Phosphatidylserine flux into mitochondria unveiled by organelle-targeted Escherichia coli phosphatidylserine synthase PssA.

Most phospholipids are synthesised in the endoplasmic reticulum and distributed to other cellular membranes. Although the vesicle transport contributes to the phospholipid distribution among the endomembrane system, exactly how phospholipids are transported to, from and between mitochondrial membranes remains unclear. To gain insights into phospholipid transport routes into mitochondria, we expressed the Escherichia coli phosphatidylserine (PS) synthase PssA in various membrane compartments with distinct membrane topologies in yeast cells lacking a sole PS synthase (Cho1). Interestingly, PssA could complement loss of Cho1 when targeted to the endoplasmic reticulum (ER), peroxisome, or lipid droplet membranes. Synthesised PS could be converted to phosphatidylethanolamine (PE) by Psd1, the mitochondrial PS decarboxylase, suggesting that phospholipids synthesised in the peroxisomes and low doses (LDs) can efficiently reach mitochondria. Furthermore, we found that PssA which has been integrated into the mitochondrial inner membrane (MIM) from the matrix side could partially complement the loss of Cho1. The PS synthesised in the MIM was also converted to PE, indicating that PS flops across the MIM to become PE. These findings expand our understanding of the intracellular phospholipid transport routes via mitochondria.

Yeast transformation stress, together with loss of Pah1, phosphatidic acid phosphatase, leads to Ty1 retrotransposon insertion into the INO4 gene.

Most phospholipids are synthesized via modification reactions of a simple phospholipid phosphatidic acid (PA). PA and its modified phospholipids travel between organelle membranes, for example, the endoplasmic reticulum (ER) and mitochondrial inner membrane, to be converted to the other phospholipids. To gain insight into mechanisms of the phospholipid biosynthetic pathways, we searched for factors whose loss affects the phospholipid synthesis using an in vitro phospholipid transport assay. Among the various factors that were tested, we noticed that a lack of Pah1, which is a phosphatidic acid phosphatase, led to severe defects in phospholipid synthesis, which was not rescued by re-expression of wild-type Pah1. These results indicated other mutations in addition to the deletion of Pah1. Interestingly, we found that stress conditions associated with the yeast transformation process triggered a disruption of the INO4 gene by insertion of the Ty1 retrotransposon in pah1∆ strains. Additionally, we noticed that loss of the diacylglycerol kinase Dgk1, which has an opposing function to Pah1, suppressed the insertional mutation of INO4. These findings suggest that normal Pah1 function is critical for the suppression of insertional mutations by retrotransposon elements.

Maintenance of Cardiolipin and Crista Structure Requires Cooperative Functions of Mitochondrial Dynamics and Phospholipid Transport.

Mitochondria are dynamic organelles that constantly fuse and divide to maintain their proper morphology, which is essential for their normal functions. Energy production, a central role of mitochondria, demands highly folded structures of the mitochondrial inner membrane (MIM) called cristae and a dimeric phospholipid (PL) cardiolipin (CL). Previous studies identified a number of factors involved in mitochondrial dynamics, crista formation, and CL biosynthesis, yet it is still enigmatic how these events are interconnected and cooperated. Here, we first report that mitochondrial fusion-division dynamics are important to maintain CL abundance. Second, our genetic and biochemical analyses revealed that intra-mitochondrial PL transport plays an important role in crista formation. Finally, we show that simultaneous defects in MIM fusion and intra-mitochondrial PL transport cause a drastic decrease in crista structure, resulting in CL depletion. These results expand our understanding of the integrated functional network among the PL transport, crista formation, and CL biogenesis.