Abstract
Of the four separate PE biosynthetic pathways in eukaryotes, one occurs in the mitochondrial inner membrane (IM) and is executed by phosphatidylserine decarboxylase (Psd1). Deletion of Psd1 is lethal in mice and compromises mitochondrial function. We hypothesize that this reflects inefficient import of non-mitochondrial PE into the IM. Here, we test this by re-wiring PE metabolism in yeast by re-directing Psd1 to the outer mitochondrial membrane or the endomembrane system and show that PE can cross the IMS in both directions. Nonetheless, PE synthesis in the IM is critical for cytochrome bc1 complex (III) function and mutations predicted to disrupt a conserved PE-binding site in the complex III subunit, Qcr7, impair complex III activity similar to PSD1 deletion. Collectively, these data challenge the current dogma of PE trafficking and demonstrate that PE made in the IM by Psd1 support the intrinsic functionality of complex III.
Highlights
Of the four separate PE biosynthetic pathways in eukaryotes, one occurs in the mitochondrial inner membrane (IM) and is executed by phosphatidylserine decarboxylase (Psd[1])
We tested the growth of wildtype (WT), psd1Δ, psd2Δ, and psd1Δpsd2Δ yeast in synthetic complete ethanol-glycerol (SCEG) or SC-lactate medium with or without ethanolamine supplementation (Fig. 1a)
Given the capacity of Psd[1] to produce up to 70% of PE in the cell[11], it has long been appreciated that PE made in the IM can be transported across the intermembrane space (IMS) to other membrane-bound compartments
Summary
Of the four separate PE biosynthetic pathways in eukaryotes, one occurs in the mitochondrial inner membrane (IM) and is executed by phosphatidylserine decarboxylase (Psd[1]). The lethal consequence of PISD deletion in mice and the failure of supplemental ethanolamine to rescue the respiratory defects of psd1Δ yeast suggest that PE made outside of the mitochondrion cannot compensate for the absence of Psd[11,5,10,15]. We further characterize ER-Psd[1], together with an OM-targeted chimeric Psd[1] (OM-Psd1), to test if the cytosol, IMS, or both, are barriers that prevent nonmitochondrially produced PE from functionally rescuing the absence of PE made in the IM Alongside strains expressing these re-directed Psd[1] constructs, we compare the mitochondrial function of psd1Δpsd2Δ yeast grown in the non-fermentable carbon source, lactate, with or without exogenous ethanolamine supplementation, to evaluate the ability of the ER-localized Kennedy pathway to support mitochondrial function. We conclude that in the context of bi-directional PE transport across the IMS, IM-localized Psd[1] is required to support electron transport chain activity
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