Neuraminidase, or lysosomal sialidase , has a dual physiologic function: it participates in intralysosomal catabolism of sialated glycoconjugates and is involved in cellular immune response. The enzyme occurs in a complex with beta-galactosidase (GLB1) and protective protein/cathepsin A (CTSA).The cDNA predicted a 415-amino acid protein showing extensive homology to other mammalian and bacterial neuraminidases. After the cleavage of a 47-amino acid N-terminal signal peptide and glycosylation, it becomes a 48.3-kD mature active enzyme similar to that found in the multienzyme lysosomal complex. Transient expression of cDNA in deficient human fibroblasts showed that the enzyme is compartmentalized in lysosomes and restores neuraminidase activity in a PPCA-dependent manner.