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The lipid droplet protein Pgc1 controls the subcellular distribution of phosphatidylglycerol

The biosynthesis of yeast phosphatidylglycerol (PG) takes place in the inner mitochondrial membrane. Outside mitochondria, the abundance of PG is low. Here we present evidence that the subcellular distribution of PG is maintained by locally controlled enzymatic activity of the PG-specific phospholipase, Pgc1. We document that the Pgc1 absence leads to spreading of PG over various cellular membranes. Fluorescently labeled Pgc1 protein strongly accumulates at the surface of lipid droplets (LD). We show, however, that LD are not only dispensable for Pgc1-mediated PG degradation, but even host no phospholipase activity of Pgc1. Our in vitro assays document the capability of LD-accumulated Pgc1 to degrade PG upon entry to membranes of the endoplasmic reticulum, mitochondria, and even of artificial phospholipid vesicles. FRAP analysis confirms continuous exchange of GFP-Pgc1 within individual LD in situ, suggesting that a steady-state equilibrium exists between LD and membranes to regulate immediate phospholipase activity of Pgc1. In this model, LD serve as storage place and shelter Pgc1 preventing untimely degradation, while both phospholipase activity and degradation of the enzyme occur in membranes.