Role Of Dimer Interface On The Cooperativity And Misfolding In Sod1 Studied By Single Molecule Force Spectroscopy

Superoxide dismutase 1 (SOD1) is a dimeric cytosolic antioxidant that can undergo prion-like misfolding in the context of the disease ALS. We studied the role of the dimer interface in folding and misfolding at the level of single molecules using optical tweezers, by comparing the behavior of dimers to that of isolated monomers. The unfolding and refolding of dimers was less cooperative than reported from ensemble studies, with numerous intermediate states that could be resolved. We also identified several distinct misfolded states. Several notable differences in the folding of the dimer were observed. The number of intermediates in dimer folding was noticeably less would be expected if the monomer domains acted independently, indicating that the dimer interface increases the cooperativity. Moreover, the initial steps during unfolding were different for the dimer compared to individual monomers whereas the final unfolding steps were similar, suggesting that the dimer interface modulates the folding pathways by changing the relative stabilities of the portions of the monomer structure close to the interface. Finally, we found that the dimer misfolded roughly half as much as would be expected from the rate at which monomers misfold. These results suggest that the dimer interface helps protect SOD1 against misfolding, in part by reducing the prevalence of partially-folded intermediates that lead to misfolded states.