Cyclic AMP receptor protein from yeast mitochondria: submitochondrial localization and preliminary characterization

The Yeast Saccharomyces cerevisiae as a Model for Understanding RAS Proteins and their Role in Human Tumorigenesis

The exploitation of the yeast Saccharomyces cerevisiae as a biological model for the investigation of complex molecular processes conserved in multicellular organisms, such as humans, has allowed fundamental biological discoveries. When comparing yeast and human proteins, it is clear that both amino acid sequences and protein functions are often very well conserved. One example of the high degree of conservation between human and yeast proteins is highlighted by the members of the RAS family. Indeed, the study of the signaling pathways regulated by RAS in yeast cells led to the discovery of properties that were often found interchangeable with RAS proto-oncogenes in human pathways, and vice versa. In this work, we performed an updated critical literature review on human and yeast RAS pathways, specifically highlighting the similarities and differences between them. Moreover, we emphasized the contribution of studying yeast RAS pathways for the understanding of human RAS and how this model organism can contribute to unveil the roles of RAS oncoproteins in the regulation of mechanisms important in the tumorigenic process, like autophagy.

A mitochondrial CO2-adenylyl cyclase-cAMP signalosome controls yeast normoxic cytochrome c oxidase activity

Mitochondria, the major source of cellular energy in the form of ATP, respond to changes in substrate availability and bioenergetic demands by employing rapid, short-term, metabolic adaptation mechanisms, such as phosphorylation-dependent protein regulation. In mammalian cells, an intramitochondrial CO2-adenylyl cyclase (AC)-cyclic AMP (cAMP)-protein kinase A (PKA) pathway regulates aerobic energy production. One target of this pathway involves phosphorylation of cytochrome c oxidase (COX) subunit 4-isoform 1 (COX4i1), which modulates COX allosteric regulation by ATP. However, the role of the CO2-sAC-cAMP-PKA signalosome in regulating COX activity and mitochondrial metabolism and its evolutionary conservation remain to be fully established. We show that in Saccharomyces cerevisiae, normoxic COX activity measured in the presence of ATP is 55% lower than in the presence of ADP. Moreover, the adenylyl cyclase Cyr1 activity is present in mitochondria, and it contributes to the ATP-mediated regulation of COX through the normoxic subunit Cox5a, homologue of human COX4i1, in a bicarbonate-sensitive manner. Furthermore, we have identified 2 phosphorylation targets in Cox5a (T65 and S43) that modulate its allosteric regulation by ATP. These residues are not conserved in the Cox5b-containing hypoxic enzyme, which is not regulated by ATP. We conclude that across evolution, a CO2-sAC-cAMP-PKA axis regulates normoxic COX activity.

ASC1/RAS2 suppresses the growth defect on glycerol caused by the atp1-2 mutation in the yeast Saccharomyces cerevisiae

To better define the regulatory role of the F(1)-ATPase alpha-subunit in the catalytic cycle of the ATP synthase complex, we isolated suppressors of mutations occurring in ATP1, the gene for the alpha-subunit in Saccharomyces cerevisiae. First, two atp1 mutations (atp1-1 and atp1-2) were characterized that prevent the growth of yeast on non-fermentable carbon sources. Both mutants contained full-length F(1)alpha-subunit proteins in mitochondria, but in lower amounts than that in the parental strain. Both mutants exhibited barely measurable F(1)-ATPase activity. The primary mutations in atp1-1 and atp1-2 were identified as Thr(383) --> Ile and Gly(291) --> Asp, respectively. From recent structural data, position 383 lies within the catalytic site. Position 291 is located near the region affecting subunit-subunit interaction with the F(1)beta-subunit. An unlinked suppressor gene, ASC1 (alpha-subunit complementing) of the atp1-2 mutation (Gly(291) --> Asp) restored the growth defect phenotype on glycerol, but did not suppress either atp1-1 or the deletion mutant Deltaatp1. Sequence analysis revealed that ASC1 was allelic with RAS2, a G-protein growth regulator. The introduction of ASC1/RAS2 into the atp1-2 mutant increased the F(1)-ATPase enzyme activity in this mutant when the transformant was grown on glycerol. The possible mechanisms of ASC1/RAS2 suppression of atp1-2 are discussed; we suggest that RAS2 is part of the regulatory circuit involved in the control of F(1)-ATPase subunit levels in mitochondria.

Analysis of the mechanism of activation of cAMP-dependent protein kinase through the study of mutants of the yeast regulatory subunit

Spontaneous mutations in the gene which encodes the regulatory subunit of cAMP-dependent protein kinase (PKA) of Saccharomyces cerevisiae (BCY1) have been isolated previously [Cannon, J. F., Gibbs, J. B. & Tatchell, K. (1986) Genetics 113, 247-264] by selection of ras2::LEU2 revertants that grew on non-fermentable carbon sources. The revertants were placed into groups of increasing severity based on the number of PKA-dependent traits affected [Cannon, J. F., Gitan, R. & Tatchell, K. (1990) J. Biol. Chem. 265, 11897-11904]. In this work the ras2 mutation has been crossed out in each bcy1 allele and the phenotypes of these mutants have been assessed. The order of severity of the mutants in both genetic backgrounds is maintained but the severity of each mutant in the normal background is higher than in the ras2::LEU2 background. Total catalytic-subunit and regulatory-subunit activities were measured in crude extracts of the bcy1 ras2::LEU2 mutants. With one exception (bcy1-6) the calculated regulatory subunit/catalytic subunit ratios of the bcy1 mutants relative to that of wild-type cells were greater than one. The dependence of PKA activity on cAMP was measured in permeabilized cells. The strains show an activity ratio in the absence and presence of cAMP in the range 0.5-1 for Kemptide phosphorylation. Overexpression of the high-affinity cAMP phosphodiesterase gene (PDE2) in the bcy1 ras2::LEU2 strains did not alter their PKA-dependent phenotypes. However, transformants were not observed from the parental ras2::LEU2 strain and the bcy1-6 ras2::LEU2 strain. The results are discussed with respect to a hypothesis for the molecular mechanism of the differential reversal of ras2 phenotypes by the bcy1 alleles. Mutations in the regulatory subunit are predicted to affect the structure of the holoenzyme such that the catalytic subunit is capable of maintaining an active catalytic state, without the need to dissociate from the regulatory subunit.

Strain-dependent occurrence of functional GTP:AMP phosphotransferase (AK3) in Saccharomyces cerevisiae

The gene for yeast GTP:AMP phosphotransferase (PAK3) was found to encode a nonfunctional protein in 10 laboratory strains and one brewers' strain. The protein product showed high similarity to vertebrate AK3 and was located exclusively in the mitochondrial matrix. The deduced amino acid sequence revealed a protein that was shorter at the carboxyl terminus than all other known adenylate kinases. Introduction of a +1 frameshift into the 3'-terminal region of the gene extended homology of the deduced amino acid sequence to other members of the adenylate kinase family including vertebrate AK3. Frameshift mutations obtained after in vitro and in vivo mutagenesis were capable of complementing the adk1 temperature-conditional deficiency in Escherichia coli, indicating that the frameshift led to the expression of a protein that could phosphorylate AMP. Some yeasts, however, including strain D273-10B, two wine yeasts, and two more distantly related yeast genera, harbored an active allele, named AKY3, which contained a +1 frameshift close to the carboxyl terminus as compared with the laboratory strains. The encoded protein exhibited GTP:AMP and ITP:AMP phosphotransferase activities but did not accept ATP as phosphate donor. Although single copy in the haploid genome, disruption of the AKY3 allele displayed no phenotype, excluding the possibility that laboratory and brewers' strains had collected second site suppressors. It must be concluded that yeast mitochondria can completely dispense with GTP:AMP phosphotransferase activity.

Transcription of the yeast mitochondrial genome requires cyclic AMP

Using various mutant strains and nutritional manipulations, we investigated a potential role for cyclic AMP (cAMP) in the regulation of mitochondrial (mt) gene expression in the yeast Saccharomyces cerevisiae. In RAS mutants known to have either abnormally low or high cellular levels of this nucleotide, we show that both mt transcription rate and overall mt transcript levels vary directly with cellular cAMP levels. We further show that nutritional downshift of actively growing cells causes a severe, rapid fall in cAMP levels, and that this fall is concomitant with the stringent mt transcriptional curtailment that we and others have previously shown to follow this nutritional manipulation. In in vitro mt transcription assays using intact organelles from downshifted and actively growing cells, stringently curtailed mt gene expression can be restored to 75% of control levels by addition of cAMP to the assay mix. Consistent with these observations a RAS2vall9 mutant strain, which cannot adjust cAMP levels in response to external stimuli, shows no mt stringent response following nutritional downshift. We also demonstrate a significant but transient increase in both mt transcript levels and mt transcription rate following shift of actively respiring wild-type cells to glucose-based medium, a manipulation known to cause a short-lived pulse of cAMP in yeast; similar manipulation of the RAS2vall9 mutant strain generates no such response. Taken together all these observations indicate that cellular cAMP levels are involved in the regulation of mt transcription in yeast. Moreover, the lack of a mt stringent transcriptional response following downshift in a strain in which the BCY1 gene had been insertionally inactivated suggests that cAMP may influence mt transcription via a mt cAMP-dependent protein kinase. These results link mt gene expression with mechanisms governing growth control and nutrient adaptation in yeast, and they provide a means by which mt gene expression might be coordinated with that of related nuclear genes.

The sulphonylurea drug, glimepiride, stimulates release of glycosylphosphatidylinositol-anchored plasma-membrane proteins from 3T3 adipocytes

Sulphonylurea drugs stimulate glucose transport and metabolism in muscle and fat cells in vitro. The molecular basis for the insulin-mimetic extrapancreatic effects of these oral antidiabetic therapeutic agents is unknown at present. Here we demonstrate that incubation of 3T3 adipocytes with the novel sulphonylurea, glimepiride, causes a time- and concentration-dependent release of the glycosylphosphatidylinositol (GPI)-anchored ecto-proteins, 5'-nucleotidase, lipoprotein lipase and a 62 kDa cyclic AMP (cAMP)-binding protein from the plasma membrane into the culture medium. The change in the localization is accompanied by conversion of the membrane-anchored amphiphilic proteins into their soluble hydrophilic versions, as judged by pulse-chase experiments and Triton X-114 partitioning, and by appearance of anti-cross-reacting determinant (CRD) immunoreactivity of the released proteins as shown by Western blotting. Metabolic labelling of cells with myo-[14C]inositol demonstrates that inositol is retained in the major portion of released lipoprotein lipase and cAMP-binding ectoprotein. The identification of inositol phosphate after deamination of these proteins with nitrous acid suggests cleavage of their GPI membrane anchor by a GPI-specific phospholipase C. However, after longer incubation with glimepiride the amount of soluble versions of the GPI-proteins lacking inositol and anti-CRD immunoreactivity increases, which may be caused by additional drug-stimulated hydrolytic events within their GPI structure or C-termini. Since insulin also stimulates membrane release of these GPI-modified proteins, and in combination with glimepiride in a synergistic manner, sulphonylurea drugs may exert their peripheral actions in adipose tissue by using (part of) the insulin postreceptor signalling cascade at the step of activation of a GPI-specific phospholipase C.

The adenylate kinase family in yeast: identification of URA6 as a multicopy suppressor of deficiency in major AMP kinase

The gene URA6 encoding uridylate kinase (UK) from Saccharomyces cerevisiae was isolated as a multicopy suppressor of the respiratory-deficient phenotype of an S. cerevisiae mutant defective in the gene AKY2 encoding AMP kinase (AK). The URA6 gene also restored temperature resistance to two different temperature-sensitive mutations in the gene encoding Escherichia coli AK. By contrast, the gene encoding UK of Dictyostelium discoideum on a multicopy yeast shuttle plasmid, expressed under control of the constitutive yeast AKY2 promoter, failed to complement the deficiency in yeast, although such transformants expressed high UK activity. We show that yeast UK exerts significant AK activity which is responsible for the complementation and is absent in the analogous enzyme from D. discoideum. Since UK also significantly phosphorylates CMP (but not GMP), it must be considered an unspecific short-form nucleoside monophosphate kinase. Wild-type mitochondria lack UK activity, but import AKY2. Since multicopy transformation with URA6 heals the Pet- phenotype of AKY2 disruption mutants, the presence of AKY2 in the mitochondrial intermembrane space is not required to maintain respiratory competence. However, furnishing UK with the bipartite intermembrane space-targeting presequence of cytochrome c1 improves the growth rates of AKY2 mutants with nonfermentable substrates, suggesting that AK activity in mitochondria is helpful, though not essential for oxidative growth.

A new member of the adenylate kinase family in yeast: PAK3 is highly homologous to mammalian AK3 and is targeted to mitochondria

Making use of the polymerase chain reaction primed by oligonucleotides corresponding to regions conserved between members of the nucleoside monophosphate kinase family, we have isolated the yeast gene PAK3. Pak3p belongs to the subgroup of long-form adenylate kinase isozymes (deduced molecular mass 25.3 kDa) and exhibits highest sequence similarity to bovine AK3 rather than to the yeast isozyme, Aky2p. The gene is shown to be non-essential because haploid disruption mutants are viable, both in the presence and absence of a functional AKY2 allele. It maps on chromosome V upstream of RAD3. Its expression level is low when cells are grown on glucose or other fermentable carbon sources and about threefold higher on glycerol, but can be significantly induced by ethanol. A PAK3/mouse dihydrofolate reductase fusion construct expressed in yeast is targeted to mitochondria. Transformation with PAK3 on a multicopy plasmid complements neither adenylate kinase deficiency in an aky2-disrupted yeast strain nor in Escherichia coli cells conditionally defective in adenylate kinase.

Two lipid-anchored cAMP-binding proteins in the yeast Saccharomyces cerevisiae are unrelated to the R subunit of cytoplasmic protein kinase A

We show that the yeast, Saccharomyces cerevisiae, contains two cAMP-binding proteins in addition to the well-characterized regulatory (R) subunit of cytoplasmic cAMP-dependent protein kinase (PKA). We provide evidence that they comprise a new type of cAMP receptor, membrane-anchored by covalently attached lipid structures. They are genetically not related to the cytoplasmic R subunit. The respective proteins can be detected in sral mutants, in which the gene for the R subunit of PKA has been disrupted and a monoclonal antibody raised against the cytoplasmic R subunit does not cross-react with the two membrane-bound cAMP-binding proteins. In addition, they differ from the cytoplasmic species also with respect to their location and the peptide maps of the photoaffinity-labeled proteins. Although they differ from one another in molecular mass and subcellular location, peptide maps of the cAMP-binding domains resemble each other and both proteins are membrane-anchored by lipid structures, one to the outer surface of the plasma membrane, the other to the outer surface of the inner mitochondrial membrane. Both anchors can be metabolically labeled by Etn, myo-Ins and fatty acids. In addition, the anchor structure of the cAMP receptor from plasma membranes can be radiolabeled by GlcN and Man. After cleavage of the anchor with glycosylphosphatidylinositol-specific phospholipase C from trypanosomes, the solubilized cAMP-binding protein from plasma membranes reacts with antibodies which specifically recognize the cross-reacting determinant from soluble trypanosomal coat protein, suggesting similarity of the anchors. Degradation studies also point to the glycosylphosphatidylinositol nature of the anchor from the plasma membrane, whereas the mitochondrial counterpart is less complex in that it lacks carbohydrates. The plasma membrane cAMP receptor is, in addition, modified by an N-glycosidically linked carbohydrate side chain, responsible mainly for its higher molecular mass.

A cAMP-binding ectoprotein in the yeast Saccharomyces cerevisiae

Purified plasma membranes from the yeast Saccharomyces cerevisiae bind about 1.2 pmol of cAMP/mg of protein with high affinity (Kd = 6 nM). By using photoaffinity labeling with 8-N3-[32P]cAMP, we have identified in plasma membrane vesicles a cAMP-binding protein (Mr = 54,000) that is present also in bcy1 disruption mutants, lacking the cytoplasmic R subunit of protein kinase A (PKA). This argues that it is genetically unrelated to PKA. Neither high salt, nor alkaline carbonate, nor cAMP extract the protein from the membrane, suggesting that it is not peripherally bound. The observation that (glycosyl)phosphatidylinositol-specific phospholipases (or nitrous acid) release the amphiphilic protein from the membrane, thereby converting it to a hydrophilic form, indicates anchorage by a glycolipidic membrane anchor. Treatment with N-glycanase reduces the Mr to 44,000-46,000 indicative of a modification by N-linked carbohydrate side chain(s). In addition to the action of a phospholipase, the efficient release from the membrane requires the removal of the carbohydrate side chain(s) or the presence of high salt or methyl alpha-mannopyranoside, suggesting complex interactions with the membrane involving not only the glycolipidic anchor but also the glycan side chain(s). Topological studies show that the protein is exposed to the periplasmic space, raising intriguing questions for the function of this protein.

An amphitropic cAMP-binding protein in yeast mitochondria. 1. Synergistic control of the intramitochondrial location by calcium and phospholipid

A cAMP-binding protein is found to be integrated into the inner mitochondrial membrane of the yeast Saccharomyces cerevisiae under normal conditions. It resists solubilization by high salt and chaotropic agents. The protein is, however, converted to a soluble form which then resides in the intermembrane space, when isolated mitochondria are incubated with low concentrations of calcium. Phospholipids or diacylglycerol (or analogues) dramatically increases the efficiency of receptor release from the inner membrane, whereas these compounds alone are ineffective. Also, cAMP does not effect or enhance liberation from the membrane of the cAMP-binding protein. Photoaffinity labeling with 8-N3-[32P]cAMP followed by mitochondrial subfractionation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis does not reveal differences in the apparent molecular weight between the membrane-bound and the soluble form of the cAMP receptor. The two forms differ, however, in their partitioning behavior in Triton X-114 as well as in their protease resistance, indicating that the release from the membrane is accompanied by a change in lipophilicity and conformation of the receptor protein. Evidence is presented that a change of the intramitochondrial location of the yeast cAMP-binding protein also occurs in vivo and leads to the activation of a mitochondrial cAMP-dependent protein kinase. The cAMP-binding protein is the first example of a mitochondrial protein with amphitropic character; i.e., it has the property to occur in two different locations, as a membrane-embedded and a soluble form.

An amphitropic cAMP-binding protein in yeast mitochondria. 3. Membrane release requires both Ca2(+)-dependent phosphorylation of the cAMP-binding protein and a phospholipid-activated mitochondrial phospholipase

The amphitropic cAMP-binding protein in mitochondria of the yeast Saccharomyces cerevisiae is released from the inner membrane into the intermembrane space by the degradation of its lipid membrane anchor consisting of or containing phosphatidylinositol. The releasing reaction depends on the presence of an N-ethylmaleimide-sensitive protein (releasing factor) in the intermembrane space and is controlled by Ca2+ and phospholipid (or lipid derivatives). Here we demonstrate that these two effector molecules act on different activation steps within a complex releasing pathway involving both the cAMP receptor and the releasing factor: Ca2(+)-dependent phosphorylation of the receptor protein seems to be prerequisite for its subsequent lipolytic liberation from the inner membrane. In the presence of phospholipid (or lipid derivatives) the previously soluble releasing factor, which may be identical with a soluble diacylglycerol-binding protein in the mitochondrial intermembrane space, associates with the inner membrane. This change in the intramitochondrial location of the releasing factor, which thus exhibits amphitropic behavior itself, may be required for (direct or indirect) activation of the mitochondrial phospholipase which then releases the cAMP receptor from the inner membrane in a form liable to dissociation from the C subunit by cAMP.

An amphitropic cAMP-binding protein in yeast mitochondria. 2. Phospholipid nature of the membrane anchor

We describe the first example of a mitochondrial protein with a covalently attached phosphatidylinositol moiety acting as a membrane anchor. The protein can be metabolically labeled with both stearic acid and inositol. The stearic acid label is removed by phospholipase D whereupon the protein with the retained inositol label is released from the membrane. This protein is a cAMP receptor of the yeast Saccharomyces cerevisiae and tightly associated with the inner mitochondrial membrane. However, it is converted into a soluble form during incubation of isolated mitochondria with Ca2+ and phospholipid (or lipid derivatives). This transition requires the action of a proteinaceous, N-ethylmaleimide-sensitive component of the intermembrane space and is accompanied by a decrease in the lipophilicity of the cAMP receptor. We propose that the component of the intermembrane space triggers the amphitropic behavior of the mitochondrial lipid-modified cAMP-binding protein through a phospholipase activity.

The ras oncogene--an important regulatory element in lower eucaryotic organisms

The ras proto-oncogene in mammalian cells encodes a 21-kilodalton guanosine triphosphate (GTP)-binding protein. This gene is frequently activated in human cancer. As one approach toward understanding the mechanisms of cellular transformation by ras, the function of this gene in lower eucaryotic organisms has been studied. In the yeast Saccharomyces cerevisiae, the RAS gene products serve as essential function by regulating cyclic adenosine monophosphate metabolism. Stimulation of adenylyl cyclase is dependent not only on RAS protein complexed to GTP, but also on the CDC25 and IRA gene products, which appear to control the RAS GTP-guanosine diphosphate cycle. Although analysis of RAS biochemistry in S. cerevisiae has identified mechanisms central to RAS action, RAS regulation of adenylyl cyclase appears to be strictly limited to this particular organism. In Schizosaccharomyces pombe, Dictyostelium discoideum, and Drosophila melanogaster, ras-encoded proteins are not involved with regulation of adenylyl cyclase, similar to what is observed in mammalian cells. However, the ras gene product in these other lower eucaryotes is clearly required for appropriate responses to extracellular signals such as mating factors and chemoattractants and for normal growth and development of the organism. The identification of other GTP-binding proteins in S. cerevisiae with distinct yet essential functions underscores the fundamental importance of G-protein regulatory processes in normal cell physiology.

Yeast adenylate kinase is active simultaneously in mitochondria and cytoplasm and is required for non-fermentative growth

Displacement of the single copy structural gene for yeast adenylate kinase (long version) by a disrupted nonfunctional allele is tolerated in haploid cells. Since adenylate kinase activity is a pre-requisite for cell viability, the survival of haploid disruption mutants is indicative of the presence of an adenylate kinase isozyme in yeast, capable of forming ADP from AMP and, thus, of complementing the disrupted allele. The phenotype of these disruption mutants is pet, showing that complementation occurs only under fermentative conditions. Even on glucose, growth of the disruption mutants is slow. Adenylate kinase activity is found both in mitochondria and cytoplasm of wild type yeast. The disruption completely destroys the activity in mitochondria, whereas in the cytoplasmic fraction about 10% is retained. An antibody raised against yeast mitochondrial adenylate kinase recognizes cross-reacting material both in mitochondria and cytoplasm of the wild type, but fails to do so in each of the respective mutant fractions. The data indicate that yeast adenylate kinase (long version, AKY2) simultaneously occurs and is active in mitochondria and cytoplasm of the wild type. Nevertheless, it lacks a cleavable pre-sequence for import into mitochondria. A second, minor isozyme, encoded by a separate gene, is present exclusively in the cytoplasm.

Localization of the regulatory subunit of cAMP-dependent protein kinase in Saccharomyces cerevisiae

The subcellular distribution of the regulatory subunit of cAMP-dependent protein kinase in Saccharomyces cerevisiae cells was determined by subcellular fractionation and indirect immunofluorescence microscopy using the bcy1 mutant deficient in the regulatory subunit as control. The regulatory subunit of cAMP-dependent protein kinase showing cAMP-binding activity was identified as a single protein of 50 kDa by photoaffinity labeling and immunoblotting. The regulatory subunit was concentrated in a nuclear fraction in addition to a cytoplasmic fraction. By comparison of the regulatory subunit distribution with the DNA localization, the area detected by the indirect immunofluorescence was identified as the nucleus.

Protein phosphorylation in yeast mitochondria: cAMP-dependence, submitochondrial localization and substrates of mitochondrial protein kinases

We describe the identification and submitochondrial localization of four protein kinases and of their target proteins in derepressed yeast mitochondria. The activity of one of the kinases depends on the presence of cyclic AMP (cAMP). It is soluble and localized in the mitochondrial intermembrane space. Its natural target is a polypeptide of 40 kDa molecular mass, which is bound to the inner membrane. Besides this natural target this kinase phosphorylates acidic heterologous proteins, like casein, with high efficiency. The other protein kinases identified so far are cAMP-independent. At least one is localized in the matrix having its natural substrates (49 and 24 kDa) in the same compartment. Two others are firmly bound to the inner membrane phosphorylating target proteins in the inner membrane (52.5 kDa) and in the intermembrane space (17.5 kDa), respectively.

A direct evidence of the localization of mitochondrial calmodulin

The presence and localization of mitochondrial calmodulin was directly proved immuno-electron microscopically by the protein A-gold technique. In the ultra-pure mitochondria the complexes of anti-calmodulin antibody and protein A-gold clearly showed the localization of mitochondrial calmodulin on the inner membrane and in the matrix space.

Isolation of a cAMP receptor protein from yeast mitochondria (Mr 45000) and comparison with mitochondrial RNA polymerase (Mr 45000)

We have isolated a cAMP-binding protein from highly purified yeast mitochondria by affinity chromatography. It is a lipophilic protein of molecular mass 45 000 Da, which is tightly membrane-bound and localized on the outer surface of the inner membrane. It can be solubilized in active form under mild conditions. The cAMP receptor resembles mitochondrial RNA polymerase prepared as described by Levens et al. [(1981) J. Biol. Chem. 256, 1474] in a surprisingly large number of properties including molecular mass. Comparison of the two proteins revealed that the polypeptide previously considered as RNA polymerase is, in fact, a mitochondrial cAMP receptor protein.