Evidence from Electron Paramagnetic Resonance for Function‐Related Conformation Changes in the Anion‐Transport Protein of Human Erythrocytes

The erythrocyte membrane protein involved in anion transport (band 3) wits isolated in its native lipid milieu in the form of leaky vesicles and then was spin‐labelled with N‐(1‐oxyl‐2,2,6,6‐tetramethyl‐4‐piperidinyl)‐maleimide (MalMe₄PipO). The resulting electron paramagnetic resonance spectrum of band‐3‐bound MalMe₄PipO was resolved into rapid tumbling component and another, relatively immobile component. The percentage of the signal contributed by the mobile component (Q), was sensitive to various characteristic factors known to affect erythrocyte anion transport: Q was a hyperbolic function of chloride concentration displaying a half saturation constant K_1/2 similar to that of chloride transport. On the other hand Q showed a biphasic response to sulfate concentration, in line with the relatively high affinity of sulfate for the anion modifier site. Q was a saturable function of pH, either in presence of Cl⁻ or SO⁻²₄, showing a pK_a between pH 6.0 and 6.5, in analogy with the pH titration curve of Cl⁻ and SO⁻²₄ transport. Spin‐labelled vesicles treated with a covalent inhibitor of anion transport, 4‐acetamido‐4′‐isothiocyanostilbene‐2,2′‐disulfonic acid, were markedly less susceptible to changes in CI⁻ concentration. It is suggested that the electron paramagnetic resonance spectrum of MalMe₄PipO covalently bound to the band‐3 protein, reports conformational changes which are related to the anion‐transport function of this protein.