Functional expression and purification of histidine-tagged rat renal Na/Phosphate (NaPi-2) and Na/Sulfate (NaSi-1) cotransporters.

Two mammalian sodium-dependent anion-cotransporters (NaPi-2 for phosphate and NaSi-1 for sulfate) have been expressed in Sf9 insect cells using the baculovirus expression system. A histidine tag was introduced at the C-termini in order to facilitate purification by metal-affinity chromatography. Sf9 cells infected with the histidine-tagged Ni/Pi-cotransporter exhibited more than 60-fold higher sodium-dependent transport of phosphate compared to noninfected cells. Expressed Na/Pi-cotransport exhibited a Km of Pi of 0.21 mm and an apparent Km of sodium of 92 mm. Infected cells expressed a 65 kDa polypeptide as detected by Western blotting and immunoprecipitation. Sf9 cells infected with the histidine-tagged NaSi-1 or untagged NaSi-1 protein expressed sodium-dependent sulfate cotransport up to 60-fold higher compared to noninfected cells. Transport of sulfate was highly dependent on sodium exhibiting a Km of SO2-4 of about 0.3-0.4 mm and a Km of sodium of 55 mm. By Western blotting and immunoprecipitation expressed NaSi-1 proteins were detected at 55-60 kDa. These studies demonstrate that histidine tagged proximal tubular Na-dependent cotransporters for phosphate and sulfate can be expressed functionally in Sf9 cells and that the kinetic characteristics were not altered by the introduction of a histidine tag at the C-termini. Furthermore, it is demonstrated that after solubilization under denaturing conditions histidine-tagged cotransporter proteins can be purified by metal-chelate affinity chromatography.