Cytochrome <i>c</i> Polymer: Polymerization Mechanism Discovered in a Water-soluble Globular Protein

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  • シトクロム<i>c</i>ポリマー：水溶性球状タンパク質に見出された多量化機構
  • シトクロム cポリマー : スイヨウセイ キュウジョウ タンパクシツ ニ ミイダサレタ タリョウカ キコウ

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Abstract

Cytochrome c (cyt c) is a stable globular protein which 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 half a century, it has been known that cyt c forms polymers, but the polymerization mechanism remains unknown. 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 (NH4)2HPO4. The absorption and CD spectra of high order oligomers (∼40 mer) were similar to those of dimeric and trimeric cyt c but different from those of monomeric cyt c. These results show 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. Successive domain swapping may be a common mechanism of protein polymerization.

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