title: Identification of variant molecules of Bacillus thermoproteolyticus ferredoxin: crystal structure reveals bound coenzyme A and an unexpected [3Fe-4S] cluster associated with a canonical [4Fe-4S] ligand motif.
abstract: During the purification of recombinant Bacillus thermoproteolyticus ferredoxin (BtFd) from Escherichia coli, we have noted that some Fe-S proteins were produced in relatively small amounts compared to the originally identified BtFd carrying a [4Fe-4S] cluster. These variants could be purified into three Fe-S protein components (designated as V-I, V-II, and V-III) by standard chromatography procedures. UV-vis and EPR spectroscopic analyses indicated that each of these variants accommodates a [3Fe-4S] cluster. From mass spectrometric and protein sequence analyses together with native and SDS gel electrophoresis, we established that V-I and V-II contain the polypeptide of BtFd associated with acyl carrier protein (ACP) and with coenzyme A (CoA), respectively, and that V-III is a BtFd dimer linked by a disulfide bond. The crystal structure of the BtFd-CoA complex (V-II) determined at 1.6 A resolution revealed that each of the four complexes in the crystallographic asymmetric unit possesses a [3Fe-4S] cluster that is coordinated by Cys(11), Cys(17), and Cys(61). The polypeptide chain of each complex is superimposable onto that of the original [4Fe-4S] BtFd except for the segment containing Cys(14), the fourth ligand to the [4Fe-4S] cluster of BtFd. In the variant molecules, the side chain of Cys(14) is rotated away to the molecular surface, forming a disulfide bond with the terminal sulfhydryl group of CoA. This covalent modification may have occurred in vivo, thereby preventing the assembly of the [4Fe-4S] cluster as observed previously for Desulfovibrio gigas ferredoxin. Possibilities concerning how the variant molecules are formed in the cell are discussed.
authors.affiliation: Department of Biology, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan.
authors.forename: Tadayoshi
authors.initials: T
authors.lastname: Shirakawa
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Takahashi
authors.forename: Kei
authors.initials: K
authors.lastname: Wada
authors.forename: Junko
authors.initials: J
authors.lastname: Hirota
authors.forename: Toshifumi
authors.initials: T
authors.lastname: Takao
authors.forename: Daijiro
authors.initials: D
authors.lastname: Ohmori
authors.forename: Keiichi
authors.initials: K
authors.lastname: Fukuyama
mesh_terms: D001407:Bacillus / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D001665:Binding Sites; D002852:Chromatography, Ion Exchange; D003065:Coenzyme A / Q000737:chemistry / Q000378:metabolism; D005288:Ferredoxins / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D014644:Genetic Variation; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D011487:Protein Conformation
chemical_list: D001426:Bacterial Proteins; D005288:Ferredoxins; D007506:Iron-Sulfur Proteins; D008024:Ligands; D003065:Coenzyme A