Phosphatidylserine Transport to the Mitochondria Is Regulated by Ubiquitination

Mitochondrial membrane biogenesis requires the interorganelle transport of phospholipids. Phosphatidylserine (PtdSer) synthesized in the endoplasmic reticulum and related membranes (mitochondria-associated membrane (MAM)) is transported to the mitochondria by unknown gene products and decarboxylated to form phosphatidylethanolamine at the inner membrane by PtdSer decarboxylase 1 (Psd1p). We have designed a screen for strains defective inPtdSer transport (pstAmutants) between the endoplasmic reticulum and Psd1p that relies on isolating ethanolamine auxotrophs in suitable (psd2Δ) genetic backgrounds. Following chemical mutagenesis, we isolated an ethanolamine auxotroph that we designatepstA1-1. Using in vivo and in vitro phospholipid synthesis/transport measurements, we demonstrate that the pstA1-1 mutant is defective in PtdSer transport between the MAM and mitochondria. The gene that complements the growth defect and PtdSer transport defect of the pstA1-1 mutant is MET30, which encodes a substrate recognition subunit of the SCF (suppressor of kinetochore protein 1, cullin,F-box) ubiquitin ligase complex. Reconstitution of different permutations of MAM and mitochondria from wild type andpstA1-1 strains demonstrates that theMET30 gene product affects both organelles. These data provide compelling evidence that interorganelle PtdSer traffic is regulated by ubiquitination.