Biosynthesis of CL and PE. (A) CL production starts with PA, which can be sourced from multiple pathways (Box 1). PA needs to be trafficked to the matrix side of the IMM, where CL biosynthetic enzymes reside. PA shuttling from the OMM to the IMM is performed by Ups1-Mdm35/PRELID1-TRIAP1. Trafficking of CL or PA between OMM and IMM may also involve proteins at contact sites such as NDPK-D and MtCK. PLSCR3 promotes CL externalization to the OMM. It may transport CL or PA from the IMM to OMM or facilitate CL flip-flop from the inner to outer leaflet of the IMM. (B) PE is produced in the ER and mitochondria. ER pathways include the CDP–ethanolamine/Kennedy pathway (1), lysophosphatidylethanolamine acylation in yeast (2), and head group base exchange with PS in mammals (3). In mammals, PS can also be formed via base exchange with PC by PSS2, while in yeast, it is generated from CDP-DAG and serine by Cho1. PS produced in the MAM by PSS1/PSS2/Cho1 is then translocated to the OMM with the help of ER-mitochondrion MCS, such as ERMES in yeast. Ups2-Mdm35/PRELID3b-TRIAP1 transports PS to the IMM, where Psd1/PISD decarboxylates it to PE (4). Tethering of IMM and OMM by MICOS may allow Psd1/PISD to convert PS to PE in the OMM (5). PE made in the mitochondrion is exported to the ER, where it serves as a substrate for PC production. PE from the ER or OMM can also reach the IMM to a limited extent. Unresolved mechanisms are depicted by red arrows. MOG, monoacylglycerol.