Transport of human lysosomal neuraminidase to mature lysosomes requires protective protein cathepsin A

Human lysosomal N-acetyl-alpha-neuraminidase is deficient in two lysosomal storage disorders, sialidosis, caused by structural mutations in the neuraminidase gene, and galactosialidosis, in which a primary defect of protective protein/cathepsin A (PPCA) leads to a combined deficiency of neuraminidase and beta-D-galactosidase, These three glycoproteins can be isolated in a high molecular weight multi-enzyme complex, and the enzymatic activity of neuraminidase is contingent on its interaction with PPCA,To explain the unusual need of neuraminidase for an auxiliary protein, we examined, in transfected COS-1 cells, the effect of PPCA expression on post-translational modification, turnover and intracellular localization of neuraminidase, In pulse-chase studies, we show that the enzyme is synthesized as a 46 kDa glycoprotein, which is poorly phosphorylated, does not undergo major proteolytic processing and is secreted, Importantly, its half-life is not altered by the presence of PPCA, However, neuraminidase associates with the PPCA precursor shortly after synthesis, since the latter protein coprecipitates with neuraminidase using anti-neuraminidase antibodies, We further demonstrate by subcellular fractionation of transfected cells that neuraminidase segregates to mature lysosomes only when accompanied by wild-type PPCA, but not by transport-impaired PPCA mutants, These data suggest a novel role for PPCA in the activation of lysosomal neuraminidase, that of an intracellular transport protein.