Cytochrome c1 exhibits two binding sites for cytochrome c in plants

Abstract

In plants, channeling of cytochrome c molecules between complexes III and IV has been purported to shuttle electrons within the supercomplexes instead of carrying electrons by random diffusion across the intermembrane bulk phase. However, the mode plant cytochrome c behaves inside a supercomplex such as the respirasome, formed by complexes I, III and IV, remains obscure from a structural point of view. Here, we report ab-initio Brownian dynamics calculations and nuclear magnetic resonance-driven docking computations showing two binding sites for plant cytochrome c at the head soluble domain of plant cytochrome c1, namely a non-productive (or distal) site with a long heme-to-heme distance and a functional (or proximal) site with the two heme groups close enough as to allow electron transfer. As inferred from isothermal titration calorimetry experiments, the two binding sites exhibit different equilibrium dissociation constants, for both reduced and oxidized species, that are all within the micromolar range, thus revealing the transient nature of such a respiratory complex. Although the docking of cytochrome c at the distal site occurs at the interface between cytochrome c1 and the Rieske subunit, it is fully compatible with the complex III structure. In our model, the extra distal site in complex III could indeed facilitate the functional cytochrome c channeling towards complex IV by building a "floating boat bridge" of cytochrome c molecules (between complexes III and IV) in plant respirasome.

Structural basis for inhibition of the histone chaperone activity of SET/TAF-Iβ by cytochromec

“…Such a role would also explain the Cc-mediated inhibition observed here.The results of ITC and NMR titrations herein performed reveal the oncoprotein SET/TAF-Iβ as binding specifically to Cc. Interestingly, Cc uses the surface residues surrounding its heme group to interact with the histone chaperone, as it does with its respiratory partners cytochrome c oxidase (28) and cytochrome bc 1 (29) and its apoptotic partner Apaf-1 (31). Mutagenesis and NMR-based docking analysis carried out here demonstrated that Cc docks between the two histone-binding domains of SET/TAF-Iβ, thereby preventing the binding of the latter to core histones.…”

“…3B). Notably, a similar surface patch of Cc is involved in the interactions with cytochrome c oxidase (28) and cytochrome bc 1 (29). Interestingly, the interaction between Cc and the SET/TAF-Iβ chaperone implicated 10 lysine residues-namely 5, 7, 8, 22, 25, 53, 72, 73, 86, and 88 (Fig.…”

“…3B)-thereby evidencing the key role of electrostatic forces in the formation of Cc:SET/TAF-Iβ complexes. Similarly, lysine residues are also involved in the interaction with cytochrome bc 1 (29,30). Given the well-known "lysine masking activity" of SET/TAF-Iβ that prevents the acetylation of histone lysines inside the INHAT complex (3), it is plausible that SET/TAF-Iβ also recognizes the lysine residues from Cc.…”

Respiratory complexes III and IV can each bind two molecules of cytochrome c at low ionic strength

“…Human cytochrome c 1 (hCc 1 ) model was built following the same procedure reported for pCc 1 model [29]. The electrostatic potential surfaces of pCc, pCc 1 , hCc, and hCc 1 structures were calculated using DelPhi [37] and Chimera [38].…”

“…pCc and pCc 1 models were built in previous work [29]. hCc structure was taken from Protein Data Bank (PDB ID: 3zcf) [36].…”