The N-terminal 11 amino acids of human erythrocyte band 3 are critical for aldolase binding and protein phosphorylation: Implications for band 3 function

The 911 amino acid band 3 (SLC4A1) is the major intrinsic membrane protein of red cells and is the principal Cl^-/HCO₃ ^- exchanger. The N-terminal cytoplasmic domain of band 3 anchors the spectrin-based membrane skeleton to the lipid bilayer through its interaction with ankyrin and also binds glycolytic enzymes and hemoglobin. We identified a son of a consanguineous marriage with severe anemia in association with marked deficiency of band 3 (12% ± 4% of normal). Direct nucleotide sequencing of SLC4A1 gene demonstrated a single base substitution (T→C) at position + 2 in the donor splice site of intron 2, resulting in the generation of a novel mutant protein. Biochemical characterization of the mutant protein showed that it lacked the first 11 N-terminal amino acids (band 3 Neapolis). The expression of the mutant protein resulted in the complete absence of membrane-bound aldolase, and the mutant band 3 could not be tyrosine phosphorylated. The ability of the malarial parasite P falciparum to invade these red cells was significantly decreased. The identification of a novel band 3 mutant and its structural and functional characterization enabled us to identify pivotal roles for the 11 N-terminal amino acids in several protein functions and, in turn, in red-cell physiology.