We have shown that diethyl pyrocarbonate (DEPC) inhibits band 3-mediated anion exchange and that the inhibition occurs only when histidine residue(s) is (are) modified with DEPC from the cytosolic surface of resealed ghosts [Izuhara et al. (1989) Biochemistry 28, 4725-4728]. In the present study, we have identified the DEPC-modified histidine residue as His834 using liquid chromatography with electrospray ionization mass spectrometry (LC/ESI-MS). This mild, rapid, sensitive, and quantitative method was successfully applied to analysis of the unstable DEPC-histidine adduct. The DEPC modification of His834 was pH dependent and 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNDS) sensitive as previously shown. After DEPC modification, band 3-mediated anion exchange is inhibited. Consistent with previous results, we confirmed that His834 was located on the cytosolic side of the membrane and the DEPC modification of His834 had allosteric effects on the extracellular DNDS-binding site of band 3. Therefore, we conclude that His834 is located at the cytosolic surface of band 3 and is an essential residue for band 3-mediated anion exchange. We will discuss important roles of the region from TM12 to TM14 in the conformational changes that occur during the band 3-mediated anion exchange.
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