8.A.224. The Splicing factor, Arginine/Serine-rich 19 (SCAF1) Family
The enzymes of the mitochondrial electron transport chain associate into supercomplexes. Supercomplexes CIII2CIV1-2, CICIII2 and CICIII2CIV (respirasome) exist in mammals, but in contrast to CICIII2 and the respirasome, CIII2CIV requires a specific assembly factor (SCAF1) to be formed. Dr. Irene Vercellino solved the structures of mammalian (mouse and ovine) CIII2CIV and its assembly intermediates in different conformations (Vercellino and Sazanov 2021). These allowed description of the assembly of CIII2CIV from the CIII2 precursor to the final CIII2CIV conformation, driven by the insertion of the N terminus of SCAF1 deep into CIII2, while its C terminus is integrated into CIV. The structures confirmed that SCAF1 is exclusively required for the assembly of CIII2CIV and has no role in the assembly of the respirasome. Further, CIII2 is asymmetric due to the presence of only one copy of subunit 9, which straddles both monomers and prevents the attachment of a second copy of SCAF1 to CIII2, explaining the presence of one copy of CIV in CIII2CIV in mammals. Biochemical analyses showed that CIII2 and CIV gain catalytic advantage when assembled into the supercomplex, suggesting a role for CIII2CIV in fine tuning the efficiency of electron transfer in the electron transport chain (Vercellino and Sazanov 2021).