Construction of a Triangle-Shaped Trimer and a Tetrahedron Using an α-Helix-Inserted Circular Permutant of Cytochrome c₅₅₅

Highly-ordered protein structures have gained interest for future uses for biomaterials. Herein, we constructed a building block protein (BBP) by the circular permutation of the hyperthermostable Aquifex aeolicus cytochrome (cyt) c₅₅₅ , and assembled BBP into a triangle-shaped trimer and a tetrahedron. The angle of the intermolecular interactions of BBP was controlled by cleaving the domain-swapping hinge loop of cyt c₅₅₅ and connecting the original N- and C-terminal α-helices with an α-helical linker. We obtained BBP oligomers up to ≈40 mers, with a relatively large amount of trimers. According to the X-ray crystallographic analysis of the BBP trimer, the N-terminal region of one BBP molecule interacted intermolecularly with the C-terminal region of another BBP molecule, resulting in a triangle-shaped structure with an edge length of 68 Å. Additionally, four trimers assembled into a unique tetrahedron in the crystal. These results demonstrate that the circular permutation connecting the original N- and C-terminal α-helices with an α-helical linker may be useful for constructing organized protein structures.