Saposin C is required for normal resistance of acid β-glucosidase to proteolytic degradation

Saposins (A, B, C, and D) are small sphingolipid activator proteins that are derived by proteolytic processing of a common precursor, prosaposin. In the lysosomal sphingolipid degradation pathway, acid beta-glucosidase (GCase) requires saposin C for optimal in vitro and in vivo hydrolysis of glucocerebroside. The deficiency of prosaposin/saposins (PS-/-) in humans and mice leads to a decrease of GCase activity in selected tissues. Concordant decreases (>50%) of GCase protein and in vitro activity were detected in extracts of cultured fibroblasts and hepatocytes from PS-/- mice and human prosaposin-deficient fibroblasts. GCase RNA in the PS-/- cells was at wild-type levels. Compared with that in wild-type cells (t(1/2)>24 h), the GCase protein in the PS-/- cells had a faster disappearance rate (t(1/2)similar to1 h in mouse and similar to8 h in human) as determined by metabolic labeling and immunoprecipitation with anti-GCase antibodies. Treatment of PS-/- cells with leupeptin, an inhibitor of cysteine proteases, led to significant increases (similar to2-fold) in GCase protein and in vitro activity. Loading saposin C to human PS-/- fibroblasts resulted in an enhancement of GCase protein and in vitro activity. Saposin D loading had no effect. These data indicate that saposin C is required for GCase resistance to proteolytic degradation in the cell. Thus, diminished in vivo GCase activity would be greater than expected only from the lack of GCase activation by saposin C. These results indicate a new property for saposin C, an anti-proteolytic protective function toward GCase.