (HN), an anti-apoptotic peptide of 24 amino acids encoded in mammalian genomes. HN prevents the translocation of Bax from cytosol to mitochondria. Conversely, reducing HN expression by small interfering RNAs sensitizes cells to Bax and increases Bax translocation to membranes. HN peptides also block Bax association with isolated mitochondria, and suppress cytochrome c release in vitro. Notably, the mitochondrial genome contains an identical open reading frame, and the mitochondrial version of HN can also bind and suppress Bax. We speculate therefore that HN arose from mitochondria and transferred to the nuclear genome, providing a mechanism for protecting these organelles from Bax.     

基于人类线粒体基因功能网络的线粒体蛋白功能预测

本研究旨在利用生物信息学的方法建立高可信度线粒体基因清单,通过整合14个全基因组和线粒体水平的异质组学证据,利用朴素贝叶斯模型建立了线粒体基因功能网络,然后基于此网络预测线粒体相关KEGG pathway新的组件,确定部分线粒体基因功能.本研究预测得到78个线粒体相关KEGG pathway新的组件.     

Mitochondrial endonuclease G is important for apoptosis in C. elegans.

Programmed cell death (apoptosis) is a tightly regulated process of cell disassembly in which dying cells and their nuclei shrink and fragment and the chromosomal DNA is degraded into internucleosomal repeats. Here we report the characterization of the cps-6 gene, which appears to function downstream of, or in parallel to, the cell-death protease CED-3 of Caenorhabditis elegans in the DNA degradation process during apoptosis. cps-6 encodes a homologue of human mitochondrial endonuclease G, and its protein product similarly localizes to mitochondria in C. elegans. Reduction of cps-6 activity caused by a genetic mutation or RNA-mediated interference (RNAi) affects normal DNA degradation, as revealed by increased staining in a TUNEL assay, and results in delayed appearance of cell corpses during development in C. elegans. This observation provides in vivo evidence that the DNA degradation process is important for proper progression of apoptosis. CPS-6 is the first mitochondrial protein identified to be involved in programmed cell death in C. elegans, underscoring the conserved and important role of mitochondria in the execution of apoptosis.