Molecular role of sulfated glycoprotein-1 (SGP-1/prosaposin) in Sertoli cells.
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Sulfated Glycoprotein-1 (SGP-1) is a major polypeptide secreted by rat Sertoli cells. Sequence analysis revealed a 70% sequence similarity with human prosaposin and a 80% similarity with mouse prosaposin. Both human and mouse prosaposin are 65-70 kDa proteins cleaved in the lysosomes into four 10-15 kDa proteins designated saposins A, B, C and D. Lysosomal saposins function as enzymatic activators that promote the hydrolysis of certain glycolipids. SGP-1 (70 kDa) was first considered as being exclusively secreted to the extracellular space. However, recent immunocytochemical studies using an anti SGP-1 antibody demonstrated the presence of this protein in Sertoli cell lysosomes. In addition Sertoli cell lysosomes isolated by cellular fractionation were found to contain a 65 kDa form of SGP-1 or lysosomal prosaposin, as well as, the 15 kDa saposins. Morphological and immunocytochemical evidences also indicated that both prosaposin and saposins may reach Sertoli cell phagosomes by lysosomal fusion. These phagosomes contain cytoplasmic residual bodies detached from spermatids during spermiation. Thus, prosaposin and their derived saposins must play a role in the hydrolysis of membrane glycolipids present in phagocytosed residual bodies. On the other hand, the function of the secreted form of SGP-1 is still unclear. However, SGP-1 was seen to interact with the plasma membrane of developing spermatids. Due to its capacity to bind certain types of gangliosides, SGP-1 appears to act as glycolipid transfer between Sertoli cells and the developing spermatids.
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