Mft52, an acid-bristle protein in the cytosol that delivers precursor proteins to yeast mitochondria

MnSOD in oxidative stress response-potential regulation via mitochondrial protein influx

SIGNIFICANCE

The mitochondrial antioxidant manganese superoxide dismutase (MnSOD) is encoded by genomic DNA and its dismutase function is fully activated in the mitochondria to detoxify free radical O2(•-) generated by mitochondrial respiration. Accumulating evidence shows an extensive communication between the mitochondria and cytoplasm under oxidative stress. Not only is the MnSOD gene upregulated by oxidative stress, but MnSOD activity can be enhanced via the mitochondrial protein influx (MPI).

RECENT ADVANCES

A cluster of MPI containing cytoplasmic/nuclear proteins, such as cyclins, cyclin-dependent kinases, and p53 interact with and alter MnSOD activity. These proteins modulate MnSOD superoxide scavenging activity via post-translational modifications in the mitochondria. In addition to well-established pathways in gene expression, recent findings suggest that MnSOD enzymatic activity can also be enhanced by phosphorylation of specific motifs in mitochondria. This review attempts to discuss the pre- and post-translational regulation of MnSOD, and how these modifications alter MnSOD activity, which induces a cell adaptive response to oxidative stress.

CRITICAL ISSUES

MnSOD is biologically significant to aerobic cells. Its role in protecting the cells against the deleterious effects of reactive oxygen species is evident. However, the exact network of MnSOD-associated cellular adaptive reaction to oxidative stress and its post-translational modifications, especially its enzymatic enhancement via phosphorylation, is not yet fully understood.

FUTURE DIRECTIONS

The broad discussion of the multiple aspects of MnSOD regulation, including gene expression, protein modifications, and enzymatic activity, will shed light onto the unknown mechanisms that govern the prosurvival networks involved in cellular and mitochondrial adaptive response to genotoxic environment.

Import of nuclear-encoded mitochondrial proteins: a cotranslational perspective

A growing amount of evidence suggests that the cytosolic translation of nuclear-encoded mitochondrial proteins and their subsequent import into mitochondria are tightly coupled in a process termed cotranslational import. In addition to the original posttranslational view of mitochondrial protein import, early literature also provides both in vitro and in vivo experimental evidence supporting the simultaneous existence of a cotranslational protein-import mechanism in mitochondria. Recent investigations have started to reveal the cotranslational import mechanism which is initiated by transporting either a translation complex or a translationally competent mRNA encoding a mitochondrial protein to the mitochondrial surface. The intracellular localization of mRNA to the mitochondrial surface has emerged as the latest addition to our understanding of mitochondrial biogenesis. It is mediated by targeting elements within the mRNA molecule in association with potential mRNA-binding proteins.

Characterisation of the nascent polypeptide-associated complex in fission yeast

The nascent polypeptide-associated complex (NAC) is an abundant and phylogenetically conserved protein complex. It is composed of two subunits and interacts with nascent polypeptide chains emerging from the ribosome. It has been proposed to protect the nascent chains from premature interaction with other cell proteins, but has also been found to associate with DNA junctions, and to be involved in other processes including transcription regulation and mitochondrial protein import.Here, we characterize NAC in fission yeast. We find that NAC is associated with ribosomes, while a significant fraction remains in a free form. The NAC alpha subunit contains a ubiquitin-associated (UBA) domain, which is found in several proteins involved in the ubiquitin-proteasome pathway for protein degradation. However, NAC does not associate with ubiquitin chains and mutants lacking NAC did not exhibit any obvious defects in protein degradation. Accordingly, we find that the NAC UBA domain belongs to an ancient and distinct subgroup of the UBA family. In contrast to the situation with budding yeast, fission yeast cells devoid of NAC were not temperature sensitive. However, they displayed resistance to the amino acid analogue canavanine, in accordance with the idea that NAC is involved in protein quality control.

The cellular chaperone heat shock protein 90 facilitates Flock House virus RNA replication in Drosophila cells

The assembly of viral RNA replication complexes on intracellular membranes represents a critical step in the life cycle of positive-strand RNA viruses. We investigated the role of the cellular chaperone heat shock protein 90 (Hsp90) in viral RNA replication complex assembly and function using Flock House virus (FHV), an alphanodavirus whose RNA-dependent RNA polymerase, protein A, is essential for viral RNA replication complex assembly on mitochondrial outer membranes. The Hsp90 chaperone complex transports cellular mitochondrial proteins to the outer mitochondrial membrane import receptors, and thus we hypothesized that Hsp90 may also facilitate FHV RNA replication complex assembly or function. Treatment of FHV-infected Drosophila S2 cells with the Hsp90-specific inhibitor geldanamycin or radicicol potently suppressed the production of infectious virions and the accumulation of protein A and genomic, subgenomic, and template viral RNA. In contrast, geldanamycin did not inhibit the activity of preformed FHV RNA replication complexes. Hsp90 inhibitors also suppressed viral RNA and protein A accumulation in S2 cells expressing an FHV RNA replicon. Furthermore, Hsp90 inhibition with either geldanamycin or RNAi-mediated chaperone downregulation suppressed protein A accumulation in the absence of viral RNA replication. These results identify Hsp90 as a host factor involved in FHV RNA replication and suggest that FHV uses established cellular chaperone pathways to assemble its RNA replication complexes on intracellular membranes.

Mature DIABLO/Smac is produced by the IMP protease complex on the mitochondrial inner membrane

DIABLO/Smac is a mitochondrial protein that can promote apoptosis by promoting the release and activation of caspases. To do so, DIABLO/Smac must first be processed by a mitochondrial protease and then released into the cytosol, and we show this in an intact cellular system. We propose that the precursor form of DIABLO/Smac enters the mitochondria through a stop-transfer pathway and is processed to its active form by the inner membrane peptidase (IMP) complex. Catalytic subunits of the mammalian IMP complex were identified based on sequence conservation and functional complementation, and the novel sequence motif RX(5)P in Imp1 and NX(5)S in Imp2 distinguish the two catalytic subunits. DIABLO/Smac is one of only a few specific proteins identified as substrates for the IMP complex in the mitochondrial intermembrane space.

Delivery of nascent polypeptides to the mitochondrial surface

Thousands of polypeptides with diverse biochemical properties, some of which are extremely hydrophobic, are targeted from cytoplasmic ribosomes to the surface of mitochondria. Localised synthesis, as well as transient interactions with a wide array of molecular chaperones and other cytoplasmic factors, can promote productive interaction of mitochondrial proteins with the TOM complex to initiate protein import into mitochondria.

The nascent polypeptide-associated complex (NAC) promotes interaction of ribosomes with the mitochondrial surface in vivo

The nascent polypeptide-associated complex (NAC) is a peripheral component of cytoplasmic ribosomes, and interacts with nascent chains as they leave the ribosome. Yeast mutants lacking NAC translate polypeptides normally, but have fewer ribosomes associated with the mitochondrial surface. The mutants lacking NAC suffer mitochondrial defects and have decreased levels of proteins like fumarase, normally targeted to mitochondria co-translationally. NAC might contribute to a ribosomal environment in which amino-terminal, mitochondrial targeting sequences can effectively adopt their appropriate conformation.

A protein complex containing Tho2, Hpr1, Mft1 and a novel protein, Thp2, connects transcription elongation with mitotic recombination in Saccharomyces cerevisiae

Transcription-induced recombination has been reported in all organisms from bacteria to mammals. We have shown previously that the yeast genes HPR1 and THO2 may be keys to the understanding of transcription-associated recombination, as they both affect transcription elongation and hyper-recombination in a concerted manner. Using a yeast strain that has the wild-type THO2 gene replaced by one encoding a His(6)-HA-tagged version, we have isolated an oligomeric complex containing four proteins: Tho2, Hpr1, Mft1 and a novel protein that we have named Thp2. We have reciprocally identified a complex containing Hpr1, Tho2 and Mft1 using anti-Mft1 antibodies in immunoprecipitation experiments. The protein complex is mainly nuclear; therefore, Tho2 and Hpr1 are physically associated. Like hpr1Delta and tho2Delta cells, mft1Delta and thp2Delta cells show mitotic hyper- recombination and impaired transcription elongation, in particular, through the bacterial lacZ sequence. Hyper-recombination conferred by mft1Delta and thp2Delta is only observed in DNA regions under transcription conditions. We propose that this protein complex acts as a functional unit connecting transcription elongation with the incidence of mitotic recombination.

Targeting and insertion of nuclear-encoded preproteins into the mitochondrial outer membrane

Most mitochondrial proteins are synthesized in the cytosol as preproteins with a cleavable presequence and are delivered to the import receptors on the mitochondria by cytoplasmic import factors. The proteins are then imported to the intramitochondrial compartments by the import systems of the outer and inner membranes, TOM and TIM. Mitochondrial outer membrane proteins are synthesized without a cleavable presequence and most of them contain hydrophobic transmembrane domains, which, in conjunction with the flanking segments, function as the mitochondria import signals. Some of the proteins are inserted into the outer membrane by the TOM machinery; the import signal probably arrests further translocation and is released from the translocation channel to the lipid bilayer. The other proteins are inserted into the membrane by a novel pathway independent of the TOM machinery. This article reviews recent developments in the biogenesis of mitochondrial outer membrane proteins.

Signals and receptors--the translocation machinery on the mitochondrial surface

Most proteins involved in mitochondrial biogenesis are encoded by the genome of the nucleus. They are synthesized in the cytosol and have to be transported toward and, subsequently, imported into the organelle. This targeting and import process is initiated by the specific mitochondrial targeting signal, which differs pending on the final localization of the protein. The preprotein will be recognized by cytosolic proteins, which function in transport toward the mitochondria and in maintaining the import competent state of the preprotein. The precursor will be transferred onto a multicomponent complex on the outer mitochondrial membrane, formed by receptor proteins and the general insertion pore (GIP). Some proteins are directly sorted into the outer membrane whereas the majority will be transported over the outer membrane through the import channel followed by further distribution of those proteins.

Mechanisms of protein translocation into mitochondria

Mitochondrial biogenesis utilizes a complex proteinaceous machinery for the import of cytosolically synthesized preproteins. At least three large multisubunit protein complexes, one in the outer membrane and two in the inner membrane, have been identified. These translocase complexes cooperate with soluble proteins from the cytosol, the intermembrane space and the matrix. The translocation of presequence-containing preproteins through the outer membrane channel includes successive electrostatic interactions of the charged mitochondrial targeting sequence with a chain of import components. Translocation across the inner mitochondrial membrane utilizes the energy of the proton motive force of the inner membrane and the hydrolysis of ATP. The matrix chaperone system of the mitochondrial heat shock protein 70 forms an ATP-dependent import motor by interaction with the polypeptide chain in transit and components of the inner membrane translocase. The precursors of integral inner membrane proteins of the metabolite carrier family interact with newly identified import components of the intermembrane space and are inserted into the inner membrane by a second translocase complex. A comparison of the full set of import components between the yeast Sacccharomyces cerevisiae and the nematode Caenorhabditis elegans demonstrates an evolutionary conservation of most components of the mitochondrial import machinery with a possible greater divergence for the import pathway of the inner membrane carrier proteins.

Nascent polypeptide-associated complex stimulates protein import into yeast mitochondria

To identify yeast cytosolic proteins that mediate targeting of precursor proteins to mitochondria, we developed an in vitro import system consisting of purified yeast mitochondria and a radiolabeled mitochondrial precursor protein whose C terminus was still attached to the ribosome. In this system, the N terminus of the nascent chain was translocated across both mitochondrial membranes, generating a translocation intermediate spanning both membranes. The nascent chain could then be completely chased into the mitochondrial matrix after release from the ribosome. Generation of this import intermediate was dependent on a mitochondrial membrane potential, mitochondrial surface proteins, and was stimulated by proteins that could be released from the ribosomes by high salt. The major salt-released stimulatory factor was yeast nascent polypeptide-associated complex (NAC). Purified NAC fully restored import of salt-washed ribosome-bound nascent chains by enhancing productive binding of the chains to mitochondria. We propose that ribosome-associated NAC facilitates recognition of nascent precursor chains by the mitochondrial import machinery.

Overexpression of yeast karyopherin Pse1p/Kap121p stimulates the mitochondrial import of hydrophobic proteins in vivo

During evolution, cellular processes leading to the transfer of genetic information failed to send all the mitochondrial genes into the nuclear genome. Two mitochondrial genes are still exclusively located in the mitochondrial genome of all living organisms. They code for two highly hydrophobic proteins: the apocytochrome b and the subunit I of cytochrome oxidase. Assuming that the translocation machinery could not efficiently transport long hydrophobic fragments, we searched for multicopy suppressors of this physical blockage. We demonstrated that overexpression of Pse1p/Kap121p or Kap123p, which belong to the superfamily of karyopherin beta proteins, facilitates the translocation of chimeric proteins containing several stretches of apocytochrome b fused to a reporter mitochondrial gene. The effect of PSE1/KAP121 overexpression (in which PSE1 is protein secretion enhancer 1) on mitochondrial import of the chimera is correlated with an enrichment of the corresponding transcript in cytoplasmic ribosomes associated with mitochondria. PSE1/KAP121 overexpression also improves the import of the hydrophobic protein Atm1p, an ABC transporter of the mitochondrial inner membrane. These results suggest that in vivo PSE1/KAP121 overexpression facilitates, either directly or indirectly, the co-translational import of hydrophobic proteins into mitochondria.

A toxic fusion protein accumulating between the mitochondrial membranes inhibits protein assembly in vivo

When overexpressed in Saccharomyces cerevisiae, beta-galactosidase fusion proteins directed to the mitochondria are toxic, preventing growth of yeast cells on non-fermentable carbon sources (Emr, S. D., Vassarotti, A., Garrett, J., Geller, B. L., Takeda, M., and Douglas, M. G. (1986) J. Cell Biol. 102, 523-533). We show that such fusion proteins interfere with the assembly of respiratory complexes in the mitochondrial inner membrane, without blocking protein translocation. The gene YME1, encoding an ATP-dependent metalloprotease of the mitochondrial inner membrane, acts as a suppressor of this defect; a 3-fold overexpression of Yme1p is sufficient to restore respiratory complex assembly and mitochondrial function. Detailed knowledge of the topology and effect of the toxic beta-galactosidase fusion proteins will permit the identification and characterization of components that control protein sorting and protein assembly within the mitochondrial inner membrane.

Role of the negative charges in the cytosolic domain of TOM22 in the import of precursor proteins into mitochondria

TOM22 is an essential mitochondrial outer membrane protein required for the import of precursor proteins into the organelles. The amino-terminal 84 amino acids of TOM22 extend into the cytosol and include 19 negatively and 6 positively charged residues. This region of the protein is thought to interact with positively charged presequences on mitochondrial preproteins, presumably via electrostatic interactions. We constructed a series of mutant derivatives of TOM22 in which 2 to 15 of the negatively charged residues in the cytosolic domain were changed to their corresponding amido forms. The mutant constructs were transformed into a sheltered Neurospora crassa heterokaryon bearing a tom22::hygromycin R disruption in one nucleus. All constructs restored viability to the disruption-carrying nucleus and gave rise to homokaryotic strains containing mutant tom22 alleles. Isolated mitochondria from three representative mutant strains, including the mutant carrying 15 neutralized residues (strain 861), imported precursor proteins at efficiencies comparable to those for wild-type organelles. Precursor binding studies with mitochondrial outer membrane vesicles from several of the mutant strains, including strain 861, revealed only slight differences from binding to wild-type vesicles. Deletion mutants lacking portions of the negatively charged region of TOM22 can also restore viability to the disruption-containing nucleus, but mutants lacking the entire region cannot. Taken together, these data suggest that an abundance of negative charges in the cytosolic domain of TOM22 is not essential for the binding or import of mitochondrial precursor proteins; however, other features in the domain are required.

The yeast nascent polypeptide-associated complex initiates protein targeting to mitochondria in vivo

The yeast nascent polypeptide-associated complex (NAC) is encoded by two genes, EGD1 and EGD2, and is associated with cytoplasmic ribosomes. Yeast mutants lacking NAC (Deltaegd2) are viable but suffer slight defects in the targeting of nascent polypeptides to several locations including the endoplasmic reticulum and mitochondria. If both NAC and Mft52p are missing from yeast cells, inefficient targeting of mitochondrial precursor proteins leads to defects in both mitochondrial function and morphology. We suggest that NAC provides a ribosomal environment for nascent mitochondrial targeting sequences to achieve secondary structure, thereby enhancing the efficiency of protein targeting.

Actin-binding verprolin is a polarity development protein required for the morphogenesis and function of the yeast actin cytoskeleton

Yeast verprolin, encoded by VRP1, is implicated in cell growth, cytoskeletal organization, endocytosis and mitochondrial protein distribution and function. We show that verprolin is also required for bipolar bud-site selection. Previously we reported that additional actin suppresses the temperature-dependent growth defect caused by a mutation in VRP1. Here we show that additional actin suppresses all known defects caused by vrp1-1 and conclude that the defects relate to an abnormal cytoskeleton. Using the two-hybrid system, we show that verprolin binds actin. An actin-binding domain maps to the LKKAET hexapeptide located in the first 70 amino acids. A similar hexapeptide in other acting-binding proteins was previously shown to be necessary for actin-binding activity. The entire 70- amino acid motif is conserved in novel higher eukaryotic proteins that we predict to be actin-binding, and also in the actin-binding proteins, WASP and N-WASP. Verprolin-GFP in live cells has a cell cycle-dependent distribution similar to the actin cortical cytoskeleton. In fixed cells hemagglutinin-tagged Vrp1p often co-localizes with actin in cortical patches. However, disassembly of the actin cytoskeleton using Latrunculin-A does not alter verprolin's location, indicating that verprolin establishes and maintains its location independent of the actin cytoskeleton. Verprolin is a new member of the actin-binding protein family that serves as a polarity development protein, perhaps by anchoring actin. We speculate that the effects of verprolin upon the actin cytoskeleton might influence mitochondrial protein sorting/function via mRNA distribution.

Temporal regulation of in vitro import of precursor proteins into tobacco mitochondria

Protein import into isolated tobacco mitochondria was investigated using mitochondria from leaves harvested at different times of the day and night. Efficient import was only detected with mitochondria isolated from leaves harvested during the dark period of the growth cycle, only low levels of import were detected from leaves harvested during the light period. However, this temporal difference seen in import did not appear to be circadian in nature. This implies that the protein import process in mitochondria isolated from leaves is not constitutive. This has important implications for targeting specificity studies performed in transgenic plants, as unless the plants are tested at the time when import is occurring, the true in vivo targeting abilities of chimeric constructs will not be measured.

The protein encoded by the MFT1 gene is a targeting factor for mitochondrial precursor proteins, and not a core ribosomal protein

Yeast cells harboring mft1 mutations are compromised in mitochondrial protein targeting, and Mft1p has previously been identified as a ribosomal protein. However, two genes, PLC2 and YML062C, are present in the MFT1 locus, and we show that mft1 mutant cells are compromised in the function of the cytosolic protein encoded by YML062C. The ribosomal protein (YS3a) is actually encoded by the tightly linked PLC2 gene, and does not play a role in targeting proteins to the mitochondria.

Highways for protein delivery to the mitochondria

Messenger RNA (mRNA) localisation is one of the prime mechanisms to ensure protein localisation in the cytoplasm of polarised embryonic cells, and has been well-studied in the development of Xenopus and Drosophila embryos. But what of other cells? Here, we discuss whether the directed transport of mRNA out of the nucleus, following cytoplasmic highways to a specified organelle in the cytoplasm, might also contribute to the exquisite fidelity of protein targeting observed in all eukaryotic cells.