MOLECULAR ROLE OF SULFATED GLYCOPROTEIN-1 (SGP-1 PROSAPOSIN) IN SERTOLI CELLS/

Sulfated Glycoprotein-1 (SGP-1) is a major polypeptide secreted by rat Sertoli cells. Sequence analysis revealed a 70% sequence similarity w ith human prosaposin and a 80% similarity with mouse prosaposin. Both human and mouse prosaposin are 65-70 kDa proteins cleaved in the lysos omes into four 10-15 kDa proteins designated saposins A, B, C and D. L ysosomal saposins function as enzymatic activators that promote the hy drolysis of certain glycolipids. SGP-1 (70 kDa) was first considered a s being exclusively secreted to the extracellular space. However, rece nt immunocytochemical studies using an anti SGP-1 antibody demonstrate d the presence of this protein in Sertoli cell lysosomes. In addition Sertoli cell lysosomes isolated by cellular fractionation were found t o contain a 65 kDa form of SGP-1 or lysosomal prosaposin, as well as, the 15 kDa saposins. Morphological and immunocytochemical evidences al so indicated that both prosaposin and saposins may reach Sertoli cell phagosomes by lysosomal fusion. These phagosomes contain cytoplasmic r esidual bodies detached from spermatids during spermiation. Thus, pros aposin and their derived saposins must play a role in the hydrolysis o f membrane glycolipids present in phagocytosed residual bodies. On the other hand, the function of the secreted form of SGP-1 is still uncle ar. However, SGP-1 was seen to interact with the plasma membrane of de veloping spermatids. Due to its capacity to bind certain types of gang liosides, SGP-1 appears to act as glycolipid transfer between Sertoli cells and the developing spermatids.