Direct observation of the transmembrane recruitment of band 3 transport sites by competitive inhibitors. A 35Cl NMR study.

Numerous models describing anion exchange across the red cell membrane by band 3 have been discussed in literature. These models are readily distinguished from one another by an experiment which tests the ability of band 3 transport sites to be recruited to one side of the membrane. In order to observe directly the transmembrane recruitment of transport sites, we have developed 35Cl NMR techniques that resolve the two transport site populations on opposite sides of the membrane. Using these techniques, we show that the inhibitors 4,4'- dinitrostilbene -2,2'-disulfonate and p- nitrobenzensulfonate each recruit all of the transport sites on both sides of the membrane to the extracellular facing conformation. This result indicates that band 3 has an alternating site transport mechanism: each band 3 transport unit possesses a single functional transport site which is alternately exposed first to one side of the membrane then to the other.