A morphological view on mitochondrial protein targeting

Stomatin-like protein 2 deficiency results in impaired mitochondrial translation

Mitochondria translate the RNAs for 13 core polypeptides of respiratory chain and ATP synthase complexes that are essential for the assembly and function of these complexes. This process occurs in close proximity to the mitochondrial inner membrane. However, the mechanisms and molecular machinery involved in mitochondrial translation are not fully understood, and defects in this process can result in severe diseases. Stomatin-like protein (SLP)-2 is a mainly mitochondrial protein that forms cardiolipin- and prohibitin-enriched microdomains in the mitochondrial inner membrane that are important for the formation of respiratory supercomplexes and their function. Given this regulatory role of SLP-2 in processes closely associated with the mitochondrial inner membrane, we hypothesized that the function of SLP-2 would have an impact on mitochondrial translation. ³⁵S-Methionine/cysteine pulse labeling of resting or activated T cells from T cell-specific Slp-2 knockout mice showed a significant impairment in the production of several mitochondrial DNA-encoded polypeptides following T cell activation, including Cytb, COXI, COXII, COXIII, and ATP6. Measurement of mitochondrial DNA stability and mitochondrial transcription revealed that this impairment was at the post-transcriptional level. Examination of mitochondrial ribosome assembly showed that SLP-2 migrated in sucrose-density gradients similarly to the large ribosomal subunit but that its deletion at the genetic level did not affect mitochondrial ribosome assembly. Functionally, the impairment in mitochondrial translation correlated with decreased interleukin-2 production in activated T cells. Altogether, these data show that SLP-2 acts as a general regulator of mitochondrial translation.

Mitochondrial pleomorphy in plant cells is driven by contiguous ER dynamics

Mitochondria are pleomorphic, double membrane-bound organelles involved in cellular energetics in all eukaryotes. Mitochondria in animal and yeast cells are typically tubular-reticulate structures and several micro-meters long but in green plants they are predominantly observed as 0.2-1.5 μm punctae. While fission and fusion, through the coordinated activity of several conserved proteins, shapes mitochondria, the endoplasmic reticulum (ER) has recently been identified as an additional player in this process in yeast and mammalian cells. The mitochondria-ER relationship in plant cells remains largely uncharacterized. Here, through live-imaging of the entire range of mitochondria pleomorphy we uncover the underlying basis for the predominantly punctate mitochondrial form in plants. We demonstrate that mitochondrial morphology changes in response to light and cytosolic sugar levels in an ER mediated manner. Whereas, large ER polygons and low dynamics under dark conditions favor mitochondrial fusion and elongation, small ER polygons result in increased fission and predominantly small mitochondria. Hypoxia also reduces ER dynamics and increases mitochondrial fusion to produce giant mitochondria. By observing elongated mitochondria in normal plants and fission-impaired Arabidopsis nmt1-2 and drp3a mutants we also establish that thin extensions called matrixules and a beads-on-a-string mitochondrial phenotype are direct consequences of mitochondria-ER interactions.

A matter of life, death and diseases: mitochondria from a proteomic perspective

Mitochondria are highly ordered, integrated organelles that energize cellular activities and contribute to programmed death by initiating disciplined apoptotic cascades. This review seeks to clarify our understanding of mitochondrial structural-functional integrity beyond the resolved nuclear genome by unraveling the dynamic mitochondrial proteome and elucidating proteome/genome interplay. The roles of mechanochemical coupling between mitoskeleton and cytoskeleton and crosstalk with other organelles in orchestrating cellular outcomes are explained. The authors also review the modulation of mitochondrial-related oxidative stress on apoptosis and cancer development and the context is applied to interpret pathogenetic events in neurodegenerative disorders and cardiovascular diseases. The accumulated proteomics evidence is used to describe the integral role that mitochondria play and how they influence other intracellular organelles. Possible mitochondrial-targeted therapeutic interventions are also discussed.

Biogenesis of mitochondrial proteins

Depending on the organism, mitochondria consist approximately of 500-1,400 different proteins. By far most of these proteins are encoded by nuclear genes and synthesized on cytosolic ribosomes. Targeting signals direct these proteins into mitochondria and there to their respective subcompartment: the outer membrane, the intermembrane space (IMS), the inner membrane, and the matrix. Membrane-embedded translocation complexes allow the translocation of proteins across and, in the case of membrane proteins, the insertion into mitochondrial membranes. A small number of proteins are encoded by the mitochondrial genome: Most mitochondrial translation products represent hydrophobic proteins of the inner membrane which-together with many nuclear-encoded proteins-form the respiratory chain complexes. This chapter gives an overview on the mitochondrial protein translocases and the mechanisms by which they drive the transport and assembly of mitochondrial proteins.

Single molecule tracking fluorescence microscopy in mitochondria reveals highly dynamic but confined movement of Tom40

Tom40 is an integral protein of the mitochondrial outer membrane, which as the central component of the Translocase of the Outer Membrane (TOM) complex forms a channel for protein import. We characterize the diffusion properties of individual Tom40 molecules fused to the photoconvertable fluorescent protein Dendra2 with millisecond temporal resolution. By imaging individual Tom40 molecules in intact isolated yeast mitochondria using photoactivated localization microscopy with sub-diffraction limited spatial precision, we demonstrate that Tom40 movement in the outer mitochondrial membrane is highly dynamic but confined in nature, suggesting anchoring of the TOM complex as a whole.

Muscle intermediate filaments and their links to membranes and membranous organelles

Intermediate filaments (IFs) play a key role in the integration of structure and function of striated muscle, primarily by mediating mechanochemical links between the contractile apparatus and mitochondria, myonuclei, the sarcolemma and potentially the vesicle trafficking apparatus. Linkage of all these membranous structures to the contractile apparatus, mainly through the Z-disks, supports the integration and coordination of growth and energy demands of the working myocyte, not only with force transmission, but also with de novo gene expression, energy production and efficient protein and lipid trafficking and targeting. Desmin, the most abundant and intensively studied muscle intermediate filament protein, is linked to proper costamere organization, myoblast and stem cell fusion and differentiation, nuclear shape and positioning, as well as mitochondrial shape, structure, positioning and function. Similar links have been established for lysosomes and lysosome-related organelles, consistent with the presence of widespread links between IFs and membranous structures and the regulation of their fusion, morphology and stabilization necessary for cell survival.

Preconditioning: The Mitochondrial Connection

Over the past decade there has been considerable progress in elucidating the signaling pathways involved in cardioprotection. Considerable recent data suggest that many of these signaling pathways converge on the mitochondria, where such pathways alter the activity of key mitochondrial proteins, leading to reduced apoptosis and necrosis. Inhibition of the mitochondrial permeability transition pore is emerging as a central mechanism in cardioprotection. This review focuses on mechanisms by which cardioprotection alters mitochondrial proteins and channels that regulate cell death and survival.

Dynamic subcompartmentalization of the mitochondrial inner membrane

The inner membrane of mitochondria is organized in two morphologically distinct domains, the inner boundary membrane (IBM) and the cristae membrane (CM), which are connected by narrow, tubular cristae junctions. The protein composition of these domains, their dynamics, and their biogenesis and maintenance are poorly understood at the molecular level. We have used quantitative immunoelectron microscopy to determine the distribution of a collection of representative proteins in yeast mitochondria belonging to seven major processes: oxidative phosphorylation, protein translocation, metabolite exchange, mitochondrial morphology, protein translation, iron-sulfur biogenesis, and protein degradation. We show that proteins are distributed in an uneven, yet not exclusive, manner between IBM and CM. The individual distributions reflect the physiological functions of proteins. Moreover, proteins can redistribute between the domains upon changes of the physiological state of the cell. Impairing assembly of complex III affects the distribution of partially assembled subunits. We propose a model for the generation of this dynamic subcompartmentalization of the mitochondrial inner membrane.

Bcl-2 regulation of mitochondrial energetics

Recent data suggest that in addition to regulating apoptosis, Bcl-2 (an anti-apoptotic protein overexpressed in B-cell lymphoma) and Bcl-2 family members also regulate mitochondrial and cell physiology. t-Bid, a Bcl-2 family member, has been shown to modulate reorganization of mitochondrial cristae. Bcl-2 appears to regulate voltage-dependent anion channel permeability, which has important consequences for mitochondrial transport of adenine nucleotides, Ca(2+), and other metabolites. BAD, a pro-apoptotic Bcl-2 family member, is required for the binding of glucokinase to a mitochondrial complex, and BAD null mice have altered glucose homeostasis. It has been suggested that Bcl-2 family members may regulate important mitochondrial/cell functions and serve as sentinels to detect abnormalities in these pathways and, when the abnormalities are severe enough, to initiate or facilitate cell death. Understanding the physiologic processes controlled by Bcl-2 will be important in understanding cell regulation, and it may also provide new insights into the regulation of apoptosis.

The relevance of mitochondrial membrane topology to mitochondrial function

This review summarizes recent findings from electron tomography about the three-dimensional shape of mitochondrial membranes and its possible influence on a range of mitochondrial functions. The inner membrane invaginations called cristae are pleomorphic, typically connected by narrow tubular junctions of variable length to the inner boundary membrane. This design may restrict intra-mitochondrial diffusion of metabolites such as ADP, and of soluble proteins such as cytochrome c. Tomographic images of a variety of mitochondria suggest that inner membrane topology reflects a balance between membrane fusion and fission. Proteins that can affect cristae morphology include tBid, which triggers cytochrome c release in apoptosis, and the dynamin-like protein Mgm1, involved in inter-mitochondrial membrane fusion. In frozen-hydrated rat-liver mitochondria, the space between the inner and outer membranes contains 10-15 nm particles that may represent macromolecular complexes involved in activities that span the two membranes.

The m-AAA protease defective in hereditary spastic paraplegia controls ribosome assembly in mitochondria

AAA proteases comprise a conserved family of membrane bound ATP-dependent proteases that ensures the quality control of mitochondrial inner-membrane proteins. Inactivation of AAA proteases causes pleiotropic phenotypes in various organisms, including respiratory deficiencies, mitochondrial morphology defects, and axonal degeneration in hereditary spastic paraplegia (HSP). The molecular basis of these defects, however, remained unclear. Here, we describe a regulatory role of an AAA protease for mitochondrial protein synthesis in yeast. The mitochondrial ribosomal protein MrpL32 is processed by the m-AAA protease, allowing its association with preassembled ribosomal particles and completion of ribosome assembly in close proximity to the inner membrane. Maturation of MrpL32 and mitochondrial protein synthesis are also impaired in a HSP mouse model lacking the m-AAA protease subunit paraplegin, demonstrating functional conservation. Our findings therefore rationalize mitochondrial defects associated with m-AAA protease mutants in yeast and shed new light on the mechanism of axonal degeneration in HSP.

Stability and Association with the Cytomatrix of Mitochondrial DNA in Spontaneously Immortalized Mouse Embryo Fibroblasts Containing or Lacking the Intermediate Filament Protein Vimentin

To extend previous observations demonstrating differences in number, morphology, and activity of mitochondria in spontaneously immortalized vim(+) and vim(-) fibroblasts derived from wild-type and vimentin knockout mice, some structural and functional aspects of mitochondrial genome performance and integrity in both types of cells were investigated. Primary Vim(+/+) and Vim(-/-) fibroblasts, which escaped terminal differentiation by immortalization were characterized by an almost twofold lower mtDNA content in comparison to that of their primary precursor cells, whereby the average mtDNA copy number in two clones of vim(+) cells was lower by a factor of 0.6 than that in four clones of vim(-) cells. However, during serial subcultivation up to high passage numbers, the vim(+) and vim() fibroblasts increased their mtDNA copy number 1.5- and 2.5-fold, respectively. While early-passage cells of the vim(+) and vim(-) fibroblast clones differed only slightly in the ratio between mtDNA content and mitochondrial mass represented by mtHSP70 protein, after ca. 300 population doublings the average mtDNA/mtmass ratio in the vim(+) and vim() cells was increased by a factor of 2 and 4.5, respectively. During subcultivation, both types of cells acquired the fully transformed phenotype. These findings suggest that cytoskeletal vimentin filaments exert a strong influence on the mechanisms controlling mtDNA copy number during serial subcultivation of immortalized mouse embryo fibroblasts, and that vimentin deficiency causes a disproportionately enhanced mtDNA content in high-passage vim(-) fibroblasts. Such a role of vimentin filaments was supported by the stronger retention potential for mtDNA and mtDNA polymerase (gamma) detected in vim(+) fibroblasts by Triton X-100 extraction of mitochondria and agaroseembedded cells. Moreover, although the vim(+) and vim(-) fibroblasts were equally active in generating free radicals, the vim(-) cells exhibited higher levels of immunologically detectable 8-oxoG and mismatch repair proteins MSH2 and MLH1 in their mitochondria. Because in vim(-) fibroblasts only one point mutation was detected in the mtDNA D-loop control region, these cells are apparently able to efficiently remove oxidatively damaged nucleobases. On the other hand, a number of large-scale mtDNA deletions were found in high-passage vim(-) fibroblasts, but not in low-passage vim(-) cells and vim(+) cells of both low and high passage. Large mtDNA deletions were also induced in young vim(-) fibroblasts by treatment with the DNA intercalator ethidium bromide, whereas no such deletions were found after treatment of vim(+) cells. These results indicate that in immortalized vim(-) fibroblasts the mitochondrial genome is prone to large-scale rearrangements, probably due to insufficient control of mtDNA repair and recombination processes in the absence of vimentin.

Bcl-2 overexpression corrects mitochondrial defects and ameliorates inherited desmin null cardiomyopathy

One of the hallmarks of cardiomyopathy and heart failure is pronounced and progressive cardiomyocyte death. Understanding the mechanisms involved in cardiomyocyte cell death is a topic of great interest for treatment of cardiac disease. Mice null for desmin, the muscle-specific member of the intermediate filament gene family, develop cardiomyopathy characterized by extensive cardiomyocyte death, fibrosis, calcification, and eventual heart failure. The earliest ultrastructural defects are observed in mitochondria. In the present study, we have demonstrated that these mitochondrial abnormalities are the primary cause of the observed cardiomyopathy and that these defects can be ameliorated by overexpression of bcl-2 in desmin null heart. Overexpression of bcl-2 in the desmin null heart results in correction of mitochondrial defects, reduced occurrence of fibrotic lesions in the myocardium, prevention of cardiac hypertrophy, restoration of cardiomyocyte ultrastructure, and significant improvement of cardiac function. Furthermore, we have found that loss of desmin also diminishes the capacity of mitochondria to resist exposure to calcium, a defect that can be partially restored by bcl-2 overexpression. These results point to a unique function for desmin in protection of mitochondria from calcium exposure that can be partially rescued by overexpression of bcl-2. We show that bcl-2 cardiac overexpression has provided significant improvement of an inherited form of cardiomyopathy, revealing the potential for bcl-2, and perhaps other genes in the family, as therapeutic agents for heart disease of many types, including inherited forms.

Insight into mitochondrial structure and function from electron tomography

In recent years, electron tomography has provided detailed three-dimensional models of mitochondria that have redefined our concept of mitochondrial structure. The models reveal an inner membrane consisting of two components, the inner boundary membrane (IBM) closely apposed to the outer membrane and the cristae membrane that projects into the matrix compartment. These two components are connected by tubular structures of relatively uniform size called crista junctions. The distribution of crista junction sizes and shapes is predicted by a thermodynamic model based upon the energy of membrane bending, but proteins likely also play a role in determining the conformation of the inner membrane. Results of structural studies of mitochondria during apoptosis demonstrate that cytochrome c is released without detectable disruption of the outer membrane or extensive swelling of the mitochondrial matrix, suggesting the formation of an outer membrane pore large enough to allow passage of holo-cytochrome c. The possible compartmentation of inner membrane function between the IBM and the cristae membrane is also discussed.

Contact sites between the outer and inner membrane of mitochondria-role in protein transport

Many essential functions of mitochondrial metabolism have been studied in the past three decades in considerable depth: oxidative phosphorylation, catabolism of fatty acids, role in nitrogen metabolism, and amino acid metabolism. More recently, other aspects attracted much attention like protein translocation into mitochondria, inheritance of mitochondrial DNA, movement of mitochondria, their fusion and fission, and their involvement in apoptosis, ageing, cancer and other cellular processes. Together with these new views on the function of mitochondria, new ideas on the structure of mitochondria emerged. Here we will discuss the current knowledge about how the membranes of mitochondria are organized and how they interact. Interactions between components of the inner and the outer membrane are necessary for a number of central mitochondrial functions such as the channeling of metabolites, coordinated fusion and fission of mitochondria, and protein transport. Some of these interactions appear stable such as the so-called morphological contact sites; others are quite dynamic. Direct evidence that a certain protein is part of morphologically defined contact sites is lacking. Nevertheless, protein translocase complexes of the outer and the inner membrane exhibit stable interactions between the two membranes when precursor proteins are arrested during import into mitochondria. Finally, we discuss possible roles of cristae junctions, another morphologically defined membrane structure in mitochondria.

Isolation of hepatic mitochondrial contact sites: previously unrecognized inner membrane components

An improved, fast, and relatively simple procedure for isolation of hepatic mitochondrial contact sites is described. These contact sites include conventional outer membrane, but the inner membrane component (which we term fusion patches) has a unique biochemical composition characterized by a clustering of three specific inner membrane proteins of 54, 52, and 31 kDa identified by proteomics, respectively, as the alpha and beta subunits of ATP synthase and the liver isoform of adenine nucleotide transferase. The contact site fraction was prepared using a discontinuous sucrose gradient from crude outer membranes derived from swollen/shrunk rat liver mitochondria. The resultant contact sites were analyzed using a continuous sucrose density gradient, revealing an apparent heterogeneity due to varying amounts of retained fusion patches in relation to the unvarying outer membrane component. By electron microscopy, contact sites consist of small vacuoles that contain one or several tiny vesicles, many of which are composed of multiple, closely packed lamellae. The contact site subfraction morphology is consistent with the biochemical variation. Thus, contact sites are not haphazard fusions of outer and inner membrane, but consist in part of regions of inner membrane of novel composition (fusion patches) and of conventional outer membrane.

Mitochondrial dysfunction in cardiac disease: ischemia--reperfusion, aging, and heart failure

Mitochondria contribute to cardiac dysfunction and myocyte injury via a loss of metabolic capacity and by the production and release of toxic products. This article discusses aspects of mitochondrial structure and metabolism that are pertinent to the role of mitochondria in cardiac disease. Generalized mechanisms of mitochondrial-derived myocyte injury are also discussed, as are the strengths and weaknesses of experimental models used to study the contribution of mitochondria to cardiac injury. Finally, the involvement of mitochondria in the pathogenesis of specific cardiac disease states (ischemia, reperfusion, aging, ischemic preconditioning, and cardiomyopathy) is addressed.

Connection of the mitochondrial outer and inner membranes by Fzo1 is critical for organellar fusion

Mitochondrial membrane fusion is a process essential for the maintenance of the structural integrity of the organelle. Since mitochondria are bounded by a double membrane, they face the challenge of fusing four membranes in a coordinated manner. We provide evidence that this is achieved by coupling of the mitochondrial outer and inner membranes by the mitochondrial fusion machinery. Fzo1, the first known mediator of mitochondrial fusion, spans the outer membrane twice, exposing a short loop to the intermembrane space. The presence of the intermembrane space segment is required for the localization of Fzo1 in sites of tight contact between the mitochondrial outer and inner membranes. Mutations in the intermembrane space domain of yeast Fzo1 relieve the association with the inner membrane. This results in a loss of function of the protein in vivo. We propose that the mitochondrial fusion machinery forms membrane contact sites that mediate mitochondrial fusion. A fusion machinery that is in contact with both mitochondrial membranes appears to be functionally important for coordinated fusion of four mitochondrial membranes.

Fatty acid import into mitochondria

The mitochondrial carnitine system plays an obligatory role in beta-oxidation of long-chain fatty acids by catalyzing their transport into the mitochondrial matrix. This transport system consists of the malonyl-CoA sensitive carnitine palmitoyltransferase I (CPT-I) localized in the mitochondrial outer membrane, the carnitine:acylcarnitine translocase, an integral inner membrane protein, and carnitine palmitoyltransferase II localized on the matrix side of the inner membrane. Carnitine palmitoyltransferase I is subject to regulation at the transcriptional level and to acute control by malonyl-CoA. The N-terminal domain of CPT-I is essential for malonyl-CoA inhibition. In liver CPT-I activity is also regulated by changes in the enzyme's sensitivity to malonyl-CoA. As fluctuations in tissue malonyl-CoA content are parallel with changes in acetyl-CoA carboxylase activity, which in turn is under the control of 5'-AMP-activated protein kinase, the CPT-I/malonyl-CoA system is part of a fuel sensing gauge, turning off and on fatty acid oxidation depending on the tissue's energy demand. Additional mechanism(s) of short-term control of CPT-I activity are emerging. One proposed mechanism involves phosphorylation/dephosphorylation dependent direct interaction of cytoskeletal components with the mitochondrial outer membrane or CPT-I. We have proposed that contact sites between the outer and inner mitochondrial membranes form a microenvironment which facilitates the carnitine transport system. In addition, this system includes the long-chain acyl-CoA synthetase and porin as components.

Mitochondria-targeted GFP highlights the heterogeneity of mitochondrial shape, size and movement within living plant cells

Little is known concerning the heterogeneity of mitochondrial shape, size, number, cytoplasmic distribution, and motility in planta. Ultrastructural studies using the electron microscope have shown a variety of mitochondrial shapes and sizes within fixed cells, however, it is not possible to dismiss the possibility that any heterogeneity observed resulted from preparation or fixation artefacts. Unambiguous demonstration of the extent and nature of mitochondrial heterogeneity in vivo necessitates the use of a truly in vivo mitochondrial detection system. Green fluorescent protein is an excellent in vivo marker for gene expression and protein localization studies. It is particularly useful for real-time spatiotemporal analysis of intracellular protein targeting and dynamics and as such is an ideal marker for analysing mitochondria in planta. Stably transformed Arabidopsis lines have been generated with GFP targeted to the mitochondria using either of two plant mitochondrial signal sequences from the beta-ATPase subunit or the mitochondrial chaperonin CPN-60. Mitochondrially targeted GFP, which is easily detectable using an epifluorescent or confocal microscope, highlights heterogeneity of mitochondrial shape, size, position, and dynamic within living plant cells.

Recent structural insight into mitochondria gained by microscopy

Novel applications of microscopy have recently provided new insights into mitochondrial structures. Diverse techniques such as high resolution scanning electron microscopy, transmission electron microscopy, electron microscope tomography and light microscopy have contributed a better understanding of mitochondrial compartmentalization, dynamic networks of mitochondria, intermembrane bridges, segregation of mitochondrial DNA and contacts with the endoplasmic reticulum among other aspects. This review focuses on advances reported in the last five years concerning aspects of mitochondrial substructure or dynamics gained through new techniques, whether they be novel microscope methods or new ways to prepare or label specimens. Sometimes these advances have produced surprising results and more often than not, they have challenged current conceptions of how mitochondria work.

Mitochondrial cristae diversity in human Leydig cells: a revised look at cristae morphology in these steroid-producing cells

Mitochondria of steroid-producing cells are integrally involved with steroidogenesis. For decades, the mitochondrial morphology of Leydig cells, as with other steroid-producing cells, has been known to differ from typical mitochondria in that the cristae are predominately "tubular." In a few species, humans being one example, the cristae have often been further categorized as "tubular and/or lamellar," without further elaboration. In the present study, mitochondria of human Leydig cells were examined with the purpose of providing a more detailed description of "cristae" morphology in these steroid-producing cells. The cristae are found to be rather diverse in morphology, consisting of elements of anastomosing tubules in continuity with small cisternal regions as well as with stacked arrays of lamellae, referred to as "lamellar associations." The tubular elements are found to branch in a tripartite fashion and sometimes to expand into small cisternal elements at these junctures. The lamellar associations are a distinctive feature of cristae in human Leydig cells and consist of two to eight closely apposed lamellae with a consistent gap of approximately 4 nm between the membranes of apposing lamellae. Such a close association of cellular membranes is highly suggestive of an integral transmembrane linkage. Although the lamellar associations often appear isolated, evidence is present of a continuity of this compartment of the cristae with the tubular elements. The connections (termed "initial segments") of the various forms of the cristae to the inner mitochondrial membrane are typically via tubules. Mitochondria exhibiting a central region of matrix delineated by one or more cup-shaped lamellae are also present. The pleomorphic structure of mitochondrial cristae in human Leydig cells reemphasizes our present lack of knowledge of how subcellular structure relates to steroidogenesis.

Electron tomography of mitochondria from brown adipocytes reveals crista junctions

Electron microscope tomography was used to examine the membrane topology of brown adipose tissue (BAT) mitochondria prepared by cryofixation or chemical fixation techniques. These mitochondria contain an uncoupling protein which results in the conversion of energy from electron transport into heat. The three-dimensional reconstructions of BAT mitochondria provided a view of the inner mitochondrial membrane different in important features from descriptions found in the literature. The work reported here provides new insight into BAT mitochondria architecture by identifying crista junctions, including multiple junctions connecting a crista to the same side of the inner boundary membrane, in a class of mitochondria that have no tubular cristae, but only lamellar cristae. Crista junctions were defined previously as the tubular membranes of relatively uniform diameter that connect a crista membrane with the inner boundary membrane. We have also found that the cristae architecture of cryofixed mitochondria, including crista junctions, is similar to that found in chemically fixed mitochondria, suggesting that this architecture is not a fixation artifact. The stacks of lamellar cristae extended through more of the BAT mitochondrial volume than did the cristae we observed in neuronal mitochondria. Hence, the inner membrane surface area was larger in the former. In chemically fixed mitochondria, contact sites were easily visualized because the outer and inner boundary membranes were separated by an 8 nm space. However, in cryofixed mitochondria almost all the outer membrane was observed to be in close contact with the inner boundary membrane.

Population heterogeneity of higher-plant mitochondria in structure and function

Mitochondria of rapidly developing mungbean seedlings were fractionated into four populations: two density classes, each from a 1500S and a 150S pellet. Each of the four populations exhibited cytochrome c oxidase (COX) activity and contained mitochondrial DNA and cardiolipin; plastid and glyoxysome content were found to be relatively low. Five mitochondrial membrane proteins, COXII/III, ATPase alpha/beta and porin, and a matrix enzyme, manganese superoxide dismutase (MnSOD), were detected by immunoblots in all four populations. Another matrix enzyme, pyruvate dehydrogenase was detected only in the two respiratory-competent 1500S populations. The two 150S populations contained a previously unidentified organelle that lacked demonstrable respiratory capability. This organelle, which we have tentatively referred to as "slow-sedimenting (ss-) mitochondrion", was small in size (below light-optics resolution, 70-300nm, majority < or =200nm) and possessed a peculiar looking boundary membrane, ribosomes, and an occasional prominent electron-dense spot. Characteristically, ss-mitochondria were almost always in contact with a filament-aligned membrane-like structure of varying length. Cristae structure, while undetected in small ss-mitochondria, appeared in larger individuals. Typical mitochondria were found in the denser 1500S population, while the lighter 1500S population consisted of 300-800 nm mitochondria exhibiting a varying degree of size-dependent inner membrane folding. Using electron microscopy (EM) immunolocalization and serial sectioning, we have identified in situ organelles resembling in size and in fine structure the ss-mitochondria, which also exhibit a size-dependent folding of the inner membrane. These results suggest that small ss-mitochondria may undergo a progressive development in situ. Taken together, our findings demonstrate the existence of a pattern of structure-function-coordinated gross heterogeneity among mitochondria. This pattern of mitochondrial heterogeneity, characterized both in isolated mitochondria and in situ, implies that small ss-mitochondria may represent a type of "nascent mitochondria" derived from a yet unidentified mitochondria-propagation mode operating during rapid seedling growth. Mitochondrial division by binary fission, characterized by the appearance of dumbbell-shaped intermediates, was also detected.

Functional cooperation and stoichiometry of protein translocases of the outer and inner membranes of mitochondria

The qualitative relationship between preprotein translocases in the mitochondrial outer and inner membranes was determined by both a functional analysis and a determination of characteristic components of the translocases. Translocation contact sites of isolated mitochondria were saturated with intermediates of a matrix-targeted precursor of the beta-subunit of the F1-ATPase (pF1beta), and import of preproteins into the different mitochondrial subcompartments was monitored. A strong inhibition (75-95%) was observed for preproteins with an N-terminal matrix targeting signal, indicating that a significant portion of the contact sites was blocked by accumulated F1beta. Insertion of preproteins into the outer membrane and import into the intermembrane space of preproteins without matrix targeting signals was inhibited by about 45%, indicating that functional outer membrane translocases were available despite saturation of contact sites. Similarly, import of members of the mitochondrial carrier family into the inner membrane was only partly inhibited (40-50%), demonstrating that functional Tim22 translocases were available to cooperate with the Tom machinery in the import of carrier proteins. The stoichiometry of Tom40, Tim23, and Tim22 in mitochondria was determined to be 5:1:0.22. We conclude that translocases of the outer membrane are present in excess over translocases of the inner membrane.

Electron tomography of neuronal mitochondria: three-dimensional structure and organization of cristae and membrane contacts

The structure of neuronal mitochondria from chick and rat was examined using electron microscope tomography of chemically fixed tissue embedded in plastic and sliced in approximately 500 nm-thick sections. Three-dimensional reconstructions of representative mitochondria were made from single-axis tilt series acquired with an intermediate voltage electron microscope (400 kV). The tilt increment was either 1 degree or 2 degrees ranging from -60 degrees to +60 degrees. The mitochondrial ultrastructure was similar across species and neuronal regions. The outer and inner membranes were each approximately 7 nm thick. The inner boundary membrane was found to lie close to the outer membrane, with a total thickness across both membranes of approximately 22 nm. We discovered that the inner membrane invaginates to form cristae only through narrow, tubular openings, which we call crista junctions. Sometimes the cristae remain tubular throughout their length, but often multiple tubular cristae merge to form lamellar compartments. Punctate regions, approximately 14 nm in diameter, were observed in which the inner and outer membranes appeared in contact (total thickness of both membranes approximately 14 nm). These contact sites are known to a play a key role in the transport of proteins into the mitochondrion. It has been hypothesized that contact sites may be proximal to crista junctions to facilitate transport of proteins destined for the cristae. However, our statistical analyses indicated that contact sites are randomly located with respect to these junctions. In addition, a close association was observed between endoplasmic reticulum membranes and the outer mitochondrial membrane, consistent with the reported mechanism of transport of certain lipids into the mitochondrion.

In vivo zippering of inner and outer mitochondrial membranes by a stable translocation intermediate

It was previously assumed that the import of cytoplasmically synthesized precursor proteins into mitochondria occurs through a single structure spanning both outer and inner membranes at contact sites. Based on recent findings, however, the two membranes appear to contain independent translocation elements that reversibly cooperate during protein import. This feature makes it difficult to generate a means of isolating a fully integrated and functional translocation complex. To study these independent translocases in vitro and in vivo, we have constructed a chimeric protein consisting of an N-terminal authentic mitochondrial precursor (delta1-pyrroline-5-carboxylate dehydrogenase) linked, through glutathione S-transferase, to IgG binding domains derived from staphylococcal protein A. This construct becomes trapped en route to the matrix, spanning both outer and inner membranes in such a way that the entire signal-less delta1-pyrroline-5-carboxylate dehydrogenase moiety reaches the matrix, while only the folded protein A domain remains outside. During in vivo import of this precursor, outer and inner membranes of yeast mitochondria become progressively "zippered" together, forming long stretches of close contact. Using this novel intermediate, the outer and inner mitochondrial membrane channels, which normally interact only transiently, can be tightly joined (both in vitro and in vivo), forming a stable association. This suggests a method for isolating the functional translocation complex as a single entity.

Protein import into mitochondria

Mitochondria import many hundreds of different proteins that are encoded by nuclear genes. These proteins are targeted to the mitochondria, translocated through the mitochondrial membranes, and sorted to the different mitochondrial subcompartments. Separate translocases in the mitochondrial outer membrane (TOM complex) and in the inner membrane (TIM complex) facilitate recognition of preproteins and transport across the two membranes. Factors in the cytosol assist in targeting of preproteins. Protein components in the matrix partake in energetically driving translocation in a reaction that depends on the membrane potential and matrix-ATP. Molecular chaperones in the matrix exert multiple functions in translocation, sorting, folding, and assembly of newly imported proteins.

Import and insertion of proteins into the mitochondrial outer membrane

Nuclear-encoded proteins destined for insertion into the mitochondrial outer membrane, follow the same general pathway for import as proteins that are translocated to interior compartments within the organelle. This observation is true both for beta-barrel-type proteins and for proteins that contain hydrophobic alpha-helical transmembrane segments. In this review, we describe what is known about the various steps leading to protein insertion into the outer membrane, and discuss the energetics that favor vectorial translocation into and across this membrane. The selection of the outer membrane during import may involve a lateral release of the translocating polypeptide from the import machinery so that the appropriate domains of the protein become embedded in the lipid bilayer. One type of topogenic domain that can guarantee such selection of the outer membrane is a signal-anchor sequence of the type characterized for the bitopic protein Mas70p. It is suggested that a signal-anchor sequence selective for the mitochondrial outer membrane causes abrogation of polypeptide translocation and triggers the release of the transmembrane segment into the surrounding lipid bilayer, prior to any possibility for the commitment of translocation to the interior of the organelle. Specific structural features of the signal-anchor sequence specify its orientation in the membrane, and can confer on this sequence the ability to form homo-oligomers and hetero-oligomers. Strategies other than a signal-anchor sequence may be employed by other classes of proteins for selection of the outer-membrane. Of note is the ability of the outer-membrane import machinery to catalyze integration of the correct set of proteins into the outer-membrane bilayer, while allowing proteins that are destined for integration into the bilayer of the inner membrane to pass through unimpeded. Again, however, different proteins may employ different strategies. One model proposes that this can be accomplished by a combination of a matrix-targeting signal and a distal stop-transfer sequence. In this model, the formation of contact sites, which is triggered when the matrix-targeting signal engages the import machinery of the inner membrane, may prevent the outer-membrane translocon from recognizing and responding to the downstream stop-transfer domain. This allows the transmembrane segment to pass across the outer-membrane, and subsequently integrate into the inner membrane.

Significance of plasmolysis spaces as markers for periseptal annuli and adhesion sites

During hyperosmotic shock, the protoplast and stretched-out peptidoglycan layer first shrink together until the turgor pressure in the cell is relieved. Being non-compressible, the outer and inner membranes must fold their superfluous surfaces. While the protoplast contracts further, the inner membrane rearranges into plasmolysis spaces visible by phase-contrast microscopy. Two opposing theories predict a similar positioning of spaces in dividing cells and filaments: the 'periseptal annulus model', based on adhesion zones, involved in the predetermination of the division site; and a 'restricted, random model', based on physical properties of the protoplast. Strong osmotic shock causes retraction of the inner membrane over almost the entire surface forming the so-called 'Bayer bridges'. These tubular adhesion sites are preserved by chemical fixation, and can be destroyed by cryofixation and freeze-substitution of unfixed cells. Both the regular positioning of the plasmolysis spaces and the occurrence of tubular adhesion sites can be explained on the basis of physical properties of the membrane which necessitate rearrangements by membrane flow during shrinkage of the protoplast.