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Interactions between mutant and wild-type band 3 subunits in hereditary Southeast Asian ovalocytic red blood cell
1Veterans Administration Medical Center, Omaha, Nebraska, USA.
Insights
Southeast Asian ovalocytosis (SAO) involves a mutant band 3 protein affecting red cell membranes. This study reveals the mutant subunit alters wild-type band 3 conformation, impacting anion transport and H2DIDS binding kinetics.
Area of Science:
- Biochemistry
- Membrane Biology
- Genetics
Background:
- Southeast Asian ovalocytosis (SAO) is characterized by red blood cells with a unique band 3 protein mutation.
- The SAO band 3 mutation (lacking residues 400-408) results in reduced anion exchange Vmax and impaired anion transport when expressed alone.
- Approximately 50% of band 3 in SAO cells exists as wild-type/SAO heterodimers, with the remaining as wild-type homodimers.
Purpose of the Study:
- To investigate the kinetic differences in H2DIDS (4,4'-diisothiocyanatodihydro-2,2'-stilbenedisulfonate) release between wild-type and SAO band 3 in red cell membranes.
- To elucidate the structural and functional consequences of wild-type/SAO band 3 heterodimer formation on anion binding and transport.
- To understand how the SAO mutation influences the conformation and properties of the adjacent wild-type band 3 subunit.
Main Methods:
- Kinetic analysis of H2DIDS release from red cell membranes of SAO individuals and control subjects.
- Comparison of biphasic H2DIDS release kinetics in SAO membranes with monophasic kinetics in control cells.
- Examination of H2DIDS binding kinetics to assess differences in affinity between wild-type and heterodimeric band 3.
Main Results:
- H2DIDS release from wild-type band 3 in SAO membranes exhibited biphasic kinetics, with two phases in equal proportion and rate constants differing approximately fivefold.
- Control cells displayed monophasic, exponential H2DIDS release kinetics, with a rate constant similar to the fast phase observed in SAO membranes.
- The slow phase in SAO membranes was attributed to H2DIDS release from the wild-type subunit within heterodimers, suggesting altered conformation and approximately fourfold higher H2DIDS affinity compared to wild-type homodimers.
- No significant differences were observed in H2DIDS binding kinetics, indicating the conformational change primarily affects release rather than initial binding.
Conclusions:
- The SAO band 3 mutant subunit induces a conformational change in the neighboring wild-type subunit within the heterodimer, leading to altered H2DIDS affinity.
- These subunit interactions within the heterodimer are likely confined to a region involving the C-terminal subdomain of the wild-type band 3.
- The observed alterations in subunit interactions and conformation correlate with the reduced cellular anion transport function characteristic of SAO.
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.