Human lysosomal .beta.-galactosidase-cathepsin A complex: Definition of the .beta.-galactosidase-binding interface on cathepsin A

Human lysosomal beta-galactosidase is organized as a 680-kDa complex with cathepsin A (also named carboxypeptidase L and protective protein), which is necessary to protect beta-galactosidase from intralysosomal proteolysis. To understand the molecular mechanism of beta-galactosidase protection by cathepsin A, we defined the structural organization of their complex including the beta-galactosidase-binding interface on cathepsin A. Radiation inactivation analysis suggested the existence of a 168-kDa structural subunit of the complex containing both beta-galactosidase and cathepsin A. Chemical cross-linking of the complex confirmed the existence of this subunit and showed that it is composed of one cathepsin A dimer and one beta-galactosidase monomer. The modeling of the cathepsin A dimer tertiary structure based on atomic coordinates of a wheat carboxypeptidase suggested a putative beta-galactosidase-binding cavity formed by the association of two cathepsin A monomers. According to this model two exposed loops of cathepsin A bordering the cavity were chosen as part of a putative beta-galactosidase-binding interface. Synthetic peptides corresponding to these loops were found both to dissociate the complex and to inhibit its in vitro reconstitution from purified cathepsin A and beta-galactosidase. The defined location of the GAL monomer in the complex with 35% of its surface covered by the CathA dimer may explain the stabilizing effect of CathA on GAL in lysosome.