Component of the ubiquinol-cytochrome c oxidoreductase, a multisubunit transmembrane complex that is part of the mitochondrial electron transport chain which drives oxidative phosphorylation. The respiratory chain contains 3 multisubunit complexes succinate dehydrogenase (complex II, CII), ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII) and cytochrome c oxidase (complex IV, CIV), that cooperate to transfer electrons derived from NADH and succinate to molecular oxygen, creating an electrochemical gradient over the inner membrane that drives transmembrane transport and the ATP synthase. The cytochrome b-c1 complex catalyzes electron transfer from ubiquinol to cytochrome c, linking this redox reaction to translocation of protons across the mitochondrial inner membrane, with protons being carried across the membrane as hydrogens on the quinol. In the process called Q cycle, 2 protons are consumed from the matrix, 4 protons are released into the intermembrane space and 2 electrons are passed to cytochrome c. The Rieske protein is a catalytic core subunit containing a [2Fe-2S] iron-sulfur cluster. It cycles between 2 conformational states during catalysis to transfer electrons from the quinol bound in the Q(0) site in cytochrome b to cytochrome c1 (By similarity). Incorporation of UQCRFS1 is the penultimate step in complex III assembly (By similarity). Component of the ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII). UQCRFS1 undergoes proteolytic processing once it is incorporated in the complex III dimer. One of the fragments, called subunit 9, corresponds to its mitochondrial targeting sequence (MTS) (PubMed:8386158, PubMed:2996928). The proteolytic processing is necessary for the correct insertion of UQCRFS1 in the complex III dimer, but the persistence of UQCRFS1-derived fragments may prevent newly imported UQCRFS1 to be processed and assembled into complex III and is detrimental for the complex III structure and function (By similarity). a quinol + 2 Fe(III)-[cytochrome c](out) = a quinone + 2 Fe(II)-[cytochrome c](out) + 2 H(+)(out) Binds 1 [2Fe-2S] cluster per subunit. Fe-S cluster delivery to the Rieske protein is mediated by components of the iron sulfur (Fe-S) cluster assembly machinery that reside in the mitochondrial matrix (including HSC20 and LYRM7). Component of the ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII), a multisubunit enzyme composed of 11 subunits. The complex is composed of 3 respiratory subunits cytochrome b, cytochrome c1 and Rieske protein UQCRFS1, 2 core protein subunits UQCRC1/QCR1 and UQCRC2/QCR2, and 6 low-molecular weight protein subunits UQCRH/QCR6, UQCRB/QCR7, UQCRQ/QCR8, UQCR10/QCR9, UQCR11/QCR10 and subunit 9, the cleavage product of Rieske protein UQCRFS1 (PubMed:8386158). The complex exists as an obligatory dimer and forms supercomplexes (SCs) in the inner mitochondrial membrane with NADH-ubiquinone oxidoreductase (complex I, CI) and cytochrome c oxidase (complex IV, CIV), resulting in different assemblies (supercomplex SCI(1)III(2)IV(1) and megacomplex MCI(2)III(2)IV(2)) (PubMed:27830641). Incorporation of the Rieske protein UQCRFS1 is the penultimate step in complex III assembly. Interacts with TTC19, which is involved in the clearance of UQCRFS1 fragments (By similarity). Component of the ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII) (PubMed:8386158). Subunit 9 corresponds to the mitochondrial targeting sequence (MTS) of Rieske protein UQCRFS1. It is retained after processing and incorporated inside complex III, where it remains bound to the complex and localizes between the 2 core subunits UQCRC1/QCR1 and UQCRC2/QCR2 (PubMed:9651245). Proteolytic processing is necessary for the correct insertion of UQCRFS1 in the complex III dimer. Several fragments are generated during UQCRFS1 insertion, most probably due to the endogenous matrix-processing peptidase (MPP) activity of the 2 core protein subunits UQCRC1/QCR1 and UQCRC2/QCR2, which are homologous to the 2 mitochondrial-processing peptidase (MPP) subunits beta-MPP and alpha-MPP respectively. The action of the protease is also necessary for the clearance of the UQCRFS1 fragments. The Rieske protein is a high potential 2Fe-2S protein. Belongs to the Rieske iron-sulfur protein family. Several peptides are generated during UQCRFS1 insertion. According to some authors, the identification of the transit peptide as the subunit 9, does not necessary imply that it must be considered as a structural subunit of the complex III dimer as additional fragments from UQCRFS1 are also present.