This family represents mitochondrial respiratory chain complex III (or cytochrome b-c1 complex) assembly factors, including Cbp6 from yeast and its human orthologue UQCC2 (Ubiquinol-Cytochrome c Reductase Complex Assembly Factor 2) [1,2]. These proteins have diverged significantly among eukaryotes and UQCC2/Cbp6 have an overall low amino acid conservation [1]. UQCC2 interacts with UQCC1, the human orthologue of the Cbp6 binding partner Cbp3, to form a complex necessary for cytochrome b biogenesis [1]. Cbp3-Cbp6 complex interacts with mitochondrial ribosomes for efficient translation of cytochrome b transcript and it also interacts with newly synthesized cytochrome b to assist its correct assembly [2]. These proteins contain a LRY-like N-terminal sequence being related to the LYR superfamily of proteins, which function as subunits or assembly factors for respiratory complexes I, II, III, and V [3].

1: Mutations in the UQCC1-interacting protein, UQCC2, cause human complex III deficiency associated with perturbed cytochrome b protein expression. Tucker EJ, Wanschers BF, Szklarczyk R, Mountford HS, Wijeyeratne XW, van den Brand MA, Leenders AM, Rodenburg RJ, Reljic B, Compton AG, Frazier AE, Bruno DL, Christodoulou J, Endo H, Ryan MT, Nijtmans LG, Huynen MA, Thorburn DR; PLoS Genet. 2013;9:e1004034. PMID:24385928

2: Cbp3-Cbp6 interacts with the yeast mitochondrial ribosomal tunnel exit and promotes cytochrome b synthesis and assembly. Gruschke S, Kehrein K, Rompler K, Grone K, Israel L, Imhof A, Herrmann JM, Ott M; J Cell Biol. 2011;193:1101-1114. PMID:21670217

3: Structure of human Fe-S assembly subcomplex reveals unexpected cysteine desulfurase architecture and acyl-ACP-ISD11 interactions. Cory SA, Van Vranken JG, Brignole EJ, Patra S, Winge DR, Drennan CL, Rutter J, Barondeau DP; Proc Natl Acad Sci U S A. 2017;114:E5325. PMID:28634302