The reduction of sulfur and the solution chemistry of the various polysulfide products have been investigated in dimethyl sulfoxide by cyclic voltammetry, controlled-potential electrolysis, and absorption spectroscopy. At a gold electrode sulfur is reduced by two electrons at a potential of -0.6 V vs. see to produce S8 2~a nd other polysulfides. These products yield, by a variety of reactions, a blue radical species, S3~, with an absorption maximum at 618 nm. Electrolytic reduction of sulfur yields a second process at a potential of -1.29 V with an overall stoichiometry of four electrons per Ss molecule; the major stable product is the S4 2~s pecies. The dissociation of S6 2" into S3" has a pX^v alue of 2.11 on the basis of spectrophotometric measurements. Equilibrium constants of other disproportionation and dissociation reactions for the various poly sulfides also have been evaluated.

In many organisms, mitochondria import nuclear DNA-encoded small RNAs. In yeast Saccharomyces cerevisiae, one out of two cytoplasmic isoacceptor tRNAs Lys is partially addressed into the organelle. Mitochondrial targeting of this tRNA was shown to depend on interaction with the precursor of mitochondrial lysyl-tRNA synthetase, preMsk1p. However, preMsk1p alone was unable to direct tRNA targeting, suggesting the existence of additional protein factor(s). Here, we identify the glycolytic enzyme, enolase, as such a factor. We demonstrate that recombinant enolase and preMSK1p are sufficient to direct tRNA import in vitro and that depletion of enolase inhibits tRNA import in vivo. Enzymatic and tRNA targeting functions of enolase appear to be independent. Three newly characterized properties of the enolase can be related to its novel function: (1) specific affinity to the imported tRNA, (2) the ability to facilitate formation of the complex between preMsk1p and the imported tRNA, and (3) partial targeting toward the mitochondrial outer membrane. We propose a model suggesting that the cell exploits mitochondrial targeting of the enolase in order to address the tRNA toward peri-mitochondrially synthesized preMsk1p. Our results indicate an alternative molecular chaperone function of glycolytic enzyme enolase in tRNA mitochondrial targeting.

Cytoplasmic tRNA(Lys)CUU is the only nuclear‐encoded tRNA of Saccharomyces cerevisiae found to be associated with mitochondria. Selective import of this tRNA into isolated organelles requires cytoplasmic factors. Here we identify two of these factors as the cytoplasmic and mitochondrial lysyl‐tRNA synthetases. The cytoplasmic enzyme is obligatory for in vitro import of the deacylated, but not of the aminoacylated tRNA. We thus infer that it is needed for aminoacylation of the tRNA, which is a prerequisite for its import. The mitochondrial synthetase, which cannot aminoacylate tRN(Lys)CUU, is required for import of both aminoacylated and deacylated forms. Its depletion leads to a total arrest of tRNA import, in vitro and in vivo. The mitochondrial lysyl‐tRNA synthetase is able to form specific and stable RNP complexes with the amino‐acylated tRNA. Furthermore, an N‐terminal truncated form of the synthetase which cannot be targeted into mitochondria is unable to direct the import of the tRNA. We therefore hypothesize that the cytosolic precursor form of the mitochondrial synthetase has a carrier function for translocation of the tRNA across the mitochondrial membranes. However, cooperation of the two synthetases is not sufficient to direct tRNA import, suggesting the need of additional factor(s).

Abstract. Arp2p is an essential yeast actin-related protein. Disruption of the corresponding ARP2 gene leads to a terminal phenotype characterized by the presence of a single large bud. Thus, Arp2p may be important for a late stage of the cell cycle (Schwob, E., and R.P. Martin, 1992. Nature (Lond.). 355:179-182). We have localized Arp2p by indirect immunofluorescence. Specific peptide antibodies revealed punctate staining under the plasma membrane, which partially colocalizes with actin. Temperature-sensitive arp2 mutations were created by PCR mutagenesis and selected by an ade2/SUPll sectoring screen. One temperature-sensitive mutant that was characterized, arp2-H330L, was osmosensitive and had an altered actin cytoskeleton at a nonpermissive temperature, suggesting a role of Arp2p in the actin cytoskeleton. Random budding patterns were observed in both haploid and diploid arp2-H330L mutant cells. Endocytosis, as judged by Lucifer yellow uptake, was severely reduced in the mutant, at all temperatures. In addition, genetic interaction was observed between temperature-sensitive alleles arp2-H330L and cdclO-1. CDCIO is a gene encoding a neck filament-associated protein that is necessary for polarized growth and cytokinesis. Overall, the immunolocalization, mutant phenotypes, and genetic interaction suggest that the Arp2 protein is an essential component of the actin cytoskeleton that is involved in membrane growth and polarity, as well as in endocytosis.T HE yeast actin cytoskeleton is essential for maintenance of cell shape, organization and polarized growth of the cell surface, morphogenesis, and cell division (Adams and Pringle, 1984;Kilmartin and Adams, 1984;Novick and Botstein, 1985;Drubin, 1991). Analysis of actin mutants revealed pleiotropic effects on yeast growth and development. Phenotypes such as alteration of the actin distribution, random budding pattern, delocalization of chitin, sensitivity to osmotic pressure, defective septation and nuclear segregation, reduced internalization in endocytosis, and accumulation of secretory vesicles have been demonstrated by analysis of temperature-sensitive (Ts) 1 mutants (Novick and Botstein, 1985;Drubin et al., 1993; Ktibler and Riezman, 1993). While actin has a demonstrated role in all of these processes, different functions may be mediated by interaction with one or several of numerous other cytoskeletal proteins. For example, among genetically redundant cytoskeletal proteins (fimbrin and capping proteins or fimbrin and Abplp), the lack of structural and functional homology has been taken as evidence that these proteins regulate the actin cytoskeleton by different mechanisms (Adams et al., 1993).Whereas classical actins are highly conserved across eukaryotic phyla (e.g., Saccharomyces cerevisiae actin is 88% identical to rabbit skeletal et-actin), more divergent sequences that are homologous to actin have been identified in a number of organisms from yeast to humans . Although the functions of actin and an increasing number of different actin-binding proteins ...

Mitochondrial DNA (mtDNA) mutations are an important cause of human disease for which there is no efficient treatment. Our aim was to determine whether the A8344G mitochondrial tRNA(Lys) mutation, which can cause the MERRF (myoclonic epilepsy with ragged-red fibers) syndrome, could be complemented by targeting tRNAs into mitochondria from the cytosol. Import of small RNAs into mitochondria has been demonstrated in many organisms, including protozoans, plants, fungi and animals. Although human mitochondria do not import tRNAs in vivo, we previously demonstrated that some yeast tRNA derivatives can be imported into isolated human mitochondria. We show here that yeast tRNALys derivatives expressed in immortalized human cells and in primary human fibroblasts are partially imported into mitochondria. Imported tRNAs are correctly aminoacylated and are able to participate in mitochondrial translation. In transmitochondrial cybrid cells and in patient-derived fibroblasts bearing the MERRF mutation, import of tRNALys is accompanied by a partial rescue of mitochondrial functions affected by the mutation such as mitochondrial translation, activity of respiratory complexes, electrochemical potential across the mitochondrial membrane and respiration rate. Import of a tRNALys with a mutation in the anticodon preventing recognition of the lysine codons does not lead to any rescue, whereas downregulation of the transgenic tRNAs by small interfering RNA (siRNA) transiently abolishes the functional rescue, showing that this rescue is due to the import. These findings prove for the first time the functionality of imported tRNAs in human mitochondria in vivo and highlight the potential for exploiting the RNA import pathway to treat patients with mtDNA diseases.

It is impossible to predict which pathway, direct glutaminylation of tRNA Gln or tRNA-dependent transamidation of glutamyl-tRNA Gln , generates mitochondrial glutaminyl-tRNA Gln for protein synthesis in a given species. The report that yeast mitochondria import both cytosolic glutaminyl-tRNA synthetase and tRNA Gln has challenged the widespread use of the transamidation pathway in organelles. Here we demonstrate that yeast mitochondrial glutaminyl-tRNA Gln is in fact generated by a transamidation pathway involving a novel type of trimeric tRNAdependent amidotransferase (AdT). More surprising is the fact that cytosolic glutamyl-tRNA synthetase ( c ERS) is imported into mitochondria, where it constitutes the mitochondrial nondiscriminating ERS that generates the mitochondrial mischarged glutamyl-tRNA Gln substrate for the AdT. We show that dual localization of c ERS is controlled by binding to Arc1p, a tRNA nuclear export cofactor that behaves as a cytosolic anchoring platform for c ERS. Expression of Arc1p is down-regulated when yeast cells are switched from fermentation to respiratory metabolism, thus allowing increased import of c ERS to satisfy a higher demand of mitochondrial glutaminyltRNA Gln for mitochondrial protein synthesis. This novel strategy that enables a single protein to be localized in both the cytosol and mitochondria provides a new paradigm for regulation of the dynamic subcellular distribution of proteins between membrane-separated compartments.

In vivo, human mitochondria import 5 S rRNA and do not import tRNAs from the cytoplasm. We demonstrated previously that isolated human mitochondria are able to internalize a yeast tRNA Lys in the presence of yeast soluble factors. Here, we describe an assay for specific uptake of 5 S rRNA by isolated human mitochondria and compare its requirements with the artificial tRNA import. The efficiency of 5 S rRNA uptake by isolated mitochondria was comparable with that found in vivo. The import was shown to depend on ATP and the transmembrane electrochemical potential and was directed by soluble proteins. Blocking the pre-protein import channel inhibited internalization of both 5 S rRNA and tRNA, which suggests this apparatus be involved in RNA uptake by the mitochondria. We show that human mitochondria can also selectively internalize several in vitro synthesized versions of yeast tRNA Lys as well as a transcript of the human mitochondrial tRNA Lys . Either yeast or human soluble proteins can direct this import, suggesting that human cells possess all factors needed for such an artificial translocation. On the other hand, the efficiency of import directed by yeast or human protein factors varies significantly, depending on the tRNA version. Similarly to the yeast system, tRNA Lys import into human mitochondria depended on aminoacylation and on the precursor of the mitochondrial lysyl-tRNA synthetase. 5 S rRNA import was also dependent upon soluble protein(s), which were distinct from the factors providing tRNA internalization.Mitochondria, although containing their own genome, import the vast majority of their macromolecular components from the cytoplasm. If the mechanisms of pre-protein import are well understood, the import of nuclear-coded RNAs into mitochondria was investigated to a much lesser extent. Targeting of RNA into mitochondria though not universal is widely spread among organisms (1-5). Mitochondrial import of transfer RNAs was found in plants, protists, some lower animals, and fungi. The number of imported tRNA species varies from one (in yeast) to the totality (in trypanosomatids), and tRNA import mechanisms seem to differ from one organism to another. We have shown previously that, in the yeast Saccharomyces cerevisiae, import of a single tRNA CUU Lys (further referred to as tRK1) 1 occurs via formation of a complex with the precursor form of the mitochondrial lysyl-tRNA synthetase (pre-MSK) and requires the intactness of pre-protein import apparatus (6 -8).In mammalians, no tRNA import has been reported, but several other RNAs are thought to be targeted into mitochondria. One of these is the RNA component of RNase MRP, a site-specific endoribonuclease supposed to be involved in primer RNA cleavage during replication of mitochondrial DNA and to be present in the organelle in a very low amount (9). It was hypothesized that the process of mitochondrial DNA replication requires a very low number of MRP RNA molecules per mitochondrial genome (10 -12). The presence of MRP RNA in the mitochondria was recent...

Actin, a major cytoskeletal component of all eukaryotic cells, is one of the most highly conserved proteins. It is involved in various cellular processes such as motility, cytoplasmic streaming, chromosome segregation and cytokinesis. The actin from the yeast Saccharomyces cerevisiae, encoded by the essential ACT1 gene, is 89% identical to mouse cytoplasmic actin and is involved in the organization and polarized growth of the cell surface. We report here the characterization of ACT2, a previously undescribed yeast split gene encoding a putative protein (391 amino acids, relative molecular mass (Mr) 44,073) that is 47% identical to yeast actin. The requirement of the ACT2 gene for vegetative growth of yeast cells and the existence of related genes in other eukaryotes indicate an important and conserved role for these actin-like proteins. Superimposition of the Act2 polypeptide onto the three-dimensional structure of known actins reveals that most of the divergence occurred in loops involved in actin polymerization, DNase I and myosin binding, leaving the core domain mainly unaffected. To our knowledge, the Act2 protein from S. cerevisiae is the first highly divergent actin molecule described. Structural and physiological data suggest that the Act2 protein might have an important role in cytoskeletal reorganization during the cell cycle.