Cardiolipin modulates the secondary structure of the presequence peptide of cytochrome oxidase subunit IV: a 2D 1H-NMR study

Role of cardiolipin in skeletal muscle function and its therapeutic implications

Cardiolipin, a unique phospholipid predominantly present in the inner mitochondrial membrane, is critical for maintaining mitochondrial integrity and function. Its dimeric structure and role in supporting mitochondrial dynamics, energy production, and mitophagy make it indispensable for skeletal muscle health. This review provides a comprehensive overview of cardiolipin biosynthesis, remodeling processes, and essential functions within mitochondria. We explore the influences of cardiolipin on the stability of the mitochondrial complexes, cristae formation, and calcium handling, all of which are vital for efficient oxidative phosphorylation and muscle contraction. Skeletal muscle, with its high energy demands, is particularly dependent on cardiolipin for optimal performance. We discuss the impact of aging on cardiolipin levels, which correlates with a decline in mitochondrial function and muscle mass, contributing to conditions such as sarcopenia. Furthermore, we examined the relationship between cardiolipin and endurance exercise, highlighting the effects of exercise-induced increase in cardiolipin levels on the improvement of mitochondrial function and muscle health. The role of Crls1 in cardiolipin synthesis has been emphasized as a potential therapeutic target for the treatment of sarcopenia. Increasing cardiolipin levels through gene therapy, pharmacological interventions, or specific exercise and nutritional strategies holds promise for mitigating muscle atrophy and promoting muscle regeneration. By focusing on the multifaceted role of cardiolipin in mitochondria and muscle health, we aimed to provide new insights into therapeutic approaches for enhancing muscle function and combating age-related muscle decline.

Perturbations of Native Membrane Protein Structure in Alkyl Phosphocholine Detergents: A Critical Assessment of NMR and Biophysical Studies

Membrane proteins perform a host of vital cellular functions. Deciphering the molecular mechanisms whereby they fulfill these functions requires detailed biophysical and structural investigations. Detergents have proven pivotal to extract the protein from its native surroundings. Yet, they provide a milieu that departs significantly from that of the biological membrane, to the extent that the structure, the dynamics, and the interactions of membrane proteins in detergents may considerably vary, as compared to the native environment. Understanding the impact of detergents on membrane proteins is, therefore, crucial to assess the biological relevance of results obtained in detergents. Here, we review the strengths and weaknesses of alkyl phosphocholines (or foscholines), the most widely used detergent in solution-NMR studies of membrane proteins. While this class of detergents is often successful for membrane protein solubilization, a growing list of examples points to destabilizing and denaturing properties, in particular for α-helical membrane proteins. Our comprehensive analysis stresses the importance of stringent controls when working with this class of detergents and when analyzing the structure and dynamics of membrane proteins in alkyl phosphocholine detergents.

Synthesis, preferred conformation, protease stability, and membrane activity of heptaibin, a medium-length peptaibiotic

The medium-length peptaibiotics are characterized by a primary structure of 14-16 amino acid residues. Despite the interesting antibiotic and antifungal properties exhibited by these membrane-active peptides, their exact mechanism of action is still unknown. Here, we present our results on heptaibin, a 14-amino acid peptaibiotic found to exhibit antimicrobial activity against Staphylococcus aureus. We carried out the very challenging synthesis of heptaibin on solid phase and a detailed conformational analysis in solution. The peptaibiotic is folded in a mixed 3₁₀-/α-helix conformation which exhibits a remarkable amphiphilic character. We also find that it is highly stable toward degradation by proteolytic enzymes and nonhemolytic. Finally, fluorescence leakage experiments using small unilamellar vesicles of three different compositions revealed that heptaibin, although uncharged, is a selective compound for permeabilization of model membranes mimicking the overall negatively charged surface of Gram-positive bacteria. This latter finding is in agreement with the originally published antimicrobial activity data.

Interaction of peptides with biomembranes assessed by potential-sensitive fluorescent probes

Peptide-membrane interaction is an important step to be evaluated in a study of the activity and mode of action of several bioactive peptides. A variety of methods are available; however, few of them satisfy the criteria of being sensitive, biocompatible, versatile, easy to perform, and allowing real-time monitoring as the use of potential-sensitive fluorescent probes. Here we review methods for detecting the effects of membrane-active peptides, even those that are not intrinsically fluorescent, on the different types of membrane potentials, with a special emphasis on studies conducted with living cells. FPE is a probe sensitive to surface potential and detects electrostatic interactions at the water-lipid interface. Di-8-ANEPPS is sensitive to dipole potential and detects membrane incorporations. Transmembrane potential changes reveal major membrane destabilizations, such as in pore formation. The combination of the information obtained from the three potential variations can lead to a more elucidative picture of the mechanisms of the interaction of relevant peptides with biomembranes.

Visualizing the solubilization of supported lipid bilayers by an amphiphilic peptide

The effect of the presequence peptide of cytochrome c oxidase subunit IV (p25) on supported phospholipid bilayers (SPBs) was visualized using atomic force microscopy (AFM). The presequence was found to cause the complete disruption of supported bilayers containing neutral lipids. At relatively low concentrations of presequence, the peptide was found to bind to the membrane, coalescing to form microdomains within the liquid-crystalline bilayer that were located predominantly at bilayer-mica boundaries. Further increases in peptide concentration resulted in the formation of holes within the SPB that were spanned by an interpenetrating network of narrower regions of the bilayer, which, at higher applied peptide concentrations, were observed to disappear through a budding process, ultimately leading to the formation of spherical structures at yet higher peptide concentrations. Within this paper, the impact the presequence has upon the structure and order of the membrane is discussed, as is the potential implication of this apparent solubilization process on the translocation of cytochrome c oxidase into the inner mitochondrial membrane.

Regulation of cardiolipin synthase levels in Saccharomyces cerevisiae

The Saccharomyces cerevisiae cardiolipin (CL) synthase encoded by the CRD1 gene catalyses the synthesis of CL, which is localized to the inner mitochondrial membrane and plays an important role in mitochondrial function. To investigate how CRD1 expression is regulated, a lacZ reporter gene was placed under control of the CRD1 promoter and the 5'-untranslated region of its mRNA (P(CRD1)-lacZ). P(CRD1)-lacZ expression was 2.5 times higher in early stationary phase than in logarithmic phase for glucose grown cells. Non-fermentable growth resulted in a two-fold elevation in expression relative to glucose grown cells. A shift from glycerol to glucose rapidly repressed expression, whereas a shift from glucose to glycerol had the opposite effect. The derepression of P(CRD1)-lacZ expression by non-fermentable carbon sources was dependent on mitochondrial respiration. These results support a tight coordination between translation and transcription of the CRD1 gene, since similar effects by the above factors on CRD1 mRNA levels have been reported. In glucose-grown cells, P(CRD1)-lacZ expression was repressed 70% in a pgs1delta strain (lacks phosphatidylglycerol and CL) compared with wild-type and rho- cells and elevated 2.5-fold in crd1delta cells, which have increased phosphatidylglycerol levels, suggesting a role for phosphatidylglycerol in regulating CRD1 expression. Addition of inositol to the growth medium had no effect on expression. However, expression was elevated in an ino4delta mutant but not in ino2delta cells, suggesting multiple and separate functions for the inositol-responsive INO2/INO4 gene products, which normally function as a dimer in regulating gene function.

The phosphatidylglycerol/cardiolipin biosynthetic pathway is required for the activation of inositol phosphosphingolipid phospholipase C, Isc1p, during growth of Saccharomyces cerevisiae

Inositolsphingolipid phospholipase C (Isc1p) is the Saccharomyces cerevisiae member of the extended family of neutral sphingomyelinases that regulates the generation of bioactive ceramides. Recently, we reported that Isc1p is post-translationally activated in the post-diauxic phase of growth and that it localizes to mitochondria (Vaena de Avalos, S., Okamoto, Y., and Hannun, Y. A. (2004) J. Biol. Chem. 279, 11537-11545). In this study the in vivo mechanisms of activation and function of Isc1p were investigated. Deletion of ISC1 resulted in markedly lower growth in non-fermentable carbon sources. Interestingly, the growth defect of isc1Delta strains resembled that of pgs1Delta strains, lacking the committed step in the synthesis of phosphatidylglycerol (PG) and cardiolipin (CL), which were shown to activate Isc1p in vitro. Therefore, the role of Pgs1p in activation of Isc1p in vivo was investigated. The results showed that in the pgs1Delta strain, the growth-dependent activation of Isc1p was impaired as was the ISC1-dependent increase in the levels of phytoceramide during the post-diauxic phase, demonstrating that the activation of Isc1p in vivo is dependent on PGS1 and on the mitochondrial phospholipids PG/CL. Mechanistically, loss of Isc1p resulted in lower levels of mitochondrial cytochrome c oxidase subunits cox3p and cox4p, previously established targets of both PG and CL (Ostrander, D. B., Zhang, M., Mileykovskaya, E., Rho, M., and Dowhan, W. (2001) J. Biol. Chem. 276, 25262-25272), thus suggesting that Isc1p mediates at least some functions downstream of PG/CL. This study provides the first evidence for the mechanism of in vivo activation and function of Isc1p. A model with endogenous PG/CL as the in vivo activator of Isc1p is proposed.

Lack of mitochondrial anionic phospholipids causes an inhibition of translation of protein components of the electron transport chain. A yeast genetic model system for the study of anionic phospholipid function in mitochondria

Reduction of mitochondrial cardiolipin (CL) levels has been postulated to compromise directly the function of several essential enzymes and processes of the mitochondria. There is limited genetic evidence for the critical roles with which CL and its precursor phosphatidylglycerol (PG) have been associated. A null allele of the PGS1 gene from Saccharomyces cerevisiae, which encodes the enzyme responsible for the synthesis of the CL precursor PG phosphate, was created in a yeast strain in which PGS1 expression is exogenously regulated by doxycycline. The addition of increasing concentrations of doxycycline to the growth medium causes a proportional decrease to undetectable levels of PGS1 transcript, PG phosphate synthase activity, and PG plus CL. The doubling time of this strain with increasing doxycycline increases to senescence in non-fermentable carbon sources or at high temperatures, conditions that do not support growth of the pgs1Delta strain. Doxycycline addition also causes mitochondrial abnormalities as observed by fluorescence microscopy. Products of four mitochondrial encoded genes (COX1, COX2, COX3, and COB) and one nuclear encoded gene (COX4) associated with the mitochondrial inner membrane are not present when PGS1 expression is fully repressed. No translation of these proteins can be detected in cells lacking the PGS1 gene product, although transcription and splicing appear unaffected. Protein import of other nuclear encoded proteins remains unaffected. The remaining proteins encoded by mitochondrial DNA are expressed and translated normally. Thus, the molecular basis for the lack of mitochondrial function in pgs1Delta cells is the failure to translate gene products essential to the electron transport chain.

Interaction of the precursor to mitochondrial aspartate aminotransferase and its presequence peptide with model membranes

The possible contribution of the mature portion of a mitochondrial precursor protein to its interaction with membrane lipids is unclear. To address this issue, we examined the interaction of the precursor to mitochondrial aspartate aminotransferase (pmAAT) and of a synthetic peptide corresponding to the 29-residue presequence peptide (mAAT-pp) with anionic phospholipid vesicles. The affinity of mAAT-pp and pmAAT for anionic vesicles is nearly identical. Results obtained by analyzing the effect of mAAT-pp or full-length pmAAT on either the permeability or microviscosity of the phospholipid vesicles are consistent with only a shallow insertion of the presequence peptide in the bilayer. Analysis of the quenching of Trp-17 fluorescence by brominated phospholipids reveals that this presequence residue inserts to a depth of approximately 9 A from the center of the bilayer. Furthermore, in membrane-bound pmAAT or mAAT-pp, both Arg-8 and Arg-28 are accessible to the solvent. These results suggest that the presequence segment lies close to the surface of the membrane and that the mature portion of the precursor protein has little effect on the affinity or mode of binding of the presequence to model membranes. In the presence of vesicles, mAAT-pp adopts considerable alpha-helical structure. Hydrolysis by trypsin after Arg-8 results in the dissociation of the remaining 21-residue C-terminal peptide fragment from the membrane bilayer, suggesting that the N-terminal portion of the presequence is essential for membrane binding. Based on these results, we propose that the presequence peptide may contain dual recognition elements for both the lipid and import receptor components of the mitochondrial membrane.

Structure, dynamics, and insertion of a chloroplast targeting peptide in mixed micelles

Nuclear-encoded, chloroplast-destined proteins are synthesized with transit sequences that contain all information to get them inside the organelle. Different proteins are imported via a general protein import machinery, but their transit sequences do not share amino acid homology. It has been suggested that interactions between transit sequence and chloroplast envelope membrane lipids give rise to recognizable, structural motifs. In this study a detailed investigation of the structural, dynamical, and topological features of an isolated transit peptide associated with mixed micelles is described. The structure of the preferredoxin transit peptide in these micelles was studied by circular dichroism (CD) and multidimensional NMR techniques. CD experiments indicated that the peptide, which is unstructured in aqueous solution, obtained helical structure in the presence of the micelles. By NMR it is shown that the micelles introduced ill-defined helical structures in the transit peptide. Heteronuclear relaxation experiments showed that the whole peptide backbone is very flexible. The least dynamic segments are two N- and C-terminal helical regions flanking an unstructured proline-rich amino acid stretch. Finally, the insertion of the peptide backbone in the hydrophobic interior of the micelle was investigated by use of hydrophobic spin-labels. The combined data result in a model of the transit peptide structure, backbone dynamics, and insertion upon its interaction with mixed micelles.

The petite mutation in yeasts: 50 years on

Fifty years ago it was reported that baker's yeast, Saccharomyces cerevisiae, can form "petite colonie" mutants when treated with the DNA-targeting drug acriflavin. To mark the jubilee of studies on cytoplasmic inheritance, a review of the early work will be presented together with some observations on current developments. The primary emphasis is to address the questions of how loss of mtDNA leads to lethality (rho 0-lethality) in petite-negative yeasts and how S. cerevisiae tolerates elimination of mtDNA. Recent investigation have revealed that rho 0-lethality can be suppressed by specific mutations in the alpha, beta, and gamma subunits of the mitochondrial F1-ATPase of the petite-negative yeast Kluyveromyces lactis and by the nuclear ptp alleles in Schizosaccharomyces pombe. In contrast, inactivation of genes coding for F1-ATPase alpha and beta subunits and disruption of AAC2, PGS1/PEL1, and YME1 genes in S. cerevisiae convert this petite-positive yeast into a petite-negative form. Studies on nuclear genes affecting dependence on mtDNA have provided important insight into the functions provided by the mitochondrial genome and the maintenance of structural and functional integrity of the mitochondrial inner membrane.

The structural flexibility of the preferredoxin transit peptide

In order to obtain insight into the structural flexibility of chloroplast targeting sequences, the Silene pratensis preferredoxin transit peptide was studied by circular dichroism and nuclear magnetic resonance spectroscopy. In water, the peptide is unstructured, with a minor propensity towards helix formation from Val-9 to Ser-12 and from Gly-30 to Ser-40. In 50% (v/v) trifluoroethanol, structurally independent N- and C-terminal helices are stabilized. The N-terminal helix appears to be amphipathic, with hydrophobic and hydroxylated amino acids on opposite sides. The C-terminal helix comprises amino acids Met-29-Gly-50 and is destabilized at Gly-39. No ordered tertiary structure was observed. The results are discussed in terms of protein import into chloroplasts, in which the possible interactions between the transit peptide and lipids are emphasized.

The active streptococcal hyaluronan synthases (HASs) contain a single HAS monomer and multiple cardiolipin molecules

The functional sizes of the two streptococcal hyaluronan synthases (HASs) were determined by radiation inactivation analysis of isolated membranes. The native enzymes in membranes from Group A Streptococcus pyogenes HAS and Group C Streptococcus equisimilis HAS were compared with the recombinant proteins expressed in Escherichia coli membranes. Based on their amino acid sequences, the masses of these four proteins as monomers are approximately 48 kDa. In all cases, loss of enzyme activity was a simple single exponential function with increasing radiation dose. The functional sizes calculated from these data were identical for the four HASs at approximately 64 kDa. In contrast, the sizes of the proteins estimated by the loss of antibody reactivity on Western blots were essentially identical at 41 kDa for the four HAS species, consistently lower than the functional size by approximately 23 kDa. Matrix-assisted laser desorption time of flight mass spectrometry analysis of purified S. pyogenes HAS-H6 and S. equisimilis HAS-H6 gave masses that differed by <0.07% from the predicted monomer sizes, which confirms that neither protein is posttranslationally modified or covalently attached to another protein. Ongoing studies indicate that the purified HAS enzymes require cardiolipin (CL) for maximal activity and stability. When irradiated membranes were detergent solubilized and the extracts were incubated with exogenous CL, the residual level of HAS activity increased. Consequently, the calculated functional size decreased by approximately 23 kDa to the expected size of the HAS monomer. The approximately 23-kDa larger size of the functional HAS enzyme, compared with the HAS monomer, is due, therefore, to CL molecules. We propose that the active streptococcal HA synthases are monomers in complex with approximately 16 CL molecules.

Isolation and characterization of the gene (CLS1) encoding cardiolipin synthase in Saccharomyces cerevisiae

In eukaryotic cells, cardiolipin (CL) synthase catalyzes the final step in the synthesis of CL from phosphatidylglycerol and CDP-diacylglycerol. CL and its synthesis are localized predominantly to the mitochondrial inner membrane, and CL is generally thought to be an essential component of many mitochondrial processes. By using homology searches for genes potentially encoding phospholipid biosynthetic enzymes, we have cloned the gene (CLS1) encoding CL synthase in Saccharomyces cerevisiae. Overexpression of the CLS1 gene under its endogenous promoter or the inducible GAL1 promoter in yeast and expression of CLS1 in baculovirus-infected insect cells resulted in elevated CL synthase activity. Disruption of the CLS1 gene in a haploid yeast strain resulted in the loss of CL synthase activity, no detectable CL, a 5-fold elevation in phosphatidylglycerol levels, and lack of staining of mitochondria by a dye with high affinity for CL. The cls1::TRP1 null mutant grew on both fermentable and non-fermentable carbon sources but more poorly on the latter. The level and activity of cytochrome c oxidase was normal, and a dye whose accumulation is dependent on membrane proton electrochemical potential effectively stained the mitochondria. These results definitively identify the gene encoding the CL synthase of yeast.

Cardiolipin synthase from yeast

Cardiolipin synthase catalyzes the synthesis of the mitochondrial phospholipid cardiolipin. Cardiolipin synthase is a unique membrane-bound enzyme in that it utilizes two phospholipids, both insoluble in water, as substrates. Kinetic analysis suggests that the enzyme forms a ternary complex with the two lipid substrates, and that a divalent metal ion directly associates with cardiolipin synthase to form the active enzyme. While little is known about the regulation of cardiolipin synthase in yeast, activity is reduced in mutants in which the mitochondrial genome is deleted, and in mutants with defective respiratory complexes. In p0 mutants, which contain no mitochondrial DNA and are defective in the assembly of many mitochondrial membrane protein complexes, cardiolipin synthase activity is reduced by 50%. Mutants defective in respiratory complexes, particularly those incapable of cytochrome oxidase assembly, also have reduced cardiolipin synthase activity. Thus it is likely that respiration and cardiolipin formation are interdependent. The enzyme was recently purified from the budding yeast Saccharomyces cerevisiae. Enzyme activity was associated with a 25-30-kDa protein. The amino acid sequence of this protein, combined with the availability of the complete yeast genome sequence, will hopefully lead to the identification of the structural gene for this enzyme in the near future.

Molecular basis for membrane phospholipid diversity: why are there so many lipids?

Phospholipids play multiple roles in cells by establishing the permeability barrier for cells and cell organelles, by providing the matrix for the assembly and function of a wide variety of catalytic processes, by acting as donors in the synthesis of macromolecules, and by actively influencing the functional properties of membrane-associated processes. The function, at the molecular level, of phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin in specific cellular processes is reviewed, with a focus on the results of combined molecular genetic and biochemical studies in Escherichia coli. These results are compared with primarily biochemical data supporting similar functions for these phospholipids in eukaryotic organisms. The wide range of processes in which specific involvement of phospholipids has been documented explains the need for diversity in phospholipid structure and why there are so many membrane lipids.

The mitochondrial processing peptidase: function and specificity

Targeting signals of mitochondrial precursors are cleaved in the matrix during or after import by the mitochondrial processing peptidase (MPP). This enzyme consists of two nonidentical alpha- and beta-subunits each of molecular weight of about 50 kDa. In mammals and fungi, MPP is soluble in the matrix, whereas in plants the enzyme is part of the cytochrome bc1 complex. MPP is a metalloendopeptidase which has been classified as a member of the pitrilysin family on the basis of the HXXEHX76E zinc-binding motif present in beta-MPP. Both subunits of MPP are required for processing activity. The alpha-subunit of MPP, which probably recognizes a three-dimensional motif adopted by the presequence, presents the presequence to beta-MPP, which carries the catalytic active site. MPP acts as an endoprotease on chemically synthesized peptides corresponding to mitochondrial presequences. Matrix-targeting signals and MPP cleavage signals seem to be distinct, although the two signals may overlap within a given presequence. The structural element helix-turn-helix, that cleavable presequences adopt in a membrane mimetic environment, may be required for processing but is not sufficient for proteolysis. Binding of the presequence by alpha-MPP tolerates a high degree of mutations of the presequence. alpha-MPP may present a degenerated cleavage site motif to beta-MPP in an accessible conformation for processing. The conformation of mitochondrial presequences bound to MPP remains largely unknown.

The N-terminal half of a mitochondrial presequence peptide inserts into cardiolipin-containing membranes. Consequences for the action of a transmembrane potential

The orientation of a mitochondrial-presequence peptide, associated with anionic lipid-containing model membranes, was investigated. The peptide inserts with its N-terminal alpha-helical part into cardiolipin (CL) monolayers so that the N-terminal 14 residues are protected from proteinase K. In phosphatidylglycerol (PG) monolayers the inserted peptide was fully accessible to the protease. A consequence of the different orientations of the peptide was that membrane potential-dependent protection from trypsin was much faster for the peptide bound to PG-containing vesicles compared to CL-containing membranes, suggesting that in the mitochondrial protein import process other components of the import apparatus are involved in the efficient potential-driven translocation of presequences across the inner mitochondrial membranes.

Secondary structure and topology of a mitochondrial presequence peptide associated with negatively charged micelles. A 2D H-NMR study

In this study the secondary structure and topology of the peptide, corresponding to the presequence of cytochrome oxidase subunit IV (p25) in a negatively charged membrane-mimetic environment, were assessed by circular dichroism and two-dimensional nuclear magnetic resonance. The micelles used consisted of dodecylphosphoglycol (DPG), a mild anionic detergent with a headgroup resembling that of phosphatidylglycerol. The secondary structure was analyzed by interresidue nuclear Overhauser enhancement measurements and chemical shifts of backbone protons. The data revealed alpha-helix formation of the peptide upon interaction with the micelles, both in the N- and in the C-terminal halves, which are separated from each other by the proline residue at position 13. The topology of the peptide was studied by determining the effect of spin-labeled 12-doxylstearate on the line widths of the peptide proton resonances. This method revealed the insertion of hydrophobic residues of both the N- and the C-terminal halves of p25 into the hydrophobic environment of the micelles, demonstrating the orientation of the amphiphilic helix.