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Cardiolipin (CL) is a unique phospholipid which is present throughout the eukaryotic kingdom and is localized in mitochondrial membranes. Saccharomyces cerevisiae cells containing a disruption of CRD1, the structural gene encoding CL synthase, have no CL in mitochondrial membranes. To elucidate the physiological role of CL, we compared mitochondrial functions in the crd1⌬ mutant and isogenic wild type. The crd1⌬ mutant loses viability at elevated temperature, and prolonged culture at 37°C leads to loss of the mitochondrial genome. Mutant membranes have increased phosphatidylglycerol (PG) when grown in a nonfermentable carbon source but have almost no detectable PG in medium containing glucose. In glucose-grown cells, maximum respiratory rate, ATPase and cytochrome oxidase activities, and protein import are deficient in the mutant. The ADP/ATP carrier is defective even during growth in a nonfermentable carbon source. The mitochondrial membrane potential is decreased in mutant cells. The decrease is more pronounced in glucose-grown cells, which lack PG, but is also apparent in membranes containing PG (i.e. in nonfermentable carbon sources). We propose that CL is required for maintaining the mitochondrial membrane potential and that reduced membrane potential in the absence of CL leads to defects in protein import and other mitochondrial functions.1 is a structurally unique phospholipid that carries four acyl groups and two negative charges. It is thus highly hydrophobic and acidic. The biosynthesis of CL occurs in three enzymatic steps (1-3). Phosphatidylglycerolphosphate (PGP) synthase catalyzes the formation of PGP from phosphatidyl-CMP (CDP-diacylglycerol; CDP-DG) and glycerol 3-phosphate. PGP is then dephosphorylated to phosphatidylglycerol (PG) by PGP phosphatase. Eukaryotes and bacteria utilize different reactions to convert PG to CL. In prokaryotes, CL synthase catalyzes a phosphatidyl transfer between two PG molecules (4). This is a near-equilibrium (transesterification) reaction that is mainly controlled by substrate availability. In contrast, eukaryotic CL synthase catalyzes a phosphatidyl transfer from CDP-DG to PG (5-7). This is an irreversible reaction that involves cleavage of a high energy anhydride bond. This reaction can take place in the presence of low substrate concentration and is mainly regulated by CL synthase activity. The differences in these reactions probably reflect different functions of PG and CL in prokaryotes and mitochondria.In Escherichia coli, the enzymes that catalyze the synthesis of CL have been characterized biochemically, and the genes encoding these enzymes have been cloned. Although disruption of the cls gene (encoding CL synthase) is not lethal, bacterial strains bearing a null allele of pgsA (encoding PGP synthase) are inviable (8, 9). Interestingly, bacterial cls null mutants do synthesize CL, presumably by another enzyme. These experiments suggest that the anionic phospholipids PG and/or CL are essential for bacterial viability.

Tafazzin is a putative enzyme that is involved in cardiolipin metabolism, it may carry mutations responsible for Barth syndrome. To identify the biochemical reaction catalyzed by tafazzin, we expressed the full-length isoform of Drosophila melanogaster tafazzin in a baculovirus-Sf9 insect cell system. Tafazzin expression induced a new enzymatic function in Sf9 cell mitochondria, namely 1-palmitoyl-2-[ 14 C]linoleoyl-phosphatidylcholine:monolysocardiolipin linoleoyltransferase. We also found evidence for the reverse reaction, because tafazzin expression caused transfer of acyl groups from phospholipids to 1-[ 14 C]palmitoyl-2-lyso-phosphatidylcholine. An affinity-purified tafazzin construct, tagged with the maltose-binding protein, catalyzed both forward and reverse transacylations between cardiolipin and phosphatidylcholine, but was unable to utilize CoA or acyl-CoA as substrates. Whereas tafazzin supported transacylations between various phospholipid-lysophospholipid pairs, it showed the highest rate for the phosphatidylcholine-cardiolipin transacylation. Transacylation activities were about 10-fold higher for linoleoyl groups than for oleoyl groups, and they were negligible for arachidonoyl groups. The data show that Drosophila tafazzin is a CoA-independent, acylspecific phospholipid transacylase with substrate preference for cardiolipin and phosphatidylcholine.

In this article, the formation of prokaryotic and eukaryotic cardiolipin is reviewed in light of its biological function. I begin with a detailed account of the structure of cardiolipin, its stereochemistry, and the resulting physical properties, and I present structural analogs of cardiolipin that occur in some organisms. Then I continue to discuss i) the de novo formation of cardiolipin, ii) its acyl remodeling, iii) the assembly of cardiolipin into biological membranes, and iv) the degradation of cardiolipin, which may be involved in apoptosis and mitochondrial fusion. Thus, this article covers the entire metabolic cycle of this unique phospholipid. It is shown that mitochondria produce cardiolipin species with a high degree of structural uniformity and molecular symmetry, among which there is often a dominant form with four identical acyl chains. The subsequent assembly of cardiolipin into functional membranes is largely unknown, but the analysis of crystal structures of membrane proteins has revealed a first glimpse into the underlying principles of cardiolipin-protein interactions. Disturbances of cardiolipin metabolism are crucial in the pathophysiology of human Barth syndrome and perhaps also play a role in diabetes and ischemic heart disease.-Schlame, M. Cardiolipin synthesis for the assembly of bacterial and mitochondrial membranes.

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Barth syndrome is an X-linked recessive disease caused by mutations in the tafazzin gene. Patients have reduced concentration and altered composition of cardiolipin, the specific mitochondrial phospholipid, and they have variable clinical findings, often including heart failure, myopathy, neutropenia, and growth retardation. This article provides an overview of the molecular basis of Barth syndrome. It is argued that tafazzin, a phospholipid acyltransferase, is involved in acyl-specific remodeling of cardiolipin, which promotes structural uniformity and molecular symmetry among the cardiolipin molecular species. Inhibition of this pathway leads to changes in mitochondrial architecture and function.

F(1)F(0) ATP synthase forms dimers that tend to assemble into large supramolecular structures. We show that the presence of cardiolipin is critical for the degree of oligomerization and the degree of order in these ATP synthase assemblies. This conclusion was drawn from the statistical analysis of cryoelectron tomograms of cristae vesicles isolated from Drosophila flight-muscle mitochondria, which are very rich in ATP synthase. Our study included a wild-type control, a cardiolipin synthase mutant with nearly complete loss of cardiolipin, and a tafazzin mutant with reduced cardiolipin levels. In the wild-type, the high-curvature edge of crista vesicles was densely populated with ATP synthase molecules that were typically organized in one or two rows of dimers. In both mutants, the density of ATP synthase was reduced at the high-curvature zone despite unchanged expression levels. Compared to the wild-type, dimer rows were less extended in the mutants and there was more scatter in the orientation of dimers. These data suggest that cardiolipin promotes the ribbonlike assembly of ATP synthase dimers and thus affects lateral organization and morphology of the crista membrane.