CX9C is the first half of a twin Cx9C motif in eukaryotic proteins. The function of this motif is to import nuclear-encoded mitochondrial intermembrane-space-proteins into the IMS (intermembrane space), as these latter lack a mitochondrial targeting sequence. The Cx9C proteins have a disulfide-bonded alpha-hairpin conformation. Cx9C-containing proteins are thus putative substrates for the Mia40-dependent thiol-disulfide exchange mechanism that carries out an oxidative folding process resulting in the proteins being trapped in the IMS [1].

1: Structural characterization of CHCHD5 and CHCHD7: two atypical human twin CX9C proteins. Banci L, Bertini I, Ciofi-Baffoni S, Jaiswal D, Neri S, Peruzzini R, Winkelmann J; J Struct Biol. 2012;180:190-200. PMID:22842048