Regulation of glycosylation of Lamp-1 in the bovine renal epithelial cell line NBL-1 by changes in the concentration of extracellular phosphate

Abstract

We have identified the bovine renal homologue of Lamp-1 (lysosomal-associated membrane glycoprotein 1). It has very similar physical characteristics to other Lamp-1 proteins from a wide variety of tissues and species. Partial sequence analysis has shown it to be 61% identical with human Lamp-1 and about 50% identical with rat and mouse Lamp-1. The extent of glycosylation of bovine Lamp-1 alters in response to changes in the concentration of extracellular phosphate. Bovine renal epithelial cells (NBL-1) grown in normal or phosphate-starved medium contain Lamp-1 of 120 kDa. However, if cells are grown in medium containing 8-10 mM phosphate, they contain Lamp-1 of only 100 kDa. The core protein and mRNA levels have been shown to remain constant under both conditions. Therefore the only conclusion is that the extent of Lamp-1 glycosylation must be changing in response to the extracellular concentration of phosphate. Unlike Carlsson and Fukuda [(1990) J. Biol. Chem. 265, 20488-20495], who showed that the human Lamp-1 protein contained polylactosaminoglycan residues, we have been unable to demonstrate the partial deglycosylation of bovine Lamp-1 by endo-beta-galactosidase. This enzyme removes polylactosaminoglycan groups from glycoproteins, and therefore indicates that the carbohydrate structure of bovine Lamp-1 is probably different from that of other Lamp-1 proteins. At present the physiological importance of bovine renal Lamp-1 and the changes in its extent of glycosylation are unknown. In this paper we postulate that Lamp-1 may be involved in the cycling of plasma-membrane proteins to the lysosome. This is based on the finding that the only other known effect of high extracellular phosphate on NBL-1 cells is to cause a decrease in the Vmax. of plasma-membrane-associated Na(+)-dependent phosphate transport [Helps and McGivan (1991) Eur. J. Biochem. 200, 797-803].