Cytochrome c polymerization by successive domain swapping at the C-terminal helix.
Cytochrome c (cyt c) is a stable protein that functions in a monomeric state as an electron donor for cytochrome c oxidase. It is also released to the cytosol when permeabilization of the mitochondrial outer membrane occurs at the early stage of apoptosis. For nearly half a century, it has been known that cyt c forms polymers, but the polymerization mechanism remains unknown. We found that cyt c forms polymers by successive domain swapping, where the C-terminal helix is displaced from its original position in the monomer and Met-heme coordination is perturbed significantly. In the crystal structures of dimeric and trimeric cyt c, the C-terminal helices are replaced by the corresponding domain of other cyt c molecules and Met80 is dissociated from the heme. The solution structures of dimeric, trimeric, and tetrameric cyt c were linear based on small-angle X-ray scattering measurements, where the trimeric linear structure shifted toward the cyclic structure by addition of PEG and (NH(4))(2)HPO(4). The absorption and CD spectra of high-order oligomers (approximately 40 mer) were similar to those of dimeric and trimeric cyt c but different from those of monomeric cyt c. For dimeric, trimeric, and tetrameric cyt c, the DeltaH of the oligomer dissociation to monomers was estimated to be about -20 kcal/mol per protomer unit, where Met-heme coordination appears to contribute largely to DeltaH. The present results suggest that cyt c polymerization occurs by successive domain swapping, which may be a common mechanism of protein polymerization.
Hirota
Shun
S
Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan. hirota@ms.naist.jp
Hattori
Yoko
Y
Nagao
Satoshi
S
Taketa
Midori
M
Komori
Hirofumi
H
Kamikubo
Hironari
H
Wang
Zhonghua
Z
Takahashi
Isao
I
Negi
Shigeru
S
Sugiura
Yukio
Y
Kataoka
Mikio
M
Higuchi
Yoshiki
Y
D000818:Animals; D001704:Biopolymers / Q000378:metabolism*; D002152:Calorimetry, Differential Scanning; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D045304:Cytochromes c / Q000737:chemistry* / Q000378:metabolism*; D006736:Horses; D010084:Oxidation-Reduction; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D053838:Scattering, Small Angle; D012996:Solutions; D013329:Structure-Activity Relationship
D001704:Biopolymers; D012996:Solutions; D045304:Cytochromes c