Respiratory chain supercomplexes: Structures, function and biogenesis

Abstract

Over the past sixty years, researchers have made outmost efforts to clarify the structural organization and functional regulation of the complexes that configure the mitochondrial respiratory chain. As a result, the entire composition of each individual complex is practically known and, aided by notable structural advances in mammals, it is now widely accepted that these complexes stablish interactions to form higher-order supramolecular structures called supercomplexes and respirasomes. The mechanistic models and players that regulate the function and biogenesis of such superstructures are still under intense debate, and represent one of the hottest topics of the mitochondrial research field at present. Noteworthy, understanding the pathways involved in the assembly and organization of respiratory chain complexes and supercomplexes is of high biomedical relevance because molecular alterations in these pathways frequently result in severe mitochondrial disorders. The purpose of this review is to update the structural, biogenetic and functional knowledge about the respiratory chain supercomplexes and assembly factors involved in their formation, with special emphasis on their implications in mitochondrial disease. Thanks to the integrated data resulting from recent structural, biochemical and genetic approaches in diverse biological systems, the regulation of the respiratory chain function arises at multiple levels of complexity.

Keywords: Assembly factors; Respirasome; Respiratory chain function; Supercomplexes.

Figure 1. Architectures of the mammalian respirasomes

Side views along the membrane from (A) Supercomplex I+III₂, (B) the tight respirasome, and (C) the loose respirasome, according to the structures proposed by Letts et al. [22]. Images were obtained from the RCSB Protein Data Bank in combination with the NGL viewer. The structural models of CI, CIII₂, and CIV are colored in red, turquoise, and navy blue, respectively. The transmembrane region is indicated by two dashed lines. M, matrix; IM, mitochondrial inner membrane; IMS, intermembrane space.

Figure 2. Distribution of human respiratory chain complexes in supercomplexes

(A) Mitochondria isolated from cultured 143B cells (1) and cybrids (2) were analysed by BN-PAGE in combination with CI in-gel activity (IGA) assay and western blot with antibodies against the indicated MRC subunits. (B) Most CI (blue), ~half of CIII₂ (green) and ~20–30% of CIV (red) are localized in the respirasome (I+III₂+IV_0–1). SC III₂+IV₁ represents ~5% of the total amount of MRC structures, as well as CIV dimers (IV₂). CII (II) is not present in SCs. Free CI (light blue) requires to be associated in supercomplexes to minimize destabilization and ROS generation [29].

Figure 3. Respirasome biogenesis through the direct association of fully-assembled respiratory chain complexes

This model [24,86] proposes that the mammalian respirasome (I+III₂+IV₁) originates by the direct association of single preassembled CI (blue), CIII₂ (green) and CIV (red). The assembly pathways of the individual MRC complexes are depicted according to stablished models [72,86,108], and SC III₂+IV₁ is formed independently of the respirasomes [98]. Supercomplex assembly factors COX7A2L and HIGD2A are marked in green/red.

Figure 4. Respirasome biogenesis through the stepwise association of partially-assembled respiratory chain complexes and submodules

This model [45] proposes the sequential and coordinated association of submodules and free subunits from CIII₂ (green) and CIV (red) to a CI-scaffold (blue) that lacks the N catalytic module, which is incorporated at the latest assembly stage to ensure respirasome activation in the presence of all the necessary structural components. The assembly pathways of individual CIII₂ and CIV are depicted according to stablished models [72,108], and SC III₂+IV₁ is formed independently of the respirasomes [98]. Supercomplex assembly factors COX7A2L and HIGD2A are marked in green/red.