Characterization of the calorimetric C transition of the human erythrocyte membrane

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Abstract
The largest calorimetric endotherm of the human erythrocyte membrane, termed the C transition (68.degree. C) is derived from the denaturation of the membrane-spanning domain of the anion transport protein, band 3. This identification was based on the following evidence: the fluorescence properties of the highly specific convalent ligand of band 3, 4,4''-diisothiocyanostilbene-2,2''-disulfonic acid, abruptly changed during the C transition; the potent, noncovalent inhibitor of anion transport, dipyridamole, was ejected from erythrocyte membranes during the C transition; the intrinsic fluorescence of the membrane-spanning domain of band 3 decreased suddenly at the temperature of the C transition; and 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 did not contribute to the C endotherm, the thermal stability of band 3 was nevertheless sensitive to its lipid/detergent environment. The stability of the membrane-spawning domain of band 3 was also 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.