On March 6th, 2018, Prof. Yang’s group from Tsinghua University published an article titled UQCRFS1N assembles mitochondrial respiratory complex III into an asymmetric 21-subunit dimer on Protein&Cell, pointing out a universe defect of previous complex III crystal structures and proposing the imperfectly symmetric structure of complex III with a different set of subunits through recalculation of previous data. Taken their supercomplex structural data together, they discovered that complex III binds into supercomplex in two different orientations but both orientations have full biological activity, which supports the megacomplex model where both monomer of complex III is functional.
Although the first structure of mitochondrial respiratory chain complex III in high level organism were obtained more than 20 years ago, and nearly 50 structures of complex III from different organisms in total were resolved till now, Prof. Yang found a common vulnerability in previous studies concerning the components of complex III. A very strange phenomenon is that there is no clear map of full-length UQCRFS1N subunit in all previous 46 crystal structures from different species. As the saying goes, "there must be a demon in abnormal events". Prof. Yang thinks there must be an unreasonable part in dealing with the UQCRFS1N subunit.
Through recalculation the previous bovine complex III structure (PDB:2A06) and avian complex III structure (PDB:3TGU) (Figure 1), Prof. Yang found the previous norm that both the monomers of complex III contain a full-length UQCRFS1N subunit is not correct. In fact, in the two monomers of complex III only one intact UQCRFS1N subunit is present, with its N terminal inserting into one monomer, and its C terminal into the other monomer. That is to say, a complete UQCRFS1N subunit connects the two monomers of complex III. The binding pockets of UQCRFS1N in both monomers could combine either the N terminal or the C terminal of UQCRFS1N, but the N terminal and C terminal of UQCRFS1N possess different conformations so that compound III is an imperfectly symmetrical dimer. The structural analysis before is negligent of this point and the two UQCRFS1N binding pockets in the two monomers are incorrectly deemed as identical, causing the density map of UQCRFS1N to be blurred, thus unable to obtain the full-length model of UQCRFS1N.
After the conclusion that the complex III is an imperfect symmetric dimer, Prof. Yang's group realized that complex III is directional in combination to supercomplex I1III2IV2. After analysis of the electron microscopy data previously acquired by their own group, Prof. Yang found that two kinds of complex III with different directions distribute equally among the overall particles, indicating both directions are equally functional, so that both monomers of complex III have biological activity, thus supporting the formation of megacomplex I2III2IV2.
In summary, through reanalysis of previous respiratory chain complex III crystal structure data, Prof. Yang found previous traditional norm that complex III is composed of 22 subunits and forms a symmetric dimer is not accurate. In fact, complex III is composed of 21 subunits and forms a imperfectly symmetric dimers, with the of N terminal and C terminal of UQCRFS1N inserting into two pockets of the two 10 subunit monomers, respectively, and then mediate their dimerization. This discovery not only pointed out the vulnerability in previous 46 complex III structures resolved in recent 20 years, providing a perfect explanation of why so many crystal structures have not built the model of intact UQCRFS1N, and let us rethink the complex III assembly process and function mechanism. Therefore, some previous studies related to the assembly process of complex III need to reconsider the reasonableness of their conclusion.
This work was supported by funds from the National Key R&D Program of China (2017YFA0504600 and 2016YFA0501100), the National Science Fund for Distinguished Young Scholars (31625008), and the National Natural Science Foundation of China (21532004 and 31570733).
Figure 1. Amended crystal structure of respiratory complex III (PDB:2A06)
A. Overall structure of recalculated complex III map, showing the binding pocket of UQCRFS1N
B. Two directions of UQCRFS1N binding into the pocket of complex III monomers, unveiling the imperfectly symmetric nature of complex III