The inner membrane of the mitochondrion folds inwards, forming the cristae. This folding allows a greater amount of membrane to be packed into the mitochondrion. The data in this study demonstrate that subunits e and g of the mitochondrial ATP synthase are involved in generating mitochondrial cristae morphology. These two subunits are non-essential components of ATP synthase and are required for the dimerization and oligomerization of ATP synthase. Mitochondria of yeast cells de®cient in either subunits e or g were found to have numerous digitations and onion-like structures that correspond to an uncontrolled biogenesis and/or folding of the inner mitochondrial membrane. The present data show that there is a link between dimerization of the mitochondrial ATP synthase and cristae morphology. A model is proposed of the assembly of ATP synthase dimers, taking into account the oligomerization of the yeast enzyme and earlier data on the ultrastructure of mitochondrial cristae, which suggests that the association of ATP synthase dimers is involved in the control of the biogenesis of the inner mitochondrial membrane. Keywords: ATP synthase oligomer/mitochondria/ morphology/yeast IntroductionThe mitochondrion is referred to as the`power house' of the cell, because it is responsible for the synthesis of the majority of ATP under aerobic conditions. The inner membrane of the mitochondrion contains the components of the electron transport chain. Oxidation/reduction reactions along the components of the electron transport chain generate a proton gradient that is used by ATP synthase to phosphorylate ADP, thereby producing ATP. To increase the capacity of the mitochondrion to synthesize ATP, the inner membrane is folded to form cristae. These folds allow a much greater amount of electron transport chain enzymes and ATP synthase to be packed into the mitochondrion. However, until now, little was known about how the inner membrane is able to form cristae. This study provides evidence that subunits of ATP synthase are involved in cristae formation.ATP synthase, or F 1 F 0 ATP synthase, is composed of a hydrophilic catalytic unit (F 1 ), which is located in the mitochondrial matrix, and a membranous domain (F 0 ), which anchors the enzyme in the inner mitochondrial membrane and mediates the conduction of protons that participate indirectly in ATP synthesis (Fillingame, 1999;Pedersen et al., 2000). Electron microscopy of negatively stained mitochondria revealed 9 nm diameter projections in the mitochondrial matrix (Ferna Ândez-Mora Ân, 1962), which were identi®ed as the hydrophilic catalytic units (F 1 ) of the F 1 F 0 ATP synthase (Racker et al., 1965). These projections were observed by electron microscopy to be arranged in a non-random, tightly ordered pattern on tubular cristae in Paramecium multimicronucleatum mitochondria using rapid techniques of freezing followed by fracturing, etching and replication (Allen et al., 1989). In this organism, the F 1 complexes are arranged as a double row of particles along the full length ...

The N-terminal portion of the mitochondrial b-subunit is anchored in the inner mitochondrial membrane by two hydrophobic segments. We investigated the role of the first membrane-spanning segment, which is absent in prokaryotic and chloroplastic enzymes. In the absence of the first membrane-spanning segment of the yeast subunit (subunit 4), a strong decrease in the amount of subunit g was found. The mutant ATP synthase did not dimerize or oligomerize, and mutant cells displayed anomalous mitochondrial morphologies with onion-like structures. This phenotype is similar to that of the null mutant in the ATP20 gene that encodes subunit g, a component involved in the dimerization/oligomerization of ATP synthase. Our data indicate that the first membrane-spanning segment of the mitochondrial b-subunit is not essential for the function of the enzyme since its removal did not directly alter the oxidative phosphorylation. It is proposed that the unique membrane-spanning segment of subunit g and the first membrane-spanning segment of subunit 4 interact, as shown by cross-linking experiments. We hypothesize that in eukaryotic cells the b-subunit has evolved to accommodate the interaction with the g-subunit, an associated ATP synthase component only present in the mitochondrial enzyme.

We have used a combination of freeze-substitution electron microscopy and specific reaction for polysaccharides to re-evaluate glycogen structures in Saccharomyces cerevisiae. We also used mutant cells devoid of glycogen to confirm the glycogenic nature of the structures described. Previously described cytoplasmic aggregates were confirmed as glycogen granules. Moreover, an original structure was discovered. This is a ring of glycogen surrounding a finger- or pleat-like plasma membrane invagination. This structure could be physiologically significant in terms of channelling glucose to or from glycogen reserves.