A model for the action of the anion exchange protein of the red blood cell.

Kinetic information on anion transport suggests that a mobile carrier system is involved in which the carrier-anion complex is able to spontaneously traverse the membrane. A second anion-binding site, the modifier, can when occupied reduce the rate of transport. Chemical studies with inhibitory probes suggest that the carrier and modifier sites are located in a specific transmembrane protein, band 3. The modifier site is accessible only from the internal surface but the carrier (transport) site is accessible from both the external and cytoplasmic surfaces. This information is discussed in terms of a model in which the anion traverses the membrane via a protein pathway. Transport involves conformational changes such that the transport site is alternately exposed to the external and cytoplasmic sides.