[Biosynthesis, processing, and lysosome targeting of acid phosphatase]

The biosyntheses, processing, and intracellular transport of lysosomal APase were studied using pulse-chase experiments with primary cultured rat hepatocytes and subcellular fractionation techniques of rat liver after pulse-labeling with [35S] methionine in vivo. Apase was transported as a membrane-bound enzyme from the site of synthesis in the ER through the Golgi complex to lysosomes. Unlike many lysosomal enzymes which are translocated into lysosomes through the mediation of the Man-6-P receptors, transport of APase to lysosomes was independent of the Man-6-P receptor system. The transport of APase to lysosomes is dependent on the GY-motif which is located in its cytoplasmic domain. Kinetic experiments combined with subcellular fractionation of rat liver showed that after reaching the lysosomes, the membrane-bound APase (67 kDa) is subsequently released into the lysosomal matrix in the 64 kDa form, which is further processed via the 55 kDa form to the 48 kDa one, the major form of APase in rat liver lysosomal content. Our data from the in vitro experiments further showed that APase is released from lysosomal membranes into the lysosomal matrix by cathepsin D in the 65 kDa form, with release of a 1 kDa peptide, following which the released enzyme is further processed to the 64 kDa form, probably by lysosomal cysteine protease.