A new functional role of mitochondria-anchored protein ligase in peroxisome morphology in mammalian cells

Mitochondria and peroxisomes are metabolically interconnected and functionally active subcellular organelles. These two dynamic organelles, share a number of common biochemical functions such as beta-oxidation of fatty acids and detoxification of peroxides. The biogenesis and morphology of both these organelles in the mammalian cells is controlled by common transcription factors like PGC1 alpha, and by a common fission machinery comprising of fission proteins like DRP1, Mff, and hFis1, respectively. In addition, the outer membrane mitochondria-anchored protein ligase (MAPL), the first mitochondrial SUMO E3 ligase with a RING-finger domain, also regulates mitochondrial morphology inducing mitochondrial fragmentation upon its overexpression. This fragmentation is dependent on both the RING domain of MAPL and the presence of the mitochondrial fission GTPase dynamin-related protein-1 (DRP1). Earlier studies have demonstrated that mitochondrial-derived vesicles are formed independently of the known mitochondrial fission GTPase, DRP1 are enriched for MAPL and are targeted to peroxisomes. The current study shows that MAPL regulates morphology of peroxisomes in a cell-type specific manner. Fascinatingly, the peroxisome elongation caused either due to silencing of DRP1 or by addition of polyunsaturated fatty acid, docosahexaenoic acid was blocked by overexpressing MAPL in mammalian cell lines. Furthermore, the transfection and colocalisation studies of MAPL with peroxisome membrane marker, PMP70, in different cell lines clearly revealed a cell-type specificity of transport of MAPL to peroxisomes. Previous work has placed the Vps35 (retromer component) as vital for delivery of MAPL to peroxisomes, placing the retromer as critical for the formation of MAPL-positive mitochondrial-derived vesicles. The results of polyethylene glycol-based cell-cell fusion assay signified that the enrichment of MAPL in peroxisomes is through vesicles and a retromer dependent phenomenon. Thus, a novel function for MAPL in peroxisomes is established to regulate peroxisome elongation and morphology under growth conditions and thus possibly modulate peroxisome fission.