Dissection of the Molecular Consequences of a Double Mutation Causing a Human Lysosomal Disease

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Abstract
Aspartylglucosaminidase (AGA) is a lysosomal enzyme, the deficiency in which leads to human storage disease aspartylglucosaminuria (AGU). AGUFin is the most common AGU mutation in the world and is found in 98% of AGU alleles in Finland, where the population displays enrichment of the disease allele. The AGUFin allele actually contains a double mutation, both individual mutations resulting in amino acid substitutions: Arg-161 → Gln and Cys-163 → Ser. The separate consequences of these two amino acid substitutions for the intracellular processing of the AGA polypeptides were analyzed using a stable expression of mutant polypeptides in Chinese hamster ovary (CHO) cells. The synthesized polypeptides were monitored by metabolic labeling, followed by immunoprecipitation, immunofluorescence, and immunoelectron microscopy. The Arg-161 → Gin substitution did not affect the intracellular processing or transport of AGA and the fully active enzyme was correctly targeted to lysosomes. The Cys-163 → Ser substitution prevented the early proteolytic cleavage required for the activation of the precursor AGA polypeptide and the inactive enzyme was accumulated in the endoplasmic reticulum (ER). The precursors of the translation products of the AGUFin double mutant and the Cys-163 → Ser mutant were also observed in the culture medium. When cells expressing the normal AGA or AGUFin double mutation were treated with DTT to prevent the formation of disulfide bonds, both normal and mutated AGA polypeptides remained in the inactive precursor form and were not secreted into the medium. These results indicate that correct initial folding is essential for the proteolytic activation of AGA. They also suggest that a fraction of the AGUFin polypeptides remain in a conformation that is able to enter the secretory pathway, but in which the lysosomal targeting signals are hidden.