Lysosomes are a bit like the suicidal bags of cells. They help to clean cells, have an acidic pH and contain a large number of hydrolyzing enzymes.
But why don't these hydrolyzing enzymes attack and destroy lysosomes' membranes?
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Early histochemical work indicated that the internal surface of the lysosomal membrane has a glycocalyx - a layer of polysaccharide, presumed to have a protective role.
Neiss, W. F. (1984) A coat of glycoconjugates on the inner surface of the lysosomal membrane in the rat kidney. Histochemistry 80, 603–608
Subsequently it was found that major membrane proteins of the lysosomal membrane are protected from proteolysis by glycosylation:
Kundra, R. and Kornfeld, S. (1999) Asparagine-linked oligosaccharides protect Lamp-1 and Lamp-2 from intracellular proteolysis. J. Biol. Chem. 274, 31039–31046
See this paper for a more recent study of another lysosomal membrane protein:
Schieweck O. et al. (2009) NCU-G1 is a highly glycosylated integral membrane protein of the lysosome. Biochem. J. 422: 83-90
NCU-G1 is the mouse orthologue of the human C1orf85 protein. The NCU-G1 gene encodes a 404 amino acid protein. This is a Type 1 transmembrane protein - i.e. the N terminal domain is directed to the extracytoplasmic surface of the membrane. In vivo the protein is detected in 70 kDa and 80 kDa forms. The discrepancy between the polypeptide size and the molecular mass is shown to be due to extensive glycosylation. The protein sequence includes 9 potential glycosylation sites.
