The relation between human lysosomal beta-galactosidase and its protective protein.
Cultured skin fibroblasts from patients with the lysosomal storage disease galactosialidosis lack a 54-kDa protein which is a precursor of 32-kDa and 20-kDa proteins, which immunoprecipitate with human anti-beta-galactosidase antiserum. The lack of a 32-kDa "protective protein" results in a combined deficiency of beta-galactosidase and sialidase. The mechanism of protection of lysosomal beta-galactosidase against proteolytic degradation is elucidated by sucrose density gradient centrifugation and immunoprecipitation studies. In normal fibroblasts at the low intralysosomal pH, more than 85% of beta-galactosidase exists as a high molecular weight (600-700 kDa) multimer and about 10% as a monomer of 64-kDa. In mutant cells from galactosialidosis patients, the residual enzyme activity, about 10%, is present as a monomer and no multimer exists. After addition of the 54-kDa precursor form of the protective protein, the density pattern of beta-galactosidase in galactosialidosis cells is normalized. Immunoprecipitation studies after sucrose density gradient centrifugation on homogenate and on purified beta-galactosidase from normal fibroblasts show that the protective protein is associated only with the multimeric form of beta-galactosidase. We propose that intralysosomal protection against proteolysis of beta-galactosidase and sialidase is accomplished by aggregation into a high molecular weight complex consisting of multimeric beta-galactosidase, sialidase, and protective protein. The genetic deficiency of the latter, as in galactosialidosis, results in a rapid degradation of monomeric beta-galactosidase and a loss of sialidase activity.
D004591:Electrophoresis, Polyacrylamide Gel; D005347:Fibroblasts / Q000378:metabolism; D005696:Galactosidases / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D007787:Lactose Intolerance / Q000378:metabolism; D008247:Lysosomes / Q000201:enzymology*; D008970:Molecular Weight; D009439:Neuraminidase / Q000172:deficiency; D011498:Protein Precursors / Q000378:metabolism; D011506:Proteins / Q000378:metabolism*; D012867:Skin / Q000201:enzymology*; D001616:beta-Galactosidase / Q000302:isolation & purification / Q000378:metabolism*
D011498:Protein Precursors; D011506:Proteins; D005696:Galactosidases; D009439:Neuraminidase; D001616:beta-Galactosidase