Location of spectrin dimer initiation site peptides used for in vitro binding studies. The arrangement of structural motifs in an antiparallel dimer and the relationship of the recombinant peptides used in this study are shown. The β subunit contains an actin binding domain (ABD), 17 homologous segments or motifs (numbered rectangles), and a small nonhomologous phosphorylated C-terminal domain. The α subunit contains 20 homologous motifs (1 through 9 and 11 through 21), an SH-3 motif (motif 10) located in a loop between the B and C helix of motif 9, and a C-terminal domain consisting of two EF-hand type motifs (diamonds). The locations of the recombinant peptides used in this study are shown by horizontal lines. The relationships of the two gene products from the α^LELY allele to a normal α18-21 recombinant peptide are shown. The α18-21¹⁸⁵⁷ peptide contains the Leu → Val mutation at codon 1857 and contains the normally expressed 6 residues encoded by exon 46. The α18-21^1857-Δ46 peptide represents the second gene product of the α^LELY allele, ie, it has both the Leu → Val substitution at codon 1857 and it lacks the exon 46 sequence.