G328E and G409E sialin missense mutations similarly impair transport activity, but differentially affect trafficking

Two disease-associated missense mutations in the sialin gene (G328E and G409E) have recently been identified in patients with lysosomal free sialic acid storage disease. We have assessed the effect of these mutations and find complete loss of measurable transport activity with both and impaired trafficking of the G409E protein. These results suggest that the two residues are important for proper function of sialin and confirm the association of loss of transport with disease causative mutations.