Work

CcsBA is a cytochrome c synthetase that also functions in heme transport

Year: 2009

Type: article

Abstract: Little is known about trafficking of heme from its sites of synthesis to sites of heme-protein assembly. We describe an integral membrane protein that allows trapping of endogenous heme to elucidate trafficking mechanisms. We show that CcsBA, a representative of a superfamily of integral membrane proteins involved in cytochrome c biosynthesis, exports and protects heme from oxidation. CcsBA has 10 transmembrane domains (TMDs) and reconstitutes cytochrome c synthesis in the Escherichia coli periplasm; thus, CcsBA is a cytochrome c synthetase. Purified CcsBA contains heme in an "external heme binding domain" for which two external histidines are shown to serve as axial ligands that protect the heme iron from oxidation. This is likely the active site of the synthetase. Furthermore, two conserved histidines in TMDs are required for heme to travel to the external heme binding domain. Remarkably, the function of CcsBA with mutations in these TMD histidines is corrected by exogenous imidazole, a result analogous to correction of heme binding by myoglobin when its proximal histidine is mutated. These data suggest that CcsBA has a heme binding site within the bilayer and that CcsBA is a heme channel.

Source: Proceedings of the National Academy of Sciences

DOI: 10.1073/pnas.0903132106

Authors: Elaine R. Frawley, Robert G. Kranz

Institutions: Washington University in St. Louis, University of Chicago

Language: English

Topic: Hemoglobin structure and function

Subfield: Cell Biology

Field: Biochemistry, Genetics and Molecular Biology

Domain: Life Sciences

Open Access status: green

Funders: Research Councils UK, Medical Research Council, National Institute of General Medical Sciences

Award: Cytochrome c biogenesis