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Abstract
Lysosomes are major sites for intracellular, acidic hydrolase-mediated proteolysis and cellular degradation. The export of low-molecular-weight catabolic end-products is facilitated by polytopic transmembrane proteins mediating secondary active or passive transport. A number of these lysosomal transporters, however, remain enigmatic. We present a detailed analysis of MFSD1, a hitherto uncharacterized lysosomal family member of the major facilitator superfamily. MFSD1 is not N-glycosylated. It contains a dileucine-based sorting motif needed for its transport to lysosomes. Mfsd1 knockout mice develop splenomegaly and severe liver disease. Proteomics of isolated lysosomes from Mfsd1 knockout mice revealed GLMP as a critical accessory subunit for MFSD1. MFSD1 and GLMP physically interact. GLMP is essential for the maintenance of normal levels of MFSD1 in lysosomes and vice versa. Glmp knockout mice mimic the phenotype of Mfsd1 knockout mice. Our data reveal a tightly linked MFSD1/GLMP lysosomal membrane protein transporter complex.
Highlights
Lysosomes are dynamic and membrane-bound organelles ubiquitously found in eukaryotic cells
Co-immunofluorescence staining with antibodies against HA, LAMP2 and Major facilitator superfamily domain containing 1’ (MFSD1) confirmed the co-localization of MFSD1 with LAMP2 and the specificity of our MFSD1 antibody
Co-immunofluorescence staining of mouse embryonic fibroblasts (MEF) for endogenous MFSD1 with LAMP1 validated the lysosomal localization at the endogenous level (Figure 1C) and notably MFSD1 was absent from Golgi-apparatus structures
Summary
Lysosomes are dynamic and membrane-bound organelles ubiquitously found in eukaryotic cells. Their major function is the hydrolytic degradation of macromolecules like proteins, complex lipids, nucleic acids and oligosaccharides after uptake from the extracellular space by pinocytosis and endocytosis or from intracellular sources after fusion with autophagosomes (Saftig and Klumperman, 2009). Hydrolytic degradation is mediated by the concerted action of ~60 different, mostly soluble acid hydrolases. The catabolic end-products of these reactions (e.g. amino acids, peptides, monosaccharides, nucleosides) are exported by polytopic transmembrane proteins to the cytosol for anabolic processes. The vast majority of soluble proteins of lysosomes were identified due to their deficiencies in lysosomal storage disorders and by proteomics of purified lysosomes
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