Faculty Bibliography

Tafazzin is a mitochondrial enzyme that transfers fatty acids from phospholipids to lysophospholipids. Mutations in tafazzin cause abnormal molecular species of cardiolipin and the clinical phenotype of Barth syndrome. However, the mechanism by which tafazzin creates acyl specificity has been controversial. We have shown that the lipid phase state can produce acyl specificity in the tafazzin reaction but others have reported that tafazzin itself carries enzymatic specificity. To resolve this issue, we replicated and expanded the controversial experiments, i.e. the transfer of different acyl groups from phosphatidylcholine to monolyso-cardiolipin by yeast tafazzin. Our data show that this reaction requires the presence of detergent and does not take place in liposomes but in mixed micelles. In order to separate thermodynamic (lipid-dependent) from kinetic (enzyme-dependent) parameters, we followed the accumulation of cardiolipin during the reaction from the initial state to the equilibrium state. The transacylation rates of different acyl groups varied over 2 orders of magnitude and correlated tightly with the concentration of cardiolipin in the equilibrium state (lipid-dependent parameter). In contrast, the rates by which different transacylations approached the equilibrium state were very similar (enzyme-dependent parameter). Furthermore, we found that tafazzin catalyzes the remodeling of cardiolipin by combinations of forward and reverse transacylations, essentially creating an equilibrium distribution of acyl groups. These data strongly support the idea that the acyl specificity of the tafazzin reaction results from the physical properties of lipids.

Among mitochondrial lipids, cardiolipin occupies a unique place. It is the only phospholipid that is specific to mitochondria and although it is merely a minor component, accounting for 10-20% of the total phospholipid content, cardiolipin plays an important role in the molecular organization, and thus the function of the cristae. This review covers the formation of cardiolipin, a phospholipid dimer containing two phosphatidyl residues, and its assembly into mitochondrial membranes. While a large body of literature exists on this topic, the review focuses on papers that appeared in the past three years. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Cardiolipin is a specific mitochondrial phospholipid that has a high affinity for proteins and that stabilizes the assembly of supercomplexes involved in oxidative phosphorylation. We found that sequestration of cardiolipin in protein complexes is critical to protect it from degradation. The turnover of cardiolipin is slower by almost an order of magnitude than the turnover of other phospholipids. However, in subjects with Barth syndrome, cardiolipin is rapidly degraded via the intermediate monolyso-cardiolipin. Treatments that induce supercomplex assembly decrease the turnover of cardiolipin and the concentration of monolyso-cardiolipin, whereas dissociation of supercomplexes has the opposite effect. Our data suggest that cardiolipin is uniquely protected from normal lipid turnover by its association with proteins, but this association is compromised in subjects with Barth syndrome, leading cardiolipin to become unstable, which in turn causes the accumulation of monolyso-cardiolipin.

Tafazzin is a transacylase that affects cardiolipin fatty acid composition and mitochondrial function. Mutations in human tafazzin cause Barth syndrome yet the enzyme has mostly been characterized in yeast. To study tafazzin in higher organisms, we isolated mitochondria from Drosophila and mammalian cell cultures. Our data indicate that tafazzin binds to multiple protein complexes in these organisms, and that the interactions of tafazzin lack strong specificity. Very large tafazzin complexes could only be detected in the presence of cardiolipin, but smaller complexes remained intact even upon treatment with phospholipase A2. In mammalian cells, tafazzin had a half-life of only 3-6h, which was much shorter than the half-life of other mitochondrial proteins. The data suggest that tafazzin is a transient resident of multiple protein complexes.

In addition to specific intermolecular interactions, biological processes at membranes are also modulated by the physical properties of the membrane. One of these properties is membrane curvature. NMR methods are useful for studying how membrane curvature affects the binding and insertion of proteins into membranes as well as how proteins can affect membrane curvature properties. In many cases these interactions result in a marked change in protein activity. We have reviewed examples from a range of systems having varied mechanisms by which membrane curvature is linked to protein activity. Among the examples discussed are antimicrobial peptides, proteins affecting membrane fusion, rhodopsin, protein kinase C, phospholipase C-delta1, phosphatidylinositol-3 kinase-related kinases and tafazzin. This article is part of a Special Issue entitled: NMR Spectroscopy for Atomistic Views of Biomembranes and Cell Surfaces.

Since it has been recognized that mitochondria are crucial not only for energy metabolism but also for other cellular functions, there has been a growing interest in cardiolipin, the specific phospholipid of mitochondrial membranes. Indeed, cardiolipin is a universal component of mitochondria in all eukaryotes. It has a unique dimeric structure comprised of two phosphatidic acid residues linked by a glycerol bridge, which gives rise to unique physicochemical properties. Cardiolipin plays an important role in the structural organization and the function of mitochondrial membranes. In this article, we review the literature on cardiolipin biology, focusing on the most important discoveries of the past decade. Specifically, we describe the formation, the migration, and the degradation of cardiolipin and we discuss how cardiolipin affects mitochondrial function. We also give an overview of the various phenotypes of cardiolipin deficiency in different organisms.

The dynamical behavior of mitochondria has attracted much attention, but little is known about the dynamics of mitochondrial lipids, specifically cardiolipin (CL). Here, we estimated the turnover of select molecular species of CL in mammalian cell cultures and compared it to the turnover of other lipids, including phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol. Cells were labeled with myristic acid, 9,10-(2)H2-oleic acid, or d-[U-(13)C6]-glucose and analyzed by mass spectrometry at different time points of pulse-chase experiments. The turnover of glycerol groups was monitored by specific isotopologues that carried (13)C primarily in the glycerol carbons, whereas the turnover of acyl groups was monitored by molecular species that carried myristoyl or (2)H2-oleoyl groups. We found that the turnover of CL, but not of mitochondrial PC and PE, was substantially slower than the turnover of other cellular phospholipids. In dioleoyl-PC and dioleoyl-PE, the acyl turnover was faster than the glycerol turnover, indicating continuous deacylation and reacylation of the oleoyl residues. In contrast, the acyl turnover was similar to the glycerol turnover in tetraoleoyl-CL, suggesting that oleoyl remodeling did not take place continuously in endogenous CL. We conclude that CL, once assembled in mitochondrial membranes, remains largely inert to degradation and acyl remodeling.

The idea of a Cardiolipin workshop in Italy came to the meeting organizers in June 2011, during the mini-sabbatical of Angela Corcelli in New York City in the Laboratory of Michael Schlame. They thought to take advantage of the presence of the 54th International Conference on the Bioscience of Lipids (ICBL) at Bari in 2013 to organize the Cardiolipin workshop as a satellite event. The web page of the Cardiolipin Meeting was kindly supported by the Euro Fed Lipid organization. About 60 scientists attended the meeting focused on the multiple roles of cardiolipin in mitochondria in physiological and pathological states in various organisms as well as in bacterial membranes. In addition to ICBL participants, many students and colleagues of the Universities of Bari and Lecce attended the meeting, increasing the number of total participants to about 100. As defects in cardiolipin metabolism may cause Barth syndrome, the meeting also presented an occasion to establish contacts between the nascent Italian Barth Syndrome Foundation and scientists actively involved in cardiolipin research. C1 [Corcelli, Angela] Univ Bari A Moro, Dept Basic Med Sci Neurosci & Sensory Organs, Bari, Italy. [Schlame, Michael] NYU, Sch Med, New York, NY USA