Succinate dehydrogenase [ubiquinone] cytochrome b small subunit, CybS (IPR007992)

Short name: CybS

Overlapping homologous superfamilies

Family relationships



This family consists of mitochondrial succinate dehydrogenase [ubiquinone] cytochrome b small subunit CybS (also known an SQR) and import inner membrane translocase subunit Tim18. Members of this subfamily are classified as Type C SQRs because they contain two transmembrane subunits and one heme group. CybS and CybL are the two transmembrane proteins of eukaryotic SQRs. They contain heme and quinone binding sites. CybS is the eukaryotic homologue of the bacterial SdhD subunit.

CybS is a membrane-anchoring subunit of succinate dehydrogenase (SDH) that is involved in complex II of the mitochondrial electron transport chain and is responsible for transferring electrons from succinate to ubiquinone (coenzyme Q) [PMID: 15989954]. CybS is composed of a flavoprotein catalytic subunit, an iron-sulfur protein and one or two hydrophobic transmembrane subunits [PMID: 15989954]. The two-electron oxidation of succinate in the flavoprotein active site is coupled to the two-electron reduction of quinone in the transmembrane subunits via electron transport through FAD and three iron-sulfur centres [PMID: 12844553]. The reversible reduction of quinone is an essential feature of respiration, allowing transfer of electrons between respiratory complexes. Mutations in human Complex II result in various physiological disorders including hereditary paraganglioma and pheochromocytoma tumors. The gene encoding for the SdhD subunit is classified as a tumor suppressor gene [PMID: 15883710, PMID: 16288654, PMID: 16143825].

In Saccharomyces cerevisiae, Tim18 is a component of the TIM22 complex, a complex that mediates the import and insertion of multi-pass transmembrane proteins into the mitochondrial inner membrane. The TIM22 complex forms a twin-pore translocase that uses the membrane potential as external driving force. Its role in the complex is unclear but it may be involved in the assembly and stabilisation of the TIM22 complex [PMID: 10648604, PMID: 10637294, PMID: 14504216].

GO terms

Biological Process

No terms assigned in this category.

Molecular Function

No terms assigned in this category.

Cellular Component

GO:0016021 integral component of membrane
GO:0005740 mitochondrial envelope

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.