Molecular characterization of protein-dependent lipid transport during mitochondrial biogenesis
by Fereshteh Sadeqi
Date of Examination:2022-03-28
Date of issue:2022-05-18
Advisor:Prof. Dr. Michael Meinecke
Referee:Prof. Dr. Michael Meinecke
Referee:Prof. Dr. Andreas Janshoff
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EnglishMitochondria are comprised of two membranes, each with a unique lipid and protein composition, subdividing this organelle into the outer mitochondrial membrane (OMM), the intermembrane space (IMS), the inner mitochondrial membrane (IMM) and the mitochondrial matrix. The building blocks of membranes are phospholipids which are mainly synthesized in the endoplasmic reticulum (ER). Besides the ER, mitochondria also harbour enzymes residing in the IMM converting ER-derived precursors into phosphatidyl-ethanolamine (PE) or the mitochondrial signature lipid cardiolipin (CL). In contrast to the endomembrane system, phospholipid trafficking in mitochondria is not connected by vesicular transport. Hence, other lipid transfer mechanisms are required to maintain the mitochondrial membrane structure and integrity. Recently, the lipid transfer protein (LTP) Ups1/Mdm35 was found in the mitochondrial IMS of yeast. This soluble heterodimeric protein complex shuttles phosphatidic acid (PA) from the OMM to the IMM for CL synthesis. Though, the structure and binding specificities of the Ups1/Mdm35 complex were extensively investigated, little is known about its complete molecular mechanism. In this study, we used a combination of in vitro and in silico approaches to further investigate the molecular mechanism of the LTP Ups1/Mdm35. We discovered that the Ups1/Mdm35 complex displays curvature inducing abilities by deforming artificial membrane vesicles into tubulated structures in vitro. Besides that, we unravelled an unexpected role of the Ups1/Mdm35 complex to act as a membrane curvature sensor. Our findings demonstrate that the membrane curvature inducing and sensing ability of the Ups1/Mdm35 complex are key to the extraction of lipids from membranes under low energy costs. Moreover, we found that the Ups1/Mdm35 complex tethers membranes of artificial vesicles into clusters while being enriched at membrane contact sites in vitro. Lastly, we showed oligomerization of the Ups1/Mdm35 complex in this study.
Keywords: mitochondria; lipid transfer protein; membrane curvature; Ups1; Mdm35