Identification of an accessory protein necessary for the activation of the Na+/glucose cotransporter 2 (892.37)

Abstract

The Na+/glucose cotransporter SGLT2, which accounts for over 90% of renal glucose reabsorption, has become a major pharmaceutical target for type 2 diabetes treatment. Unfortunately, functional studies on SGLT2 have been hindered due to its lack of activity when expressed heterologously. Using an expression cloning strategy to identify a required accessory protein for SGLT2 function, we have identified MAP17 (membrane associated protein of 17 kDa) which is specifically expressed in the kidney proximal tubule, along with SGLT2. Using electrophysiology (oocytes) and radioactive uptakes (oocytes/OK cells), we demonstrate that MAP17 increases the activity of SGLT2 in both oocytes (150 fold) and OK cells (15 fold). Furthermore, Western blot and immunofluorescence studies using FLAG‐SGLT2 show that the cell surface density for SGLT2 is independent of MAP17 expression, which suggests that MAP17 stimulates the activity of SGLT2, but not its targeting. Finally, functional assays of disease‐causing mutations to SGLT2 (familial renal glucosuria) expressed in OK cells confirm these mutations to be non‐functional, even in the presence of MAP17. In conclusion, we have identified a cofactor responsible for the activation of SGLT2 which enables its characterization in heterologous systems.