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 o rder to understand the sorting and targeting mechanisms of the two for ms of SGP-1, we have compared their maturation, processing, and secret ion in rat Sertoli cells in vivo. Metabolic labeling experiments in vi vo demonstrated that the 65 kDa form is synthesized first, then po st- translationally modified to the 70 kDa form of SGP-1. Subcellular frac tionation of testicular homogenate was used to obtain Golgi fractions containing up to 50-fold enrichment in galactosyltransferase. Permeabi lization 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 c athepsin L from Golgi membranes. Using quantitative electron-microscop ic 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 lysoso mes 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 th at the targeting of the 65 kDa form of SGP-1 to the lysosomes involves an early association with Golgi membrane that is independent of manno se B-phosphate receptors.