Expression of human cathepsin D in Xenopus oocytes: Phosphorylation and intracellular targetting

We have obtained expression of a cDNA clone for human cathepsin D in Xenopus laevis oocytes. Biosynthetic studies with [³⁵S]methionine labeling demonstrated that most of the cathepsin D remained intracellular and underwent proteolytic cleavage, converting a precursor of M(r) 47,000 D to a mature form of M(r) 39,000 D with processing intermediates of M(r) 43,000-41,000 D. >90% of the cathepsin D synthezed by oocytes bound to a mannose 6-phosphate (Man-6-P) receptor affinity column, indicating the presence of phosphomannosyl residues. An analysis of [2-³H]mannose-labeled oligosaccharides directly demonstrated phosphomannosyl residues on cathepsin D. Sucrose-gradient fractionation, performed to define the membranous compartments that cathepsin D traversed during its biosynthesis, demonstrated that cathepsin D is targeted to a subpopulation of yolk platelets, the oocyte equivalent of a lysosome. Xenopus oocytes were able to endocytose lysosomal enzymes from the medium and this uptake was inhibited by Man-6-P, thus demonstrating the presence of Man-6-P receptors in these cells. Therefore, the entire Man-6-P dependent pathway for targeting of lysosomal enzymes is present in the oocytes. Xenopus oocytes should be a useful system for examining signals responsible for the specific targeting of lysosomal enzymes to lysosomes.