Functional Coupling of a Urate-Anion Exchanger URAT1 and Sodium-Dependent Anion Transporter SMCT2 on a PDZK1 Scaffold; Proposal of 'Transportsome' for Urate-Transport

Transporters utilize electrochemical gradients across membranes to translocate their substrates through the membranes. Many eukaryotic plasma membrane symporters transport substrates in a Na+-dependent manner by using an electrochemical Na+ gradient across plasma membranes, while exchangers couple the movement of a substrate against its electrochemical gradient with the movement of another substrate down its electrochemical gradient in the opposite direction. Although the Na+ gradient is ubiquitous and stable, the electrochemical gradients of substrates for exchangers usually depend on cells and are affected by cellular conditions. The concentrations of intracellular substrates limit the accumulation of extracellular substrates thorough exchangers. To overcome the constraint, exchangers need to couple with other proteins that help accumulation of substrates around exchangers. We have been proposing that such functional coupling of transporters is general and constitutes the functional unit of transport that we call “transportsome”. Here, we show one of the examples. Proteomics analysis following co-immunoprecipitation revealed that urate-anion exchanger URAT1 and Na+-dependent anion transporter SMCT2 formed a complex through scaffold protein PDZK1 in the apical membrane of renal proximal tubules. FRET experiment clearly indicated that PDZK1 assembled URAT1 and SMCT2 in living cells. To elucidate functional coupling of the transporters, URAT1 and SMCT2 were overexpressed in insect cells, purified and reconstituted into proteoliposomes in the presence or absence of PDZK1. The initial rate of urate-uptake was increased in the presence of Na+ when co-reconstituted with PDZK1 while Na+ did not alter the rate without PDZK1. These results suggest that SMCT2 generate an electrochemical gradient of the substrate which drives the urate-uptake via URAT1 located in close proximity to SMCT2 with PDZK1 scaffold. Taken together, the transporters and the scaffold protein formed Transportsome for urate-transport.