Fine-structure analysis of intercellular and intracellular mitochondrial diversity in Saccharomyces cerevisiae

Pathways shaping the mitochondrial inner membrane

Mitochondria are complex organelles with two membranes. Their architecture is determined by characteristic folds of the inner membrane, termed cristae. Recent studies in yeast and other organisms led to the identification of four major pathways that cooperate to shape cristae membranes. These include dimer formation of the mitochondrial ATP synthase, assembly of the mitochondrial contact site and cristae organizing system (MICOS), inner membrane remodelling by a dynamin-related GTPase (Mgm1/OPA1), and modulation of the mitochondrial lipid composition. In this review, we describe the function of the evolutionarily conserved machineries involved in mitochondrial cristae biogenesis with a focus on yeast and present current models to explain how their coordinated activities establish mitochondrial membrane architecture.

Methods to study the biogenesis of membrane proteins in yeast mitochondria

The biogenesis of mitochondrial membrane proteins is an intricate process that relies on the import and submitochondrial sorting of nuclear-encoded preproteins and on the synthesis of mitochondrial translation products in the matrix. Subsequently, these polypeptides need to be inserted into the outer and the inner membranes of the organelle where many of them assemble into multisubunit complexes. In this chapter we provide established protocols to study these different processes experimentally using mitochondria of budding yeast. In particular, methods are described in detail to purify mitochondria, to study mitochondrial protein synthesis, to follow the import of radiolabeled preproteins into isolated mitochondria, and to assess membrane association and the aggregation of mitochondrial proteins by fractionation. These protocols and a list of dos and don'ts shall enable beginners and experienced scientists to address the targeting and assembly of mitochondrial membrane proteins.

The effect of anthralin (dithranol) on mitochondria

The short-term effect of topical application of anthralin (dithranol) on normal human skin was investigated by electron microscopy. Mitochondria appeared markedly damaged. By contrast other cellular structures, particularly the nuclear and cytoplasmic membranes were unchanged. In vitro experiments were therefore performed on isolated rat liver mitochondria and it was shown that anthralin acts as an uncoupler of oxidative phosphorylation. These results suggest that anthralin can inhibit the adenosine triphosphate supply in epidermal cells. This loss of energy supply in keratinocytes could explain, at least in part, the therapeutic efficiency of anthralin in psoriasis.

Anthralin: historical and current perspectives

Anthralin was first synthesized in 1916. Earlier, a natural product, chrysarobin, originally derived from the South American araroba tree, had been used to treat psoriasis. Anthralin was first used in Germany, and later in the Ingram regimen in Britain, but it has never been popular with American dermatologists. This is probably due to the side effects of staining and irritation of the skin. Attempts to reduce these using low concentration, short contact therapy, and concomitant steroid therapy, have been only partially successful. It may be that better instruction of patients and physicians will lead to wider use of this effective topical agent for the treatment of psoriasis. The mode of action of anthralin is thought to be either through its effect on deoxyribonucleic acid (DNA), probably mitochondrial DNA, which reduces cell turnover, or through its effects on various enzyme systems, including those of polyamine synthesis and respiration. The aims of this review are to discuss historical aspects of anthralin and to update its chemistry, pharmacology, and clinical usage.

Cytoplasmic inheritance in Saccharomyces cerevisiae: comparison of zygotic mitochondrial inheritance patterns

Mitochondrial movements in Saccharomyces cerevisiae (Sc) zygotes were monitored with phase-contrast microscopy and compared to known mitochondrial inheritance systems. The mitochondria of Sc were convincingly identified by integrated use of phase-contrast, cytochemical and electron microscopic observations. Mitochondria in Sc appear to move by saltatory jumps, which appear to be oriented towards movement of mitochondria into developing buds. Tracking of mitochondria of different genotypes was made possible by positive identification of each mitochondrial population before zygosis, and by the low degree of mixing (less than 10%) of mitochondrial populations before first bud septation. A grande by grande cross demonstrated equal numbers of mitochondria from each haploid moving into the first zygotic bud. A grande by neutral petite cross gave a 2:1 ratio of grande to petite mitochondria. However, a grande by suppressive petite cross gave equal numbers of grande and petite mitochondria. Using drug resistance systems, a comparison was made of highly biased (97%) and moderately biased (71%) chloramphenicol resistant inheritance patterns. In both cases, the ratios of drug resistant to sensitive mitochondria were 1:1. When numbers of mitochondria moving into an individual bud were compared to the phenotypic content of the clone of that bud, no model could be constructed which could predict the latter from the former. The data indicate (with the exception of the neutral petite by grande cross) that the numbers of each mitochondrial type "inserted" into the first zygotic bud are equal, regardless of the degree of asymmetry of inheritance of mitochondrial markers.

Size-separation of yeast mitochondria in the zonal centrifuge

Mitochondria, released from yeast spheroplasts and subjected to rate separation through sorbitol gradients in the zonal centrifuge, migrated in a wide symmetrical zone. Electron micrographs showed that the mitochondria had been resolved within the zone according to size. The mean mitochondrial diameter at the leading edge was approximately twice that at the trailing edge of the particle zone. Activities of the enzymes cytochrome oxidase, malate dehydrogenase, and reduced nicotinamide adenine dinucleotide- and d-lactate cytochrome c reductases were essentially uniform throughout the mitochondrial zone. Mitochondria from a vegetative-petite mutant had almost the same size distribution as the isogenic wild type, but with somewhat larger mean diameter and either absent or markedly reduced enzyme activities. Mixtures of wild-type and petite mitochondria produced sedimentation profiles showing overlap of particle populations with respect to mean sedimentation rates and mitochondrial diameters, as well as intermediate levels of enzyme activities. Both cristate and noncristate organelles were present throughout the mitochondrial zone from these mixtures. Mitochondria centrifuged in sorbitol density gradients were well-preserved and yielded consistent sedimentation profiles, whereas particles in sucrose density gradients migrated more slowly, produced varied sedimentation profiles, and often showed spurious peaks, presumably due to particle aggregations.