Are all mitochondrial translation products synthesized on membrane-bound ribosomes?

Molecular crowding inhibits U-insertion/deletion RNA editing in vitro: consequences for the in vivo reaction

Mitochondrial pre-mRNAs in African trypanosomes are edited to generate functional transcripts. The reaction is typified by the insertion and deletion of U nucleotides and is catalyzed by a macromolecular complex, the editosome. Editosomes bind pre-edited mRNA/gRNA pairs and the reaction can be recapitulated in vitro by using pre-mRNA- and gRNA-mimicking oligoribonucleotides together with enriched editosome preparations. Although the in vitro assay has been instrumental in unraveling the basic steps of the editing cycle it is performed at dilute solvent conditions. This ignores the fact that editing takes place inside the highly crowded mitochondria. Here we investigate the effects of molecular crowding on RNA editing. By using neutral, macromolecular cosolutes we generate defined dilute, semidilute and crowded solvent properties and we demonstrate different thermodynamic stabilities of the pre-mRNA/gRNA hybrid RNAs at these conditions. Crowded conditions stabilize the RNAs by -30 kJ/mol. Furthermore, we show that the rate constants for the association and dissociation (kass/kdiss) of substrate RNAs to editosomes decrease, ultimately inhibiting the in vitro reaction. The data demonstrate that the current RNA editing in vitro system is sensitive to molecular crowding, which suggests that the in vivo reaction cannot rely on a diffusion-controlled, collision-based mechanism. Possible non-diffusional reaction pathways are discussed.

Mitochondrial protein synthesis, import, and assembly

The mitochondrion is arguably the most complex organelle in the budding yeast cell cytoplasm. It is essential for viability as well as respiratory growth. Its innermost aqueous compartment, the matrix, is bounded by the highly structured inner membrane, which in turn is bounded by the intermembrane space and the outer membrane. Approximately 1000 proteins are present in these organelles, of which eight major constituents are coded and synthesized in the matrix. The import of mitochondrial proteins synthesized in the cytoplasm, and their direction to the correct soluble compartments, correct membranes, and correct membrane surfaces/topologies, involves multiple pathways and macromolecular machines. The targeting of some, but not all, cytoplasmically synthesized mitochondrial proteins begins with translation of messenger RNAs localized to the organelle. Most proteins then pass through the translocase of the outer membrane to the intermembrane space, where divergent pathways sort them to the outer membrane, inner membrane, and matrix or trap them in the intermembrane space. Roughly 25% of mitochondrial proteins participate in maintenance or expression of the organellar genome at the inner surface of the inner membrane, providing 7 membrane proteins whose synthesis nucleates the assembly of three respiratory complexes.

The yeast GTPase Mtg2p is required for mitochondrial translation and partially suppresses an rRNA methyltransferase mutant, mrm2

The assembly of ribosomes involves the coordinated processing and modification of rRNAs with the temporal association of ribosomal proteins. This process is regulated by assembly factors such as helicases, modifying enzymes, and GTPases. In contrast to the assembly of cytoplasmic ribosomes, there is a paucity of information concerning the role of assembly proteins in the biogenesis of mitochondrial ribosomes. In this study, we demonstrate that the Saccharomyces cerevisiae GTPase Mtg2p (Yhr168wp) is essential for mitochondrial ribosome function. Cells lacking MTG2 lose their mitochondrial DNA, giving rise to petite cells. In addition, cells expressing a temperature-sensitive mgt2-1 allele are defective in mitochondrial protein synthesis and contain lowered levels of mitochondrial ribosomal subunits. Significantly, elevated levels of Mtg2p partially suppress the thermosensitive loss of mitochondrial DNA in a 21S rRNA methyltransferase mutant, mrm2. We propose that Mtg2p is involved in mitochondrial ribosome biogenesis. Consistent with this role, we show that Mtg2p is peripherally localized to the mitochondrial inner membrane and associates with the 54S large ribosomal subunit in a salt-dependent manner.

Evidence that synthesis of the Saccharomyces cerevisiae mitochondrially encoded ribosomal protein Var1p may be membrane localized

The 5'-untranslated leaders of mitochondrial mRNAs appear to localize translation within the organelle. VAR1 is the only yeast mitochondrial gene encoding a major soluble protein. A chimeric mRNA bearing the VAR1 untranslated regions and the coding sequence for pre-Cox2p appears to be translated at the inner membrane surface. We propose that translation of the ribosomal protein Var1p is also likely to occur in close proximity to the inner membrane.

Multiple interactions involving the amino-terminal domain of yeast mtRNA polymerase determine the efficiency of mitochondrial protein synthesis

The amino-terminal domain (ATD) of Saccharomyces cerevisiae mitochondrial RNA polymerase has been shown to provide a functional link between transcription and post-transcriptional events during mitochondrial gene expression. This connection is mediated in large part by its interactions with the matrix protein Nam1p and, based on genetic phenotypes, the mitochondrial membrane protein Sls1p. These observations led us to propose previously that mtRNA polymerase, Nam1p, and Sls1p work together to coordinate transcription and translation of mtDNA-encoded gene products. Here we demonstrate by specific labeling of mitochondrial gene products in vivo that Nam1p and Sls1p indeed work together in a pathway that is required globally for efficient mitochondrial translation. Likewise, mutations in the ATD result in similar global reductions in mitochondrial translation efficiency and sensitivity to the mitochondrial translation inhibitor erythromycin. These data, coupled with the observation that the ATD is required to co-purify Sls1p in association with mtDNA nucleoids, suggest that efficient expression of mtDNA-encoded genes in yeast involves a complex series of interactions that localize active transcription complexes to the inner membrane in order to coordinate translation with transcription.

Mitochondrial ribosomal proteins (MRPs) of yeast

Mitochondrial ribosomal proteins (MRPs) are the counterparts in that organelle of the cytoplasmic ribosomal proteins in the host. Although the MRPs fulfil similar functions in protein biosynthesis, they are distinct in number, features and primary structures from the latter. Most progress in the eludication of the properties of individual MRPs, and in the characterization of the corresponding genes, has been made in baker's yeast (Saccharomyces cerevisiae). To date, 50 different MRPs have been determined, although biochemical data and mutational analysis propose a total number which is substantially higher. Surprisingly, only a minority of the MRPs that have been characterized show significant sequence similarities to known ribosomal proteins from other sources, thus limiting the deduction of their functions by simple comparison of amino acid sequences. Further, individual MRPs have been characterized functionally by mutational studies, and the regulation of expression of MRP genes has been described. The interaction of the mitochondrial ribosomes with transcription factors specific for individual mitochondrial mRNAs, and the communication between mitochondria and the nucleus for the co-ordinated expression of ribosomal constituents, are other aspects of current MRP research. Although the mitochondrial translational system is still far from being described completely, the yeast MRP system serves as a model for other organisms, including that of humans.

A morphological view on mitochondrial protein targeting

Mitochondrial protein targeting includes both intramitochondrial sorting of proteins encoded by the organellar genome and import and subsequent sorting of nuclear encoded precursor proteins. Only a few proteins are encoded by the mitochondrial genome and synthesized in the organellar matrix. These include predominantly inner membrane proteins that are perhaps co-translationally inserted into this membrane. Biochemical data suggest that insertion into the inner membrane may be confined to the inner boundary membrane. Ultrastructurally, however, a preferential association of ribosomes with either inner boundary or cristae membranes has not been established. The majority of the mitochondrial proteins are nuclear encoded and synthesized as precursors in the cytosol. Electron microscopic studies revealed that import of precursor proteins is generally confined to sites where both mitochondrial envelope membranes are closely apposed. In line with these observations, biochemical studies indicated that precursor proteins destined for the inner membrane or matrix have to interact with the energized inner membrane to allow complete passage of the precursor through the outer membrane. As a consequence, the mitochondrial envelope membranes have to be in close proximity at protein import sites. In isolated mitochondria distinct sites (designated as contact sites) exist where both envelope membranes are closely apposed and presumably stably associated. In situ, however, mitochondrial boundary membranes are in close proximity over large areas that cover almost the entire mitochondrial periphery. Consequently, the relative area of the mitochondrial surface, where both boundary membranes are in sufficient proximity for allowing protein translocation, is generally larger in situ compared to that in isolated organelles. Immunocytochemical localization studies showed a rather random distribution of components of the mitochondrial protein translocation machinery over the entire mitochondrial surface and not confined to contact sites. Based on these ultrastructural data and recent biochemical findings we propose that mitochondrial protein import sites are dynamic in nature and include relatively labile regions of close association of the boundary membranes. In vitro, however, mitochondrial protein import may preferentially take place at or near the presumably stable contact sites.

Protein synthesis in mitochondria isolated from human skeletal muscle. Detection of polymorphism in mitochondrial translation products

The importance of the mitochondrial protein synthesizing system in the development of functional mitochondria, and thus presumably in the pathogenesis of mitochondrial cytopathies has become apparent in recent years. A procedure has been developed to allow the measurement of the protein synthetic activity in mitochondria isolated from human skeletal muscle biopsy materials. The examination of the mitochondrial protein synthesis products revealed two polymorphic variants with the electrophoretic mobilities in SDS-polyacrylamide gel of a 20/22 kDa and a 45/47 kDa protein. Since functional consideration suggests that these variants are most likely to be associated with conservative amino acid substitutions, the observation indicates that it might be possible to electrophoretically detect certain alterations in the mitochondrial translation products in mitochondrial cytopathies due to mutations in the mitochondrial genome.

Current perspectives in the study of human mitochondriopathies

Recent advances in the study of human mitochondriopathies are discussed, and the impact that modern molecular-biology techniques are likely to have on the understanding of both the disease mechanisms and the basic mechanisms of mitochondrial assembly are reviewed.