Protective protein appears to be essential for both the activity of beta-galactosidase and neuraminidase, it associates with these enzymes and exerts a protective function necessary for their stability and activity (PubMed:1907282). Also functions as an activator of the sialidase NEU1 (PubMed:37205763). This protein is also a carboxypeptidase and can deamidate tachykinins (PubMed:1694176, PubMed:1756715, PubMed:1907282). {ECO:0000269|PubMed:1694176, ECO:0000269|PubMed:1756715, ECO:0000269|PubMed:1907282, ECO:0000269|PubMed:37205763}. This is the function of CTSA (cathepsin A, Ensembl gene identifier ENSG00000064601).