Cathepsin A is a ubiquitously expressed multifunctional enzyme, with deamidase, esterase, and carboxypeptidase activities and a preference for substrates with hydrophobic amino acid residues at the P1-prime position. Association with CTSA, as part of the lysosomal multienzyme complex, is essential for stabilization of lysosomal beta-galactosidase (GLB1), as well as for activation of the lysosomal neuraminidase (NEU1) . Galjart et al. isolated cDNA encoding the human 'protective protein.' The cDNA directed the synthesis of a 452-amino acid precursor molecule that is processed in vivo to yield mature 'protective protein,' a heterodimer of 32 kD and 20 kD polypeptides held together by disulfide bridges. Because of homology to yeast carboxypeptidase Y, Galjart et al. suggested a protease activity for the 'protective protein.'