Glycoproteins of the lysosomal membrane

Three glycoprotein antigens (120, 100, and 80 kD) were detected by mono- and/or polyclonal antibodies generated by immunization with highly purified rat liver lysosomal membranes. All of the antigens were judged to be integral membrane proteins based on the binding of Triton X-114. By immunofluorescence on normal rat kidney cells, a mouse monoclonal antibody to the 120-kD antigen co-stained with a polyclonal rabbit antibody that detected the 100- and 80-kD antigens as well as with antibodies to acid phosphatase, indicating that these antigens are preferentially localized in lysosomes. Few 120-kD-positive structures were found to be negative for acid phosphatase, suggesting that the antigen was not concentrated in organelles such as endosomes, which lack acid phosphatase. Immunoperoxidase cytochemistry also showed little reactivity in Golgi cisternae, coated vesicles, or on the plasma membrane. Digestion with endo-beta-N-acetylglucosaminidase H (Endo H) and endo-beta-N-acetylglucosaminidase F (Endo F) demonstrated that each of the antigens contained multiple N-linked oligosaccharide chains, most of which were of the complex (Endo H-resistant) type. The 120-kD protein was very heavily glycosylated, having at least 18 N-linked chains. It was also rich in sialic acid, since neuraminidase digestion increased the pI of the 120-kD protein from less than 4 to greater than 8. Taken together, these results strongly suggest that the glycoprotein components of the lysosomal membrane are synthesized in the rough endoplasmic reticulum and terminally glycosylated in the Golgi before delivery to lysosomes. We have provisionally designated these antigens lysosomal membrane glycoproteins lgp120, lgp100, lgp80.