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. 2021 Oct 29;4(1):1238.
doi: 10.1038/s42003-021-02767-x.

Non-conventional octameric structure of C-phycocyanin

Takuo Minato #  1   2   3 , Takamasa Teramoto #  4 , Naruhiko Adachi  5 , Nguyen Khac Hung  1   2 , Kaho Yamada  1   2 , Masato Kawasaki  5   6 , Masato Akutsu  5 , Toshio Moriya  5 , Toshiya Senda  5   6 , Seiji Ogo  1   2   7 , Yoshimitsu Kakuta  8   9 , Ki-Seok Yoon  10   11   12
Affiliations

Non-conventional octameric structure of C-phycocyanin

Takuo Minato et al. Commun Biol. .

Abstract

C-phycocyanin (CPC), a blue pigment protein, is an indispensable component of giant phycobilisomes, which are light-harvesting antenna complexes in cyanobacteria that transfer energy efficiently to photosystems I and II. X-ray crystallographic and electron microscopy (EM) analyses have revealed the structure of CPC to be a closed toroidal hexamer by assembling two trimers. In this study, the structural characterization of non-conventional octameric CPC is reported for the first time. Analyses of the crystal and cryogenic EM structures of the native CPC from filamentous thermophilic cyanobacterium Thermoleptolyngbya sp. O-77 unexpectedly illustrated the coexistence of conventional hexamer and novel octamer. In addition, an unusual dimeric state, observed via analytical ultracentrifugation, was postulated to be a key intermediate structure in the assemble of the previously unobserved octamer. These observations provide new insights into the assembly processes of CPCs and the mechanism of energy transfer in the light-harvesting complexes.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1
Fig. 1. Crystal models of TlCPC-6 and TlCPC-8.
Top and side views of TlCPC-6 (a, c) and TlCPC-8 (b, d). Dihedral angles of monomers in TlCPC-6 (e) and TlCPC-8 (f). Angles between adjacent monomers in TlCPC-6 (g) and TlCPC-8 (h). The main and side chains of TlCPC-6 and TlCPC-8 are represented by green and cyan ribbon models, respectively. PCBs are represented by grey lines. Yellow lines are guides for measuring angles.
Fig. 2
Fig. 2. Crystal packings of TlCPC-6 and TlCPC-8.
TlCPC-6 (a) and TlCPC-8 (b) in crystals are shown as ribbon representations. Two representatives rod-like [(αβ)6]2 and [(αβ)8]2 structures are shown in red.
Fig. 3
Fig. 3. Cryo-EM analyses of TlCPC-6 and TlCPC-8.
Representative motion-corrected electron micrograph of TlCPCs (a). Typical reference-free 2D class averages from single-particle images of TlCPC-6 (b) and TlCPC-8 (c). Top and side views of cryo-EM density maps of TlCPC-6 at 3.06 Å resolution (d and f) and TlCPC-8 at 3.71 Å resolution (e and g). Contour levels of TlCPC-6 and TlCPC-8 are shown at 0.06 and 0.04, respectively.
Fig. 4
Fig. 4. Proposed oligomerization mechanism in TlCPC.
Monomeric (αβ) structures are represented by bended cuboids. Contact% was calculated by the PISA program based on the monomeric (αβ) units. ΔG0f values were calculated from ΔG0diss values, where the ΔG0f values of –4.8 and –1.4 kcal/mol in the dimer were calculated using the crystal models of TlCPC-6 and TlCPC-8, respectively.
See this image and copyright information in PMC

References

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