Sialin, encoded by the SLC17A5 gene, is a member of the SLC17 solute carrier family, a group of structurally related polytopic membrane proteins, part of the major facilitator superfamily of transporters. Sialin is a sialic acid transporter located in the lysosomal membrane. In the lysosome, sialic acid residues are sequentially removed from the carbohydrate chain by sialidases and then transported out of the lysosome by the transporter protein sialin^1,2. Sialin structure consists of 12 transmembrane helices (TMs). TM4 contains three residues (Arg-168, Glu-175, and His-183) that are highly conserved in the SLC17 family, suggesting that they may participate directly in substrate binding and translocation. In addition, TM4 contains a GXXXG helix packing motif that is important for the expression of sialin. SLC17 family includes several more GXXXG-like motifs located within TMs including GXXXG/S in TM2, G/AXXXG in TM5, and G/S/AXXXGXXXG/T in TM1³.

Proteins included in the SLC17 family are implicated with inherited neurological and metabolic diseases. Mutations in the SLC17A5 gene cause sialic acid storage diseases including infantile sialic acid storage disease and Salla disease. These conditions are autosomal recessive neurodegenerative disorders characterized by excessive accumulation of sialic acid in the lysosome^1,2.