Direct Extracellular interaction between Carbonic Anhydrase IV and the Human NBCl Sodium/Bicarbonate Co-Transporter

摘要

Sodium/bicarbonate co-transporters (NBC) are crucial in the regulation of intracellular pH (pH_i) and HCO~-_3 metabolism.Electrogenic NBC1 catalyzes HCO~-_3 fluxes in mammalian kidnkey,pancreas,and heat cells.Carbonic anhydrase IV(CAIV),which is also present in these tissues,is glycosylphosphatidyl inositol-anchored to the outer surface of the plasma membrane where it catalyzes the hydration-dehydration of CO_2/HCO~-_3.The physical and functional interactions of CAIV and NBC1 were investigated.NBC1 activity was measured by changes ofPh_i in NBC1-transfected HEK293 cells subjected to acid loads.Cotransfection of CAIV with NBC1 increased the rate of pH_i recovery by 44+-3%,as compared to NBC1-alone.In contrast,CAIV did not increase the functional activity of G767-NBC1 (mutated on the fourth extracellular loop(EC4) of NBC1),and G767T-NBC1,unlike wild-type NBC1,did not interact withCAIV in glutathione-S-transferase pull-down assays.This indicates that G767 of NBC1 is directly involved in CAIV interaction.NBC1-mediated pH_i recovery activity.We conclude that CAIV binds EC4 of NBC1,and this interaction is essential for full pH_i recovery activity.We conclude that CAIV binds EC4 of NBC1,and this interaction is essential for full NBC1 activity.The tethering of CAII and CAIV close to the NBC1 HCO~-_3 transport sit maximizes the transmembrane HCO~-_3 gradietn local to NBC1 and thereby activates the transport rate.