The Human TAZ Gene Complements Mitochondrial Dysfunction in the Yeast taz1Δ Mutant

Abstract

Barth syndrome is a genetic disorder that is caused by different mutations in the TAZ gene G4.5. The yeast gene TAZ1 is highly homologous to human TAZ, and the taz1⌬ mutant has phospholipid defects similar to those observed in Barth syndrome cells, including aberrant cardiolipin species and decreased cardiolipin levels. Subcellular fractionation studies revealed that Taz1p is localized exclusively in mitochondria, which supports the theory that tafazzins are involved in cardiolipin remodeling. Because cardiolipin plays an important role in respiratory function, we measured the energy transformation and osmotic properties of isolated mitochondria from the taz1⌬ mutant. Energy coupling in taz1⌬ mitochondria was dependent on the rate of oxidative phosphorylation, as coupling was diminished when NADH was used as a respiratory substrate but was unaffected when ethanol was the substrate. Membrane stability was compromised in taz1⌬ mitochondria exposed to increased temperature and hypotonic conditions. Mitochondria from taz1⌬ also displayed decreased swelling in response to ATP, which induces the yeast mitochondrial unspecific channel, and to alamethicin, a membrane-disrupting agent. Coupling was measured in taz1⌬ cells containing different splice variants of the human TAZ gene. Only the variant that restores wild type cardiolipin synthesis (lacking exon 5) restored coupling in hypotonic conditions and at elevated temperature. These findings may shed light on the mitochondrial deficiencies observed in Barth syndrome.

Taz1, an Outer Mitochondrial Membrane Protein, Affects Stability and Assembly of Inner Membrane Protein Complexes: Implications for Barth Syndrome

“…This was confirmed in our study because we observed growth defects of taz1⌬ cells on a nonfermentable carbon source and a reduction of mitochondrial inner membrane potential. These results agree well with previous studies, which reported that the function of the respiratory chain was compromised in taz1⌬ cells (Vaz et al, 2003;Gu et al, 2004;Ma et al, 2004). To determine how inactivation of the mitochondrial outer membrane protein Taz1 affected the respiratory chain, we visualized the respiratory chain protein complexes solubilized in the mild detergent digitonin by BN-PAGE (Cruciat et al, 2000;Schägger and Pfeiffer, 2000;Schägger et al, 2004).…”

“…These analyses and recent studies (Ma et al, 2004) indicate a defect in respiratory chain function in taz1⌬ cells. In the light of the surprising outer membrane localization of Taz1 we wondered how defects in Taz1 would translate to a reduction of respiratory chain function at the inner membrane.…”

“…Taz1 has been localized to mitochondria by several methods Ma et al, 2004;Testet et al, 2004). However, these techniques did not elucidate the mitochondrial compartment in which the protein was located.…”

“…Although the molecular function of cardiolipin is still unclear, several studies have suggested that it is required for the proper function of some proteins and protein complexes in the inner mitochondrial membrane (Hoch, 1992;Jiang et al, 2000;Pfeiffer et al, 2003;Palsdottir and Hunte, 2004;Zhang et al, 2005). Two functions have been proposed for cardiolipin: 1) participation in stabilization of the physical properties of the membrane Koshkin and Greenberg, 2002;Ma et al, 2004), for example, membrane fluidity and osmotic stability and 2) participation in protein function via direct interaction with membrane proteins Palsdottir and Hunte, 2004). Indeed, cardiolipin has been found in tight association with inner membrane protein complexes such as the cytochrome bc 1 complex (complex III).…”

Characterization of lymphoblast mitochondria from patients with Barth syndrome

“…Specific involvement of CL corresponded with the mitochondrial localization of tafazzin. 28 However, the lipid abnormalities in BTHS were not confined to CL. PC showed a distinct alteration of its fatty acid composition, which, although less drastic than in CL, was consistently present in lymphoblasts from BTHS patients.…”