Required to protect lysosomal transporter MFSD1 from lysosomal proteolysis and for MFSD1 lysosomal localization. Interacts (via lumenal domain) with lysosomal protein MFSD1; the interaction starts while both proteins are still in the endoplasmic reticulum and is required for stability and lysosomal localization of MFSD1. Detected in brain, heart, liver, kidney, lung, intestine, testis and spleen (PubMed:11444019, PubMed:19489740, PubMed:24487409). Expressed at highest levels in kidney cortex (PubMed:11444019, PubMed:19489740). However, another study reports highest expression levels in lung (PubMed:24487409). Expressed in myoblasts with expression increasing during differentiation into myotubes (PubMed:26707125). Expressed throughout embryogenesis (PubMed:11444019). After birth, there is a rapid increase in expression within the first week of life, reaching adult levels by week 2 (PubMed:27141234). Highly N-glycosylated (PubMed:19489740, PubMed:19349973). N-glycosylation is essential for GLMP stability and for MFSD1 lysosomal localization (PubMed:32959924). Viable and fertile (PubMed:24487409). Significantly reduced levels of Mfsd1 (PubMed:31661432). Splenomegaly (PubMed:24487409, PubMed:26047317). Reduced size of epididymal fat pads (PubMed:26047317). Enlargement of liver and spontaneous development of liver fibrosis which is not present at birth but develops shortly after and reaches a peak at 4 months of age (PubMed:24487409, PubMed:26047317, PubMed:27141234). The liver phenotype is associated with increased expression of markers for inflammatory responses, apoptosis and oxidative stress (PubMed:24487409, PubMed:27141234). Livers show local foci where hepatocytes are lost and liver sinusoidal endothelial cells are replaced by ordinary capillary endothelium (PubMed:31661432). Reduced liver function (PubMed:27141234). Kupffer cells have increased accumulation of iron and lipofuscin (PubMed:24487409). Decreased blood glucose and serum lipids and increased liver triacylglycerol (PubMed:26047317). Altered liver expression of genes involved in metabolism including decreased expression of Acox1, Angptl4, Fabp1, Slc2a2/Glut2, Ppara, Plin2 and Plin5 and increased expression of Apoc3, Cd36, Fasn, Pdk4, Ppard, Pparg, Scd1 and Scd2 (PubMed:26047317). Increased liver expression of Pecam31/Cd31 and Vwf (PubMed:31661432). Increased glucose and fatty acid uptake in hepatocytes and increased glucose oxidation (PubMed:26047317). Increased de novo lipogenesis in hepatocytes (PubMed:26047317). Increased hepatocyte proliferation and oval cell mobilization up to 6 months of age (PubMed:27141234). Increased frequency of liver tumors after 12 months of age (PubMed:27141234). Anemia, thrombocytopenia, and reduced levels of white blood cells (PubMed:27141234). No effect on composition of muscle fibers but myotubes metabolize glucose faster and have a larger pool of intracellular glycogen while oleic acid uptake, storage and oxidation are significantly reduced (PubMed:26707125). Increased myotube expression of Myh2, Myh4 and Scd1 and decreased expression of Cd36, Myh7, Plin2, Ppara, Ppard, Pparg, Pgc1a and Scd2 (PubMed:26707125). Belongs to the GLMP family.