The largest calorimetric endotherm of the human erythrocyte membrane, termed the C transition (68°C), was shown to derive from the denaturation of the membrane-spanning domain of the anion transport protein, band 3. This identification was based on the following evidence: (i) the fluorescence properties of the highly specific covalent ligand of band 3, 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid, abruptly changed during the C transition; (ii) the potent, noncovalent inhibitor of anion transport, dipyridamole, was ejected from erythrocyte membranes during the C transition; (iii) the intrinsic fluorescence of the membrane-spanning domain of band 3 decreased suddenly at the temperature of the C transition; and (iv) the purified 53 000-dalton, membrane-spanning domain of band 3 yielded the C transition upon reconstitution into egg phosphatidylcholine/bovine brain phosphatidylserine vesicles. Although lipid melting was shown not to contribute to the C endotherm, the thermal stability of band 3 was nevertheless observed to be sensitive to its lipid/detergent environment. The stability of the membrane-spanning domain of band 3 was also found to be unaffected by the presence or absence of glycophorin, suggesting that the putative complex between this region of band 3 and glycophorin is either weak or nonexistent.
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