The antiparallel side-to-side association of spectrin alpha and beta monomers is 
a two-step process which occurs in seconds even at 0 degrees C and at low 
concentrations. Assembly involves initial contact of complementary nucleation 
sites on each subunit, which are located near the actin binding end of the long, 
flexible heterodimer rod. The minimum nucleation sites are comprised of 
approximately four contiguous 106-residue homologous segments or repeats. Three 
repeats in the nucleation site contain an 8-residue insertion and have the 
highest homology to the four spectrin-like repeats in alpha-actinin. The 
adjacent actin binding domain on the beta subunit and the adjacent EF hand 
motifs on the alpha subunit are not required for heterodimer assembly. The 
nucleation sites probably have a specific lock and key structure which defines 
the unique side-to-side pairing of the many homologous segments in both 
subunits. Assembly of spectrin heterodimers is probably most analogous to a 
zipper. After initial nucleation site binding, the remainder of the subunits 
quickly associate along their full lengths to reconstitute a normal dimer by 
supercoiling around each other to form a rope-like, flexible rod. Assembly is 
terminated if either polypeptide is interrupted by a protease cleavage. 
Heterozygotic mutations involving either nucleation site are predicted to affect 
allele incorporation into the mature membrane skeleton.