Djp1 is a multifunctional Hsp40 cochaperone for mitochondrial phospholipid metabolism

Mitochondria are cellular energy hubs best known for ATP production via oxidative phosphorylation; however, they also serve as biosynthetic centers for phospholipids. Mitochondrial phospholipids are critical for various cellular processes, and their loss underlies myriad mitochondrial diseases. The critical enzymes underlying these biosynthetic cascades are encoded in the nucleus, translated in the cytosol, and imported into mitochondria. Understanding of mechanisms and factors that ensure precise targeting of proteins to mitochondria has been long overlooked but remains critical. Recently, the J-protein/Hsp40 cochaperone Djp1 has emerged as a key player in mitochondrial protein targeting by promoting the transfer of precursors from the endoplasmic reticulum (ER) surface to mitochondria in a pathway termed ER-SURF. Molecular details regarding how Djp1 recognizes clients and more broadly supports mitochondrial function remain unknown. Using biochemical approaches, proteomics, and thin layer chromatography, we demonstrate that Djp1 is a regulator of Phosphatidylserine decarboxylase 1 (Psd1), an inner mitochondrial membrane resident responsible for mitochondrial phosphatidylethanolamine (PE) production. This regulation of Psd1 biogenesis is dependent on its mitochondrial targeting signal and is specific to Djp1 compared to other members of the Hsp40 family or ER targeting factors. Intriguingly, the combined loss of Djp1 and Psd1 results in a synthetic sick phenotype that unexpectedly reflects a role(s) for Djp1 in proper mitochondrial phospholipid metabolism independent of Psd1. Taken together, these findings expand our understanding of Djp1-dependent mitochondrial protein regulation and unveil Djp1 as important for mitochondrial phospholipid metabolism by multiple mechanisms.