Molecular and functional characterization of a membrane contact site between peroxisomes and the endoplasmic reticulum

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In recent years, organelle contact sites have attracted considerable scientific attention. So far, this research field mainly focused on contact sites either between mitochondria and the endoplasmic reticulum (ER) or between the plasma membrane and the ER. These contact sites are not only responsible for the direct transfer of metabolites but also for interorganellar signaling modifying important cellular processes such as the endoplasmic reticulum (ER) stress response or the mitochondrial apoptosis pathway. Interestingly, close membrane appositions between peroxisomes (PO) and the ER were already documented by early electron microscopic studies. Recently, we have discovered a protein tether between PO and the ER consisting of peroxisomal acyl-CoA binding domain-containing protein 5 (ACBD5) and the vesicle-associated membrane protein-associated protein B (VAPB) of the ER. At the functional level, we hypothesize that this novel ER-PO contact site facilitates metabolite exchange (e.g. ether lipid or phospholipid precursors) between both organelles and thereby contributes to regulation of cellular lipid metabolism or PO proliferation. Importantly, patients with a deletion of ACBD5 have been recently identified, suffering from a very distinct pathological phenotype. This finding clearly underlines the functional significance of ACBD5 for human health. Thus, in order to shed light on the function of the ACBD5/VAPB contact site between ER and PO, we will address the following four specific issues: (1) We will characterize additional interaction partners of ACBD5 and its closest peroxisomal relative ACBD4 in order to gain further information on the function of this protein subfamily. (2) We will analyze the proteomic composition of ER-PO contact sites combining subcellular fractionation/BioID with quantitative mass spectrometry. (3) We will decipher the functional significance of ACBD5 for membrane lipid transfer, import of fatty acids and pexophagy. (4) We will characterize the phenotype of ACBD5 knockout mice in order to unravel pathogenetic mechanisms of ACBD5 deletion and the associated peroxisomal disease. Taken together, the current proposal aims at generating fundamental new data on the structure and function of a novel ER-PO tether.