Molecular defect in combined beta-galactosidase and neuraminidase deficiency in man.

An antiserum raised against purified β-GAL precipitated the enzyme with three previously unknown proteins of 54-, 32-, and 20-kDa from human fibroblasts.

“An antiserum raised against purified β-GAL precipitated this enzyme with three previously unknown proteins of 54-, 32-, 20-kDa from human fibroblasts 5.”

The corrective factor secreted in the medium and capable of restoring NEU1 and β-GAL activities in GS fibroblasts was discovered because of its physical association with β-GAL.

“The identity of this “corrective factor” secreted in the medium and capable of restoring NEU1 and β-GAL activities in GS fibroblasts was eventually discovered fortuitously because of its physical association with β-GAL 4,5.”

A small pool of phosphorylated PPCA precursor can be recovered extracellularly, but this secreted form can be re-internalized via mannose-6-phosphate receptor-mediated endocytosis and routed to lysosomes where it becomes catalytically active.

“A small pool of phosphorylated precursor can also be recovered extracellularly, but this secreted form can be re-internalized by cells via mannose-6-phosphate receptor-mediated endocytosis and routed to the lysosomes where it becomes catalytically active as the de novo synthesized enzyme 5,6.”

The three proteins were products of a single gene, and the 54-kDa polypeptide was the uncleaved precursor of the 32- and 20-kDa proteins.

“It was soon apparent that the three proteins were the products of a single gene and that the 54-kDa polypeptide was in fact the uncleaved precursor of the 32- and 20-kDa proteins 5.”

The 54-kDa precursor was the extracellular form that, once taken up by GS cells, restored NEU1 and β-GAL activities and their lysosomal stability.

“The 54-kDa precursor was also the form present extracellularly that once taken up by GS cells restored NEU1 and β-GAL activities and their lysosomal stability.”

The three proteins were absent in fibroblasts of a GS patient with a severe clinical presentation.

“Remarkably, these three proteins were absent in fibroblasts of a GS patient with a severe clinical presentation.”

Co-culturing fibroblasts with combined NEU1/β-GAL deficiency with fibroblasts with isolated NEU1 deficiency partially corrected NEU1 activity in the former cells.

“Yet, co-culturing of fibroblasts with a combined NEU1/β-GAL deficiency with fibroblasts with an isolated NEU1 deficiency partially corrected NEU1 activity in the former cells, raising the possibility that a third gene product was involved in GS.”

In the absence of functional PPCA, NEU1 is retained in the endosomal compartment while the pool of β-GAL reaching lysosomes is rapidly degraded; formation of the PPCA/NEU1/β-GAL complex has a reciprocal advantage for the three enzymes, which acquire active and stable conformation in lysosomes.

“In absence of functional PPCA, NEU1 is retained in the endosomal compartment while the pool of β-GAL reaching the lysosomes is rapidly degraded 36.”

NEU1 requires an accessory protein for activity-the protective protein cathepsin A (CTSA; PPCA).

“NEU1 is unique among all known mammalian, microbial, or viral sialidases, as it requires an accessory protein for activity—the protective protein cathepsin A (CTSA; PPCA) (12, 19–23).”

NEU1 is unique among all known sialidases in that it requires an accessory protein for activity.

“NEU1 is unique among all known mammalian, microbial, or viral sialidases, as it requires an accessory protein for activity—the protective protein cathepsin A (CTSA; PPCA) (12, 19–23).”