The large size of spectrin, the flexible protein promoting reversible 
deformation of red cells, has been an obstacle to elucidating the molecular 
mechanism of its function. By studying cloned fragments of the repeating unit 
domain, we have found a correspondence between positions of selected spectrin 
repeats in a tetramer with their stabilities of folding. Six fragments 
consisting of two spectrin repeats were selected for study primarily on the 
basis of the predicted secondary structures of their linker regions. Fragments 
with a putatively helical linker were more stable to urea- and heat-induced 
unfolding than those with a putatively nonhelical linker. Two of the less stably 
folded fragments, human erythroid alpha-spectrin repeats 13 and 14 
(HEalpha13,14) and human erythroid beta-spectrin repeats 8 and 9 (HEbeta8,9), 
are located opposite each other on antiparallel spectrin dimers. At least 
partial unfolding of these repeats under physiological conditions indicates that 
they may serve as a hinge. Also less stably folded, the fragment of human 
erythroid alpha-spectrin repeats 4 and 5 (HEalpha4,5) lies opposite the site of 
interaction between the partial repeats at the C- and N-terminal ends of beta- 
and alpha-spectrin, respectively, on the opposing dimer. More stably folded 
fragments, human erythroid alpha-spectrin repeats 1 and 2 (HEalpha1,2) and human 
erythroid alpha-spectrin repeats 2 and 3 (HEalpha2,3), lie nearly opposite each 
other on antiparallel spectrin dimers of a tetramer. These clusterings along the 
spectrin tetramer of repeats with similar stabilities of folding may have 
relevance for spectrin function, particularly for its well known flexibility.