Trafficking of sulfated glycoprotein-1 (prosaposin) to lysosomes or to the extracellular space in rat Sertoli cells

Sulfated glycoprotein-1 (prosaposin) exists in 2 forms: a 65 kDa form targeted to lysosomes and a 70 kDa form secreted extracellularly. In order to understand the sorting and targeting mechanisms of the two forms of SGP-1, we have compared their maturation, processing, and secretion in rat Sertoli cells in vivo. Metabolic labeling experiments in vivo demonstrated that the 65 kDa form is synthesized first, then po st-translationally modified to the 70 kDa form of SGP-1. Subcellular fractionation of testicular homogenate was used to obtain Golgi fractions containing up to 50-fold enrichment in galactosyltransferase. Permeabilization of enriched Golgi fractions with saponin released the 70 kDa form, but did not affect the 65 kDa protein. While excess free mannose 6-phosphate did not release lysosomal SGP-1, it released the 35 kDa cathepsin L from Golgi membranes. Using quantitative electron-microscopic immunocytochemistry, the lysosomal contents of SGP-1 were shown to increase significantly after the administration of tunicamycin in vivo. Therefore, the trafficking of the 65 kDa form of SGP-1 to the lysosomes appears to be independent of the M6P-receptor pathway. The 70 kDa form of SGP-1 was found to aggregate within perforated Golgi fractions in a process which depends on low pH and calcium ions. We conclude that the targeting of the 65 kDa form of SGP-1 to the lysosomes involves an early association with Golgi membrane that is independent of mannose B-phosphate receptors.