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.