Determining the Lipid-Binding Specificity of SMP Domains: An ERMES Subunit as a Case Study

Structural basis for mitoguardin-2 mediated lipid transport at ER-mitochondrial membrane contact sites

The endoplasmic reticulum (ER)-mitochondria contact site (ERMCS) is crucial for exchanging biological molecules such as phospholipids and Ca²⁺ ions between these organelles. Mitoguardin-2 (MIGA2), a mitochondrial outer membrane protein, forms the ERMCS in higher eukaryotic cells. Here, we report the crystal structures of the MIGA2 Lipid Droplet (LD) targeting domain and the ER membrane protein VAPB bound to the phosphorylated FFAT motif of MIGA2. These structures reveal that the MIGA2 LD targeting domain has a large internal hydrophobic pocket that accommodates phospholipids and that two phosphorylations of the FFAT motif are required for tight interaction of MIGA2 with VAPB, which enhances the rate of lipid transport. Further biochemical studies show that MIGA2 transports phospholipids between membranes with a strong preference for binding and trafficking phosphatidylserine (PS). These results provide a structural and molecular basis for understanding how MIGA2 mediates the formation of ERMCS and facilitates lipid trafficking at the ERMCS.

The ER-mitochondria contact sites are crucial for exchanging phospholipids. Here, Kim et al. present crystal structures of mitoguardin-2 (MIGA2) which reveal that MIGA2 directly binds phospholipids and transfers them between the ER and mitochondria.

StaR-related lipid transfer-like domain-containing protein CLDP43 affects cardiolipin synthesis and mitochondrial function in Trypanosoma brucei

Cardiolipin is known to interact with bacterial and mitochondrial proteins and protein complexes. Unlike in Escherichia coli and Saccharomyces cerevisiae, the synthesis of cardiolipin is essential for growth of Trypanosoma brucei parasites in culture. Inhibition of cardiolipin production has been shown to result in major changes in the T. brucei proteome and energy metabolism, with CLDP43, a mitochondrial protein containing a StaR-related lipid transfer (START)-like domain, being depleted in a cardiolipin-dependent way. We now show that in T. brucei procyclic forms lacking CLDP43, cardiolipin metabolism and mitochondrial function are affected. Using quantitative and qualitative lipid analyses, we found that while steady-state levels of cardiolipin were elevated in CLDP43 knock-out parasites compared to parental cells, de novo formation of cardiolipin was down-regulated. In addition, depletion of CLDP43 resulted in partial loss of mitochondrial membrane potential and decreased ATP production via substrate level phosphorylation. Recombinant CLDP43 was found to bind cardiolipin and phosphatidic acid in lipid overlay experiments, suggesting that it may be involved in transport or synthesis of cardiolipin or its precursors in T. brucei.

Mechanisms of Non-Vesicular Exchange of Lipids at Membrane Contact Sites: Of Shuttles, Tunnels and, Funnels

Eukaryotic cells are characterized by their exquisite compartmentalization resulting from a cornucopia of membrane-bound organelles. Each of these compartments hosts a flurry of biochemical reactions and supports biological functions such as genome storage, membrane protein and lipid biosynthesis/degradation and ATP synthesis, all essential to cellular life. Acting as hubs for the transfer of matter and signals between organelles and throughout the cell, membrane contacts sites (MCSs), sites of close apposition between membranes from different organelles, are essential to cellular homeostasis. One of the now well-acknowledged function of MCSs involves the non-vesicular trafficking of lipids; its characterization answered one long-standing question of eukaryotic cell biology revealing how some organelles receive and distribute their membrane lipids in absence of vesicular trafficking. The endoplasmic reticulum (ER) in synergy with the mitochondria, stands as the nexus for the biosynthesis and distribution of phospholipids (PLs) throughout the cell by contacting nearly all other organelle types. MCSs create and maintain lipid fluxes and gradients essential to the functional asymmetry and polarity of biological membranes throughout the cell. Membrane apposition is mediated by proteinaceous tethers some of which function as lipid transfer proteins (LTPs). We summarize here the current state of mechanistic knowledge of some of the major classes of LTPs and tethers based on the available atomic to near-atomic resolution structures of several "model" MCSs from yeast but also in Metazoans; we describe different models of lipid transfer at MCSs and analyze the determinants of their specificity and directionality. Each of these systems illustrate fundamental principles and mechanisms for the non-vesicular exchange of lipids between eukaryotic membrane-bound organelles essential to a wide range of cellular processes such as at PL biosynthesis and distribution, lipid storage, autophagy and organelle biogenesis.