Respiratory supercomplexes provide metabolic efficiency in zebrafish

Abstract

or J.A. 13 Enríquez (jaenriquez@cnic.es). 14 15 16 17The oxidative phosphorylation (OXPHOS) system is a dynamic system in which the 18 respiratory complexes coexist with super-assembled quaternary structures called 19 supercomplexes (SCs). The physiological role of SCs is still disputed. Here we used zebrafish to 20 study the relevance of respiratory SCs. We combined immunodetection analysis and deep data-21 independent proteomics to characterize these structures and found similar SCs to those 22 described in mice, as well as novel SCs including III2+IV2, I+IV and I+III2+IV2. To study the 23 physiological role of SCs, we generated two null allele zebrafish lines for supercomplex 24 assembly factor 1 (SCAF1). SCAF1 -/fish displayed altered OXPHOS activity due to the 25 disrupted interaction of complex III and IV. SCAF1 -/fish were smaller in size, and showed 26 abnormal fat deposition and decreased female fertility. These physiological phenotypes were 27 rescued by doubling the food supply, which correlated with improved bioenergetics and 28 alterations in the metabolic gene expression program. These results reveal that SC assembly by 29 SCAF1 modulates OXPHOS efficiency and allows for the optimization of metabolic resources. 30In the last two years, the focus of investigation on the structure of the mitochondrial electron 31 transport chain (ETC) has shifted from the dispute over the existence of supercomplexes (SCs) to 32 their putative functional role. In mammals, the best understood mechanism of respiratory complex 33 super-assembly is the interaction between complexes III (CIII) and IV (CIV) mediated by 34 supercomplex assembly factor 1 (SCAF1/COX7A2L) 1 . The carboxy-terminus of SCAF1 is very 35 similar to that of the CIV subunit COX7A2 and replaces it in the subset of CIV molecules that super-36 assemble with CIII 2 . After some initial doubts 3 , which were later dispelled 4,5 , the role of SCAF1 in 37 the super-assembly of CIII and CIV is now generally accepted 2 . The process of super-assembly 38 between CI and CIII and CI and CIV to form the respirasome is unknown, but the proposed existence 39 of I+IV SCs 6 suggests that CI-CIII and CI-CIV super-assembly might occur independent from CIII 40 and CIV assembly 7,8 . So far, the interaction between CI and CIV has been mostly studied in SCs 41 containing CI, CIII and CIV (also named respirasomes). Several forms of respirasomes (I+III2+IV) 42 migrate closely together in blue native gel electrophoresis (BNGE), although the reason for their 43 SCAF1 protein (SCAF1 111 ) lacking two amino acids as compared with functional SCAF1 63 (SCAF1 113 ). SCAF1 111 mice do not assemble III2+IV and the formation of respirasomes is affected 1,2 . 64No specific phenotype has yet been ascribed to SCAF1 111 mouse strains, and this has contributed to 65 the arguments against a physiological role for the super-assembly of CIII and CIV. Furthermore, a 66 recent study performed in cultured cells suggested that the ablation of SCAF1 has no bioenergetic 67 relevance 17 , a conclusio...