Novel insights into mitochondrial phospholipid homeostasis in a disease-relevant yeast model

Abstract

The proper function of mitochondria critically depends on their membrane lipid composition. To ensure lipid homeostasis, de novo synthesis, intracellular and intraorganellar transport, remodeling, and degradation of lipids must be tightly regulated. Several studies have emphasised the importance of the mitochondrial signature phospholipid, cardiolipin (CL) for the organelle function. The acquisition of mature CL species is catalyzed by the phospholipid acyltransferase, Tafazzin. The importance of CL remodeling is underscored by the fact that mutations in Tafazzin lead to a life-threatening genetic disorder, Barth syndrome (BTHS). Currently, the biochemical processes underlying this clinical disorder remain unclear. Deletion of the yeast homologue Taz1 results in similar phenotypes to those observed in patients suffering from BTHS, making this organism an optimal model system to study the pathomechanism of the disease. To shed light on the pathomechanism of BTHS, I searched for yeast multi-copy suppressors of the taz1Δ growth defect and identified the branched-chain amino acid transaminases (BCATs) BAT1 and BAT2 as such suppressors. Similarly, overexpression of the mitochondrial isoform BCAT2 in mammalian cells lacking TAZ improves their growth. Accordingly, supplying both yeast and mammalian cells lacking Tafazzin function with certain amino acids restored their growth behavior. Although elevated levels of Bat1 or Bat2 did not restore all the mitochondrial defects of BTHS, it could correct the higher respiration rate observed in taz1Δ cells. These findings outline that the metabolism of amino acids can influence the BTHS phenotype and has an important and disease relevant role in cells lacking Taffazzin function. In another project, I investigated the transfer of lipids between mitochondria and vacuoles. The absence of documented mitochondrial vesicular lipid exchange suggests that membrane contact sites (MCSs) facilitate lipids transport between mitochondria and other cellular membranes. Recently, it has been demonstrated that the lack of one contact site leads to the expansion of an alternative one. Specifically, loss of the ER-mitochondria encounter structure (ERMES) can be bypassed by point mutations in the vacuolar protein Vps13, or by overexpression of the mitochondrial Mdm10 complementing protein 1 (Mcp1). However, the mechanism by which this bypass support lipid homeostasis has remained unclear. In this work, I analyzed the membrane topology of Mcp1. My findings revealed that Mcp1 functions as a recruiter of Vps13 to mitochondria and promotes formation of vacuole-mitochondria MCS. I demonstrated that the N-terminal region of Mcp1 is exposed to the cytosol and mediates the recruitment of Vps13, thus establishing a functional mitochondria-vacuole MCS that compensate for the loss of ERMES. Finally, in a third project of my doctoral studies I investigated the relationship between the ERMES complex and the coenzyme Q6 (CoQ6) biosynthesis system. I observed that supplementation of yeast cells lacking functional ERMES with CoQ6 could rescue the growth retardation and the altered mitochondrial morphology of these mutated cells. Based on additional results from collaborating groups, we suggest that the ERMES complex coordinates coenzyme Q biosynthesis.