Lysosomal membrane glycoproteins: properties of LAMP-1 and LAMP-2

Several properties of the lysosomal membrane glycoproteins LAMP-1 and LAMP-2 have been analysed. Each molecule was strongly associated with lysosome membranes and was extracted only in the presence of detergent. Studies of the biosynthesis and processing of the glycoproteins showed that each contained a polypeptide core of approx. 43,000 Da as identified by use of tunicamycin and endoglycosidase H. Nascent glycoproteins pulse-labelled for 5 min with [35S]methionine were approx. 92,000 Da. These precursor molecules were processed in 30 min to highly heterogeneous mature glycoproteins of approx. 110,000 Da(LAMP-1) and 105,000 Da(LAMP-2). Concomitant with the increase in apparent Mr the molecules became endoglycosidase H resistant and acquired sialic acid residues, indicating that they were converted to complex-type oligosaccharides. The final maturation of the glycoproteins was blocked by monensin. Immunohistochemical analysis of tissues from Balb/c and Beige/J mice showed that the molecules were present on many types of cells, consistent with their presence in lysosomes. The patterns of tissue expression of LAMP-1 and LAMP-2 in the two mouse strains were the same except that the intensity of staining of LAMP-2 was less than that of LAMP-1. LAMP-2, but not LAMP-1, gave a decreased immunofluorescent staining intensity in transformed HaNIH as compared with NIH/3T3 cells. The marked similarities between the LAMP proteins raise the consideration of common functions, possibly associated with the high oligosaccharide content of the molecules.