Mechanism of super-assembly of respiratory complexes III and IV

Abstract

Respiratory chain complexes can super-assemble into quaternary structures called supercomplexes that optimize cellular metabolism. The interaction between complexes III (CIII) and IV (CIV) is modulated by supercomplex assembly factor 1 (SCAF1, also known as COX7A2L). The discovery of SCAF1 represented strong genetic evidence that supercomplexes exist in vivo. SCAF1 is present as a long isoform (113 amino acids) or a short isoform (111 amino acids) in different mouse strains. Only the long isoform can induce the super-assembly of CIII and CIV, but it is not clear whether SCAF1 is required for the formation of the respirasome (a supercomplex of CI, CIII and CIV). Here we show, by combining deep proteomics and immunodetection analysis, that SCAF1 is always required for the interaction between CIII and CIV and that the respirasome is absent from most tissues of animals containing the short isoform of SCAF1, with the exception of heart and skeletal muscle. We used directed mutagenesis to characterize SCAF1 regions that interact with CIII and CIV and discovered that this interaction requires the correct orientation of a histidine residue at position 73 that is altered in the short isoform of SCAF1, explaining its inability to interact with CIV. Furthermore, we find that the CIV subunit COX7A2 is replaced by SCAF1 in supercomplexes containing CIII and CIV and by COX7A1 in CIV dimers, and that dimers seem to be more stable when they include COX6A2 rather than the COX6A1 isoform.

Tissue‐ and Condition‐Specific Isoforms of Mammalian Cytochrome c Oxidase Subunits: From Function to Human Disease

“…These studies concluded that mice bearing the shortened form of the subunit have normal biogenesis, no related respiratory defects, and normal levels of complex IV-associated supercomplexes, although differences in levels of different supercomplex subtypes were observed. In support of the hypothesis that COX VIIaR is required for supercomplex assembly, however, another recent publication showed that the long form of COX7AR was required for interaction of complexes III and IV [194]. Here, it was shown that the individual supercomplexes employ different isoforms to achieve different stoichiometries.…”

Recent Advances in Mitochondrial Disease

“…These complexes have long been recognized as discrete entities embedded within the inner mitochondrial membrane, but they are also found as respiratory supercomplexes. The crystal structures of two functional supercomplexes (supercomplexes I and III) and the fully functional respirasome (consisting of complexes I, III, and IV) have recently been elucidated (12,32,55).…”

Inner Secrets of the Respirasome

“…Two previously implicated SC stabilizing factors are COX7A2L and HIGD2. Although COX7A2L can exist within SC(I + III 2 +IV), it is now known to primarily facilitate the CIII–CIV interaction (Cogliati et al, 2016; Pérez-Pérez et al, 2016). Different isoforms of COX7A exist including COX7A1, COX7A2, and COX7A2L.…”

“…Different isoforms of COX7A exist including COX7A1, COX7A2, and COX7A2L. The presence of COX7A2 isoform correlates primarily with unassociated monomeric CIV complex, whereas COX7A1-containing CIV species favor dimeric CIV complexes (Cogliati et al, 2016). Replacement of COX7A1 with COX7A2L (SCAFI) permits assembly of the CIII–CIV heterocomplex and SC formation in some tissues (Cogliati et al, 2016).…”

“…The presence of COX7A2 isoform correlates primarily with unassociated monomeric CIV complex, whereas COX7A1-containing CIV species favor dimeric CIV complexes (Cogliati et al, 2016). Replacement of COX7A1 with COX7A2L (SCAFI) permits assembly of the CIII–CIV heterocomplex and SC formation in some tissues (Cogliati et al, 2016). The porcine respirasome clearly lacks the COX7A2L isoform, so additional work is needed to discern the effects of CIV subunit remodeling on respirasome function.…”