We have previously shown that horse cytochrome c (cyt c) forms oligomers by domain swapping its C-terminal α-helix when interacting with ethanol. Although folding of cyt c has been studied extensively, formation of domain-swapped oligomers of cyt c during folding has never been reported. We found that domain-swapped oligomeric cyt c is produced during refolding from its guanidinium ion-induced unfolded state at high protein concentrations and low temperatures. The obtained dimer exhibited a domain-swapped structure exchanging the C-terminal α-helical region between molecules. The extent of dimer formation decreased significantly for the folding of C-terminal cyt c mutants with reduced hydrophobicity achieved by replacement of hydrophobic residues with Gly in the C-terminal region, whereas a large amount of heterodimers was generated for the folding of a mixture of N- and C-terminal mutants. These results show that cyt c oligomers are formed through intermolecular hydrophobic interaction between the N- and C-terminal α-helices during folding. A slow phase (4-5 s) was observed in addition to a 400-500 ms phase during folding of a high concentration of cyt c in the presence of 1.17 M guanidine hydrochloride. The fast phase is attributed to the intramolecular ligand exchange process, and we attribute the slow phase to the ligand exchange process in oligomers. These results show that it is important to consider formation of domain-swapped oligomeric proteins when folding at high protein concentrations.

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Many proteins, including cytochrome c (cyt c), have been shown to form domain-swapped oligomers, but the factors governing the oligomerization process remain unrevealed. We obtained oligomers of cyt c by refolding cyt c from its acid molten globule state to neutral pH state under high protein and ion concentrations. The amount of oligomeric cyt c obtained depended on the nature of the anion (chaotropic or kosmotropic) in the solution: ClO4(-) (oligomers, 11% ± 2% (heme unit)), SCN(-) (10% ± 2%), I(-) (6% ± 2%), NO3(-) (3% ± 1%), Br(-) (2% ± 1%), Cl(-) (2% ± 1%), and SO4(2-) (3% ± 1%) for refolding of 2 mM cyt c (anion concentration 125 mM). Dimeric cyt c obtained by refolding from the molten globule state exhibited a domain-swapped structure, in which the C-terminal α-helices were exchanged between protomers. According to small-angle X-ray scattering measurements, approximately 25% of the cyt c molecules were dimerized in the molten globule state containing 125 mM ClO4(-). These results indicate that a certain amount of molten globule state oligomers of cyt c convert to domain-swapped oligomers during refolding and that the intermolecular interactions necessary for domain swapping are present in the molten globule state.