Protein extensibility appears to be based broadly on conformational changes that 
can in principle be modulated by protein-protein interactions. Spectrin family 
proteins, with their extensible three-helix folds, enable evaluation of 
dimerization effects at the single molecule level by atomic force microscopy. 
Although some spectrin family members function physiologically only as 
homodimers (e.g. alpha-actinin) or are strictly monomers (e.g. dystrophin), 
alpha- and beta-spectrins are stable as monomeric forms but occur 
physiologically as alpha,beta-heterodimers bound laterally lengthwise. For short 
constructs of alpha- and beta-spectrin, either as monomers or as 
alpha,beta-dimers, sawtooth patterns in atomic force microscopy-forced extension 
show that unfolding stochastically extends repeats approximately 4-5-fold 
greater in length than native conformations. For both dimers and monomers, 
distributions of unfolding lengths appear bimodal; major unfolding peaks reflect 
single repeats, and minor unfolding peaks at twice the length reflect tandem 
repeats. Cooperative unfolding thus propagates through helical linkers between 
serial repeats (1, 2). With lateral heterodimers, however, the force 
distribution is broad and shifted to higher forces. The associated chains in a 
dimer can stay together and unfold simultaneously in addition to unfolding 
independently. Weak lateral interactions do not inhibit unfolding, but strong 
lateral interactions facilitate simultaneous unfolding analogous to serial 
repeat coupling within spectrin family proteins.