Sub-mitochondrial localization of the catalytic subunit of pyruvate dehydrogenase phosphatase.

Using a specific antibody against the PDP catalytic subunit, PDPc, precise localization of this subunit in mitochondria was performed. Sub-fractionation of purified mitochondria by controlled swelling processes led to the isolation of outer membranes, matrix space and inner membrane vesicles which were purified on a sucrose density gradient. In this study, we demonstrated that PDPc was not recovered as a soluble protein in the matrix space but was associated with the inner membrane. Moreover, Triton X-114 phase partitioning performed on inner membranes showed that PDPc behaved both as a hydrophilic and as a hydrophobic protein, thus suggesting two different forms of this enzyme.

The 8 kD product of the putative oncogene MTCP-1 is a mitochondrial protein.

An unusually small (8 kD) protein (p8MTCP-1) is coded by the putative oncogene MTCP-1 (also called c6.1B), involved in the translocation t(X;14)(q28;q11) associated with some mature T-cell proliferations. Here, we show by subcellular fractionation and by confocal microscopy that this protein is located in the mitochondria. This localization orientates toward a role of p8MTCP-1 in the mitochondrial metabolism which may be relevant for the oncogenic process.

Further characterization of mitochondrial outer membrane: evidence for the presence of two endogenous sialylated glycoproteins.

The distribution of sialic acid-containing glycoproteins was investigated in highly purified mitochondrial membranes using labeled Sambucus nigra agglutinin as a detection system. Two sialylated glycoproteins were shown to be true components of the mitochondrial outer membrane. Relative to monoamine oxidase activity, these glycoproteins were found to be preferentially located in the "free" outer membrane fraction. As sialic acid is thought to be involved in molecular recognition, a role for these glycoproteins in mediating the interactions between mitochondria and other sub-cellular organelles is considered.

Involvement of mitochondrial contact sites in the subcellular compartmentalization of phospholipid biosynthetic enzymes.

The concerted synthesis of phospholipids derived from serine involving two microsomal enzymes (phosphatidylserine synthase and phosphatidylethanolamine N-methyltransferase) and a mitochondrial one (phosphatidylserine decarboxylase) occurs in reconstituted cell-free systems. Subfractionation of crude mitochondria after swelling and separating on a sucrose density gradient resulted in the isolation of two contact site-enriched fractions from total outer membranes and inner membranes, respectively. Estimation of marker enzyme activities shows a high recovery of glucose-6-phosphate phosphatase (a marker for the endoplasmic reticulum) associated with contact site-enriched fractions. Accordingly, the linked synthesis of phosphatidylserine, phosphatidylethanolamine, and at a lesser extent phosphatidylcholine can occur. This biosynthetic pathway was absent from purified contact site-enriched fractions correlative with the absence of glucose-6-phosphate phosphatase activity. Reconstitution experiments, including contact site-enriched fractions incubated with endoplasmic reticulum-rich fraction, led to the restoration of the linked synthesis of phospholipids, thereby demonstrating that a reversible association between these two fractions can occur. These functional interactions between the endoplasmic reticulum and mitochondria are confirmed at the ultrastructural level using either chemical or physical fixation before resin embedding. These results show that the interorganelle trafficking of lipids may involve only highly specialized microdomains of both membranes, thereby allowing the maintenance of a specific lipid composition and distribution within membranes.

Evidence for the presence of alpha and beta-related T3 receptors in rat liver mitochondria.

Although the dependence of mitochondrial structure and function on thyroid hormone status is well established, several attempts to demonstrate a direct pathway of T3 action on mitochondria have been made during the last decade without being firmly conclusive. In this study, we present evidence firstly for the presence of specific binding sites for [125I]-T3 in rat liver mitochondria 5 min after injection, as assessed by ultrastructural autoradiography. In the same way, using immunocytological techniques and protein immunoblotting, T3 receptor-like immunoreactivity was revealed mainly in the nucleus and mitochondria of hepatocytes. Whereas the colloidal gold labeling over mitochondria was found to be specific at the ultrastructural level, these results were confirmed biochemically by Western blotting experiments which revealed the presence of two protein bands in mitochondria: a stronger one of 55 kDa and a weaker one of 48 kDa. At the opposite, receptor T3 mRNAs were not detected in mitochondria by ultrastructural in situ hybridization thus confirming that the synthesis of receptor T3 occurs in the cytoplasm and that nuclear-encoded T3 receptors may belong to the bulk of cytosolic precursor polypeptides which are targeted to and imported into mitochondria. These results confirm that a direct pathway of T3 action on mitochondria occurs in situ which could now explain how the rapid activation of several mitochondrial functions can take place within minutes after thyroid hormone injection.

Further evidence for both functional and structural microcompartmentation within the membranes of two associated organelles, mitochondrion and endoplasmic reticulum.

We previously demonstrated that the translocation of phospholipids between the mitochondrion and the endoplasmic reticulum occurs via highly specialized membrane microdomains of both organelles that are in situ closely associated. As understanding of the interactions between both organelles requires characterization of the translocation sites organization, we first analysed the amino acid compositions of these sites. Using principal component analysis, we have shown that the translocation sites exhibit characteristic patterns when compared with the membranes from which they are derived. The results are discussed in terms of both functional and structural microcompartmentation within the membranes of mitochondria and endoplasmic reticulum.

Use of Percoll gradients for isolation of human placenta mitochondria suitable for investigating outer membrane proteins.

Human mitochondria were isolated from placenta by a combination of differential and Percoll gradient centrifugation, resulting in a highly pure and intact preparation as assessed by marker enzyme analysis and electron microscopy. The advantages over previous methods are the rapidity of the procedure and the excellent resolution of mitochondria and lysosomes. Moreover, the high extent of intactness of the mitochondria so obtained made them particularly well suited for investigating outer membrane proteins. Taking advantage of this method, we have purified human mitochondrial porin. The purified protein consists of a single unglycosylated polypeptide of molecular mass 33 kDa.

Phospholipid import into mitochondria: possible regulation mediated through lipid polymorphism.

We previously demonstrated that the translocation of microsomal phosphatidylserine to the inner mitochondrial membrane occurs via contact sites before decarboxylation. According to the specific lipid composition of contact sites, we investigated lipid polymorphism as a possible regulation mechanism of phospholipid import into mitochondria. Phosphatidylserine import into mitochondria is increased in the presence of calcium, under conditions where non bilayer lipid-structures can be induced in cardiolipin-containing membranes. The results are discussed in terms of structural as well as functional domains heterogeneity within contact sites.

Mitochondrial dolichyl-phosphate mannose synthase. Purification and immunogold localization by electron microscopy.

Mitochondrial dolichyl-phosphate mannose synthase has been purified to homogeneity using an original procedure, reconstitution into specific phospholipid vesicles and sedimentation on a sucrose gradient as final step. The enzyme has an apparent molecular mass of 30 kDa on an SDS/polyacrylamide gel. Increased enzyme activity could be correlated with this polypeptide band. A specific antibody was raised in rabbits against this transferase. Specific IgG obtained from the immune serum removed enzymatic activity from a detergent extract of mitochondrial outer membrane and reacted specifically with the 30-kDa band on immunoblots. Furthermore, an immunocytochemical experiment proved the localization of dolichyl-phosphate mannose synthase on the cytosolic face of the outer membrane of mitochondria.

Investigation of glycosylation processes in mitochondria and microsomal membranes from human skeletal muscle.

Glycoconjugates are directly involved in major skeletal muscle functions. As little is known about glycosylation processes in muscle, we investigated glycoconjugate synthesis in subcellular fractions from human skeletal muscle tissue. Mitochondria and microsomal membranes were prepared from muscle biopsies by thorough mechanical disruption and differential centrifugations. This procedure resulted in the isolation of intact mitochondria (1 mg protein/g muscle) and of a microsomal fraction (1.5 mg protein/g muscle). Glycosyltransferases were studied in both subcellular fractions using either dolichylmonophosphate as a polyprenic acceptor or chemically modified fetuin as a glycoprotein substrate. Our results provide evidence for high rates of glycosylation in muscle. The highest activities were obtained with GDP-mannose: dilichylmonophosphate mannosyltransferase, a key enzyme in glycosylation process (220 pmol/mg per h in mitochondria and 1,550 pmol/mg per h in microsomal membranes). Substantial individual variations were observed for dolichol pathway glycosyltransferases but low individual variations were found for glycosyltransferases involved in maturation of glycoproteins. The role which glycosylation defects may play in muscle dysfunction has yet to be defined.

Involvement of contact sites in phosphatidylserine import into liver mitochondria.

The synthesis, translocation, and decarboxylation of phosphatidylserine can occur in a cell-free system (Voelker, D. R. (1989) J. Biol. Chem. 264, 8019-8025). We made use of the spatial separation of the site of biosynthesis and the site of decarboxylation of phosphatidylserine to demonstrate that mitochondrial contact sites are intimately involved in the translocation of phosphatidylserine prior to decarboxylation. In that sense, the inhibition of phosphatidylserine decarboxylase leads to an accumulation of this phospholipid in the contact site-enriched fractions without mixing the inner membrane phospholipid pool. On the other hand, newly synthesized phosphatidylethanolamine can be exported very rapidly to the mitochondrial surface in the same way, i.e. via contact sites. These data provide further evidence for the existence of a structural and functional microcompartmentation at the inner mitochondrial membrane surface.

Further characterization of mitochondrial contact sites: effect of short-chain alcohols on membrane fluidity and activity.

Two mitochondrial contact site-enriched fractions were further characterized by freeze-fracture electron microscopy. Examination of the replicas revealed that these two fractions are in the form of vesicles with membrane sheets attached to the vesicles. The physical state of these fractions has been investigated by means of steady-state fluorescence polarization to assess the effects of alcohols on their fluidity properties and activity. Comparison between intact membranes and their extracted lipids shows that butanol and hexanol induce a differential increase of the overall membrane fluidity in the two contact site-enriched fractions. This alteration in the membrane dynamics leads to a complex modulation of cytochrome c oxidase activity in both fractions. These results bring further evidence for the existence of morphologically and functionally distinct domains in mitochondrial membranes.

Mitochondrial contact sites. Lipid composition and dynamics.

Two membrane fractions of intermediate density between inner and outer mitochondrial membranes were isolated by density gradient centrifugation from osmotically lysed mitochondria and mitoplasts of liver. These fractions were characterized by the presence of both monamine oxidase and cytochrome c oxidase activities and bound hexokinase. 1) The content of the fractions in proteins and lipids was assessed by biochemical determination. Thin-layer and gas-liquid chromatography showed that the two contact site-enriched fractions contain predominantly phosphatidylcholine (31%), phosphatidylethanolamine (27%, half-unsaturated), and cardiolipin (27%, fully unsaturated). 2) The dynamics of the fractions were assessed by fluorescence polarization techniques using 1,6-diphenyl-1,3,5-hexatriene as a probe and by fluorescence decay measurements. We have verified that differences in static anisotropy cannot be exclusively attributed to differences in fluorescence lifetimes. On the contrary, the results indicated an increased lipid mobility in "inner membrane contact sites," which is probably related to a lower cholesterol to phospholipid ratio, as well as a lower saturation of the fatty acyl chains when compared with "outer membrane contact sites." Taken all together, the spectroscopic measurements confirm the biochemical results, leading to the idea that the two populations of contact sites have different physicochemical properties, which are probably mainly determined by the membrane from which they are derived. They constitute microdomains enriched either in inner or outer mitochondrial membranes.

Dolichol kinase activity: a key factor in the control of N-glycosylation in inner mitochondrial membranes.

Inner mitochondrial membranes from liver contain a dolichol kinase which required CTP as a phosphoryl donor. Kinase activity was linear with protein concentration and unlike other reported kinases, activated almost equally well by Mg2+, Mn2+ or Ca2+. Thin-layer chromatography showed that the reaction product co-migrated with authentic dolichyl monophosphate. The phosphorylation of dolichol did not occur in presence of ATP, GTP or UTP but required exogenous dolichol for maximal activity. Newly synthesized [3H]dolichyl monophosphate has been shown to be glycosylated in the presence of GDP[14C]mannose or UDP[14C]glucose. The double labeled lipids formed by the sugar nucleotide-dependent reactions were identified respectively as [14C]mannosylphosphoryl[3H]dolichol and [14C]glucosylphosphoryl [3H]dolichol. These results are discussed in terms of regulation of N-glycosylation processes in inner mitochondrial membranes from liver.

Organization of mitochondrial UDP-glucose:dolichylmonophosphate glucosyltransferase in the outer membrane.

Previous studies have shown the existence of an autonomous mitochondrial UDP-glucose: dolichylmonophosphate glucosyltransferase, located in mitochondrial outer membrane of liver cells. To improve our knowledge about the topographical aspects of glycosylation in mitochondria, we have investigated the organization of this enzyme in intact mitochondria, using controlled proteolysis with trypsin and sensitivity towards amino-acid specific reagents. Our data provides evidence: --for a mitochondrial glucosyltransferase facing the cytoplasmic side of the outer membrane --and for the involvement of histidine and tryptophan residues as well as sulfhydryl groups in the catalytic activity of the enzyme.

Glucosyltransferase activity in mitochondria. Biosynthesis of glucosyl-phosphoryl-dolichol in inner mitochondrial membranes.

1. Inner mitochondrial membranes are able to transfer [14C]glucose from UDP-[14C]glucose onto dolichylmonophosphate. 2. Synthesis of dolichyl-phosphoryl-glucose takes place only in the presence of exogenous dolichyl-monophosphate loaded into phospholipid vesicles. 3. Neutral phospholipids interact preferentially with the membrane-bound enzyme. The effect of phospholipids is not related to the length of fatty acid chains but a correlation between the activation and the degree of unsaturation of fatty acid chains has been found. 4. This enzyme required divalent cations for activity. Such a requirement might be related to lipid-protein interactions which favour a suitable conformation of glycosyltransferase.