Functional analysis and molecular modeling of a cloned urate transporter/channel

Recombinant protein, designated UAT, prepared from a cloned rat renal cDNA library functions as a selective voltage-sensitive urate transporter/channe l when fused with lipid bilayers. Since we previously suggested that UAT ma y represent the mammalian electrogenic urate transporter, UAT has been func tionally characterized in the presence and absence of potential channel blo ckers, several of which are known to block mammalian electrogenic urate tra nsport. Two substrates, oxonate (a competitive uricase inhibitor) and pyraz inoate, that inhibit renal electrogenic urate transport also block UAT acti vity. Of note, oxonate selectively blocks from the cytoplasmic side of the channel while pyrazinoate only blocks from the channel's extracellular face . Like oxonate, anti-uricase (an electrogenic transport inhibitor) also sel ectively blocks channel activity from the cytoplasmic side. Adenosine block s from the extracellular side exclusively while xanthine blocks from both s ides. These effects are consistent with newly identified regions of homolog y to uricase and the adenosine A1/A3 receptor in UAT and localize these hom ologous regions to the cytoplasmic and extracellular faces of UAT, respecti vely. Additionally, computer analyses identified four putative a-helical tr ansmembrane domains, two beta sheets, and blocks of homology to the E and B loops of aquaporin-1 within UAT. The experimental observations substantiat e our proposal that UAT is the molecular representation of the renal electr ogenic urate transporter and, in conjunction with computer algorithms, sugg est a possible molecular structure for this unique channel.