Functional role of specific amino acid residues in human thiamine transporter SLC19A2:: mutational analysis

SLC19A2 is a membrane thiamine transporter expressed in a variety of human tissues, including the gastrointestinal tract. Little is currently known about the structure/function relationship of SLC19A2. We examined the effect of introducing mutations in SLC19A2 identical to those found in thiamine-responsive megaloblastic anemia syndrome (TRMA), on functional activity and membrane expression of the transporter. We also examined the effect of mutating the only conserved anionic residue (E138) in the transmembrane (TM) domains of the SLC19A2 and that of the putative glycosylation sites (N63, N314). Northern blot analysis showed SLC19A2 mRNA was expressed at the same level in HeLa cells transfected with wild-type or mutated SLC19A2. Introducing the clinically relevant mutations (D93H, S143F, G172D) or mutation at the conserved anionic residue (E138A) of SLC19A2 led to a significant (P < 0.01) inhibition of thiamine uptake. Mutations of the two potential N-linked glycosylation sites (N63Q, N314Q) of SLC19A2 did not affect functional activity; they did, however, lead to a noticeable reduction in apparent molecular weight of protein. Western blot analysis showed all proteins (except D93H) were expressed in the membrane (not the cytoplasmic) fraction of HeLa cells. These results provide direct confirmation that clinically relevant mutations in SLC19A2 observed in TRMA cause malfunctioning of the transporter and/or a defect in its translation/stability. Results also show conserved TM anionic residue of the SLC19A2 protein is critical for its function. Furthermore, native SLC19A2 is glycosylated, but this is not important for its function.