Protective protein/cathepsin A rescues N-glycosylation defects in neuraminidase-1.
BACKGROUND
Neuraminidase-1 (NEU1) catabolizes the hydrolysis of sialic acids from sialo-glycoconjugates. NEU1 depends on its interaction with the protective protein/cathepsin A (PPCA) for lysosomal compartmentalization and catalytic activation. Murine NEU1 contains 4 N-glycosylation sites, 3 of which are conserved in the human enzyme. The expression of NEU1 gives rise to differentially glycosylated proteins.


METHODS
We generated single-point mutations in mouse NEU1 at each of the 4 N-glycosylation sites. Mutant enzymes were expressed in NEU1-deficient cells in the presence and absence of PPCA.


RESULTS
All 4 N-glycosylation variants were targeted to the lysosomal/endosomal compartment. All N-glycans, with the exception of the most C-terminal glycan, were important for maintaining stability or catalytic activity. The loss of catalytic activity caused by the deletion of the second N-glycan was rescued by increasing PPCA expression. Similar results were obtained with a human NEU1 N-glycosylation mutant identified in a sialidosis patient. The N-terminal N-glycan of NEU1 is indispensable for its function, whereas the C-terminal N-glycan appears to be non-essential. The omission of the second N-glycan can be compensated for by upregulating the expression of PPCA.


CONCLUSIONS
These findings could be relevant for the design of target therapies for patients carrying specific NEU1 mutations.
Wang
Dongning
D
Department of Genetics and Tumor Cell Biology, St. Jude Children's Research Hospital, Memphis, TN 38105-2794, USA.
Zaitsev
Slava
S
Taylor
Garry
G
d'Azzo
Alessandra
A
Bonten
Erik
E
D000818:Animals; D043402:Cathepsin A / Q000235:genetics / Q000502:physiology*; D006031:Glycosylation; D006801:Humans; D051379:Mice; D008958:Models, Molecular; D009081:Mucolipidoses / Q000235:genetics / Q000628:therapy; D020125:Mutation, Missense; D009439:Neuraminidase / Q000235:genetics*; D017354:Point Mutation; D017386:Sequence Homology, Amino Acid
D009439:Neuraminidase; C468629:CTSA protein, human; C543471:CTSA protein, mouse; D043402:Cathepsin A