Mutant forms of spectrin α-subunits in hereditary elliptocytosis

Two variant spectrins have been described in hereditary elliptocytosis (HE) and pyropoikilocytosis (HPP). Both are characterized by increased susceptibility of the αI (N-terminal) 80-kD domain to mild tryptic digestion, yielding peptides of 46-50 or 65-68 kD (T50a and T68 in our terminology). In this report we add a third unstable spectrin αI domain found in three kindreds with HE; αIT80 in this type of spectrin is cleaved by mild tryptic digestion to a 50-kD peptide (T50b) distinguished from T50a by its more basic isoelectric point. All three spectrins show impaired self-association to form oligomers. Intermediate tryptic peptides of the three unstable αI domains from HE spectrins were characterized by monoclonal immunoblotting and I¹²⁵ limit peptide mapping and affinity purified using polyclonal anti-αIT80. Partial amino acid sequences of αI domain peptides were obtained from two unrelated patients for each of the three variant spectrins. T50a results from cleavage at arginine 250 or lysine 252 of αIT80; a proline replaced the normal leucine or serine at residues 254 and 255, respectively. T50b and a 19-kD peptide result from cleavage at arginine 462 or arginine 464; a proline replaced the normal residue 465 (in T19b) in one of the two patients studied. T68 results from cleavage at arginine 131. In both 68-kD peptides examined, a leucine is inserted at residue 150. The relationship of the sequence changes to the new tryptic cleavages, to the current model of αI domain structure, and to defective spectrin self-association is discussed.