Interactions between mutant and wild-type band 3 subunits in hereditary Southeast Asian ovalocytic red blood cell membranes

Abstract

Red cell membranes from individuals with Southeast Asian ovalocytosis (SAO) contain approximately equal proportions of wild-type band 3 and a mutant SAO band 3 which lacks residues 400-408. It is known that the Vmax for anion exchange in SAO cells is reduced by about 50%, that SAO band 3 does not transport anions when expressed alone in a cellular expression system, that SAO band 3 does not bind stilbenedisulfonates, and that about 50% of the band 3 exists as wild-type/SAO heterodimers. In this report, we show that the kinetics of H2DIDS (4,4'-diisothiocyanatodihydro-2,2'-stilbenedisulfonate) release from the wild-type band 3 in SAO membranes is biphasic. The two phases were present in about equal proportions, with rate constants differing by about 5-fold. In contrast; control cells showed monophasic, exponential kinetics with a rate constant comparable to that of the fast phase of SAO membranes. We assign the fast phase in SAO membranes to H2DIDS release from wild-type subunits within homodimers and the slow phase to H2DIDS release from the wild-type subunit within the heterodimer. No differences were observed in kinetic studies of H2DIDS binding. These results suggest that the mutant band 3 subunit alters the conformation of its neighboring wild-type subunit within the heterodimer, resulting in about a 4-fold higher H2DIDS affinity. Additional evidence suggesting that the interactions in the heterodimer may be confined to a region of the wild-type subunit containing the C-terminal subdomain is presented. The relationship of these subunit interactions to the observation of a reduced cellular anion transport function is discussed.