REEXPRESSION OF THE MANNOSE 6-PHOSPHATE RECEPTORS IN RECEPTOR-DEFICIENT FIBROBLASTS - COMPLEMENTARY FUNCTION OF THE 2 MANNOSE 6-PHOSPHATE RECEPTORS IN LYSOSOMAL-ENZYME TARGETING

We have previously generated primary embryonic fibroblasts lacking eit her the cation-independent mannose 6-phosphate/insulin-like growth fac tor II receptor (MPR) or the cation-dependent MPR, two trans-membrane proteins that bind the mannose 6-phosphate (Man-6-P) recognition marke r on soluble lysosomal enzymes (Ludwig, T., Munier-Lehmann, H., Bauer, U., Hollinshead, M., Ovitt, C., Lobel, P., and Hoflack, B. (1994) EMB O J, 13, 3430-3437), These two cell types partially missort phosphoryl ated lysosomal enzymes, Using two dimensional gel electrophoresis, we show here that they secrete, in a large part, different phosphorylated ligands, In order to better understand the sorting function of the MP Rs, we have re expressed each MPR in MPR-negative fibroblasts. We show that the MPRs have similar capacities for transporting the bulk of th e newly synthesized lysosomal enzymes and that they target individual ligands with various efficiencies, However, high levels of one MPR do not fully compensate for the absence of the other, demonstrating that the two MPRs have complementary targeting functions, perhaps by recogn izing different features on lysosomal enzymes, The analysis of the pho sphorylated oligosaccharides shows that the ligands missorted in the a bsence of the cation dependent MPR are slightly but significantly depl eted in oligosaccharides with two Man-6-P residues, when compared with those missorted in the absence of the cation independent MPR, While t hese results could explain some differences between the structure and the sorting function of the two MPRs, they strongly suggest that the r eason why cells express two different but related MPRs is to maintain an efficient Man-B-P-dependent targeting process that could be potenti ally regulated by MPR expression.