Molecular mechanism of mitochondrial phosphatidate transfer by Ups1/Mdm35

Cardiolipin plays many important roles for mitochondrial physiological function and is synthesized from phosphatidic acid (PA) at inner mitochondrial membrane (IMM). PA synthesized from endoplasmic reticulum needs to transfer to IMM via outer mitochondrial membrane (OMM). The transfer of PA between IMM and OMM is mediated by Ups1/Mdm35 protein family. Although there are many structures of this family available, the detailed molecular mechanism of how PA is transferred between membranes is yet unknown. Here, we report another crystal structures of Ups1/Mdm35 in the authentic monomeric apo state and the DHPA bound state. By combining subsequent all-atom molecular dynamics simulations, extensive structural comparisons and biophysical assays, we discovered the conformational changes of Ups1/Mdm35, identified key structural elements and residues during membrane binding and PA entry. We found the monomeric Ups1 on membrane is an important transit for the success of PA transfer, and the equilibrium between monomeric Ups1 and Ups1/Mdm35 complex on membrane affects the PA transfer rate and can be regulated by many factors including environmental pH.