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. Highly N-glycosylated. N-glycosylation is essential for GLMP stability and for MFSD1 lysosomal localization. Belongs to the GLMP family.