THE RENAL BRUSH-BORDER MEMBRANE SODIUM SULFATE COTRANSPORTER FUNCTIONS SITU HOMOTETRAMER/

The functional molecular size of the renal Na+/SO42- cotransporter was analysed with the radiation inactivation and fragmentation method. Pu rified brush border membrane vesicles preserved in a cryoprotective me dium were exposed to gamma-radiations. Initial rates of SO42- influx i nto these vesicles were estimated with membranes irradiated with 0, 4 and 8 Mrad, In each case, SO42- uptake by irradiated membranes was sig nificantly reduced but remained linear during the first 5 sec of incub ation, To avoid artifacts arising from a decrease in the driving force caused by modifications in membrane permeability, this incubation per iod was chosen to measure the effect of irradiation on the SO42- trans port activity. Increasing irradiation doses resulted in a monoexponent ial decrease in transport activity allowing the molecular size to be e stimated at 238 +/- 6 kDa (SD, n = 3). Recently, a cDNA for the Na + / SO42- cotransporter was cloned and expressed in Xenopus laevis oocytes (Markovich D. et al. (1993) Proc. Natl Acad, Sci, U.S.A. 90, 8073-807 7). The deduced amino acid sequence of this cotransporter predicts a m olecular weight of 66 kDa, We suggest that the in situ activity of the renal brush border membrane Na+/SO42- cotransporter requires the pres ence of four intact and identical subunits arranged as a homotetramer. Copyright (C) 1996 Elsevier Science Ltd