HUMAN AND HAMSTER PROCATHEPSIN-D, ALTHOUGH EQUALLY TAGGED WITH MANNOSE-6-PHOSPHATE, ARE DIFFERENTIALLY TARGETED TO LYSOSOMES IN TRANSFECTEDBHK CELLS

BHK cells transfected with human cathepsin D (CD) cDNA normally segreg ate the autologous hamster cathepsin D while secreting a large proport ion of the human proenzyme. In the present work, we have utilized thes e transfectants to examine to what extent the mannose-6-phosphate-depe ndent pathway for lysosomal enzyme segregation contributes to the diff erential sorting of human and hamster CD. We report that, in recipient control BHK cells, the rate of mannose-6-phosphate-dependent endocyto sis of human procathepsin D secreted by transfected BHK cells is lower than that of hamster procathepsin D and much lower than that of human arylsulphatase A. The missorted human enzyme bears phosphorylated oli gosaccharides and most of its phosphate residues are ''uncovered'', li ke the autologous enzyme. Thus, despite both the Golgi-associated modi fications of oligosaccharides, i.e. the phosphorylation of mannose and the uncovering of mannose-6-phosphate residues, which proceed on huma n and hamster procathepsin D with comparable efficiency, only the latt er is accurately packaged into lysosomes. Ammonium chloride partially affects the lysosomal targeting of cathepsin D in control BHK cells, w hereas in transfected cells, this drug strongly inhibits the maturatio n of human procathepsin D and slightly enhances its secretion. These d ata indicate that: (1) over-expression of a lysosomal protein does not saturate the Golgi-associated reactions leading to the synthesis of m annose-6-phosphate; (2) a portion of cathepsin D is targeted independe ntly of mannose-6-phosphate receptors in the transfected BHK cells; an d (3) whichever mechanism for lysosomal delivery of autologous procath epsin D is involved, this is not saturated by the high rate of express ion of human cathepsin D.