Functional characterization and genomic organization of the human Na +-sulfate cotransporter hNaS2 gene ( SLC13A4)

Abstract

Sulfate plays an essential role in human growth and development. Here, we characterized the functional properties of the human Na +-sulfate cotransporter (hNaS2), determined its tissue distribution, and identified its gene ( SLC13A4) structure. Expression of hNaS2 protein in Xenopus oocytes led to a Na +-dependent transport of sulfate that was inhibited by thiosulfate, phosphate, molybdate, selenate and tungstate, but not by oxalate, citrate, succinate, phenol red or DIDS. Transport kinetics of hNaS2 determined a K m for sulfate of 0.38 mM, suggestive of a high affinity sulfate transporter. Na + kinetics determined a Hill coefficient of n = 1.6 ± 0.6, suggesting a Na: SO 4 2 - stoichiometry of 2:1. hNaS2 mRNA was highly expressed in placenta and testis, with intermediate levels in brain and lower levels found in the heart, thymus, and liver. The SLC13A4 gene contains 16 exons, spanning over 47 kb in length. Its 5′-flanking region contains CAAT- and GC-box motifs, and a number of putative transcription factor binding sites, including GATA-1, AP-1, and AP-2 consensus sequences. This is the first study to characterize hNaS2 transport kinetics, define its tissue distribution, and resolve its gene ( SLC13A4) structure and 5′ flanking region.