Phosphatidylcholine Affects Inner Membrane Protein Translocases of Mitochondria

Abstract

Two protein translocases transport precursor proteins into or across the inner mitochondrial membrane. The presequence translocase (TIM23 complex) sorts precursor proteins with a cleavable presequence either into the matrix or into the inner membrane. The carrier translocase (TIM22 complex) inserts multispanning proteins into the inner membrane. Both protein import pathways depend on the presence of a membrane potential, which is generated by the activity of the respiratory chain. The non-bilayer-forming phospholipids cardiolipin and phosphatidylethanolamine are required for the activity of the respiratory chain and therefore to maintain the membrane potential for protein import. Depletion of cardiolipin further affects the stability of the TIM23 complex. The role of bilayer-forming phospholipids like phosphatidylcholine (PC) in protein transport into the inner membrane and the matrix is unknown. Here, we report that import of presequence-containing precursors and carrier proteins is impaired in PC-deficient mitochondria. Surprisingly, depletion of PC does not affect stability and activity of respiratory supercomplexes, and the membrane potential is maintained. Instead, the dynamic TIM23 complex is destabilized when the PC levels are reduced, whereas the TIM22 complex remains intact. Our analysis further revealed that initial precursor binding to the TIM23 complex is impaired in PC-deficient mitochondria. We conclude that reduced PC levels differentially affect the TIM22 and TIM23 complexes in mitochondrial protein transport.Mitochondria fulfill essential functions for the survival of the cell like energy conversion to produce ATP, synthesis of amino acids, lipids, and heme, as well as the generation of iron-sulfur clusters. They contain about 1000 proteins in yeast and 1500 proteins in humans (1, 2). More than 99% of the mitochondrial proteins are synthesized as precursors on cytosolic ribosomes. Mitochondria contain a sophisticated system of protein translocases to import precursor proteins (3-9). The translocase of the outer membrane (TOM 3 complex) forms the general entry gate for most precursor proteins. After passage of the TOM channel, distinct protein translocases sort the preproteins into the different subcompartments: the outer and inner membrane as well as the two aqueous compartments, the matrix and intermembrane space.The majority of mitochondrial proteins are sorted into the inner membrane and the matrix. Two inner membrane-bound protein complexes mediate protein import. The presequence translocase (also termed TIM23 complex) transports precursor proteins with a cleavable presequence into the inner membrane and the matrix, whereas the carrier translocase (also termed TIM22 complex) inserts proteins with multiple transmembrane segments into the inner membrane (3-9). The membrane potential across the inner membrane provides the driving force for both protein import pathways and is generated by the activity of the respiratory chain. Presequence-containing preproteins are directly transferred from the ...

Arabidopsis DGD1 SUPPRESSOR1 Is a Subunit of the Mitochondrial Contact Site and Cristae Organizing System and Affects Mitochondrial Biogenesis

“…Alternatively, altered mitochondrial membrane protein abundances could be due to the altered lipid composition of mitochondria, with fewer cristae present in mitochondria that contain the dgs1-1 mutant protein ( Figure 3B). The non-bilayer-forming phospholipids CL and PE are required for cristae structures and stabilization of respiratory complexes, whereas bilayer-forming phospholipid PC stabilizes TIM complexes (Schuler et al, 2016). Together, alterations in lipid composition, protein abundance, and potentially also altered mitochondria-ER contacts (hot spots for mitochondrial fission) by dysfunctional MICOS may be part of the reasons for the altered organelle size of mitochondria, chloroplasts, and the ER as evidenced by visualization with fluorescence tagging of proteins ( Figure 3A).…”

Phospholipid ebb and flow makes mitochondria go

“…It is exclusively synthesized in the ER by two separate routes: (1) via the CDPcholine pathway, by phosphorylation of choline, binding to CDP, and attachment of CDP-choline to DAG; and (2) by a series of methylation steps with PE as precursor (van der Veen et al, 2017). Normal PC levels are vital for mitochondrial integrity, including the biogenesis of β-barrel and some α-helical OMM proteins, TIM (translocase of the inner membrane) 23-mediated mitochondrial protein translocation, mitochondrial respiration, and cristae morphogenesis (Horibata et al, 2016;Schuler et al, 2015Schuler et al, , 2016. STARD7, which contains a lipid-binding START (StAR-related lipid transfer) domain, catalyzes PC transfer between liposomes (Horibata and Sugimoto, 2010).…”

The role of nonbilayer phospholipids in mitochondrial structure and function

“…Interestingly, the mitochondrial protein import defect is not specific to the nonbilayer‐forming phospholipids. A recent report has shown that a depletion in PC levels impairs mitochondrial protein import by destabilization of TIM23 . Thus, both bilayer and nonbilayer phospholipids impact mitochondrial protein import, albeit by different mechanisms.…”