Mutations in sialidosis impair sialidase binding to the lysosomal multienzyme complex

Sialidosis is an autosomal recessive disease caused by the genetic deficien cy of lysosomal sialidase, which catalyzes the catabolism of sialoglycoconj ugates. The disease is associated with progressive impaired vision, macular cherry-red spots, and myoclonus (sialidosis type I) or with skeletal dyspl asia, Hurler-like phenotype, dysostosis multiplex, mental retardation, and hepatosplenomegaly (sialidosis type II). We analyzed the effect of the miss ense mutations G68V, S182G, G227R, F260Y, L270F, A298V, G328S, and L363P, w hich are identified in the sialidosis type I and sialidosis type II patient s, on the activity, stability, and intracellular distribution of sialidase. We found that three mutations, F260Y, L270F, and A298V, which are clustere d in the same region on the surface of the sialidase molecule, dramatically reduced the enzyme activity and caused a rapid intralysosomal degradation of the expressed protein. We suggested that this region might be involved i n sialidase binding with lysosomal cathepsin A and/or P-galactosidase in th e multienzyme lysosomal complex required for the expression of sialidase ac tivity. Transgenic expression of mutants followed by density gradient centr ifugation of cellular extracts confirmed this hypothesis and showed that si alidase deficiency in some sialidosis patients results from disruption of t he lysosomal multienzyme complex.