Molecular mechanism of mitochondrial phosphatidate transfer by Ups1

Cardiolipin, an essential mitochondrial physiological regulator, is synthesized from phosphatidic acid (PA) in the inner mitochondrial membrane (IMM). PA is synthesized in the endoplasmic reticulum and transferred to the IMM via the outer mitochondrial membrane (OMM) under mediation by the Ups1/Mdm35 protein family. Despite the availability of numerous crystal structures, the detailed mechanism underlying PA transfer between mitochondrial membranes remains unclear. Here, a model of Ups1/Mdm35-membrane interaction is established using combined crystallographic data, all-atom molecular dynamics simulations, extensive structural comparisons, and biophysical assays. The alpha 2-loop, L2-loop, and alpha 3 helix of Ups1 mediate membrane interactions. Moreover, non-complexed Ups1 on membranes is found to be a key transition state for PA transfer. The membrane-bound non-complexed Ups1/ membrane-bound Ups1 ratio, which can be regulated by environmental pH, is inversely correlated with the PA transfer activity of Ups1/Mdm35. These results demonstrate a new model of the fine conformational changes of Ups1/Mdm35 during PA transfer. Combining crystallographic data, MD simulations, mutagenesis, lipid transfer and liposome binding assays, Lu et al. determine the mechanism of PA transfer between mitochondrial membranes, mediated by the Ups1/Mdm35 protein family. In particular, they describe role of the conformational change of the omega loop or alpha 2 loop of Ups1 in the Ups1 PA transfer activity.