Featured Article - October 2008
Short description: J. Biol. Chem., doi:10.1074/jbc.M804746200 (2008)
Iron–sulfur proteins have essential roles in many biological processes. These include respiration and photosynthesis, nitrogen fixation and co-enzyme biosynthesis. They can also act as detectors of iron levels and as oxygen sensors within the cell.
These metalloproteins are ancient and widely conserved from bacteria to humans. The functions of iron–sulfur proteins have been studied for many years, but it is only recently that their biogenesis has really begun to be understood.
Three systems for the biosynthesis of iron–sulfur clusters have been identified. Of these, the best characterized is the isc (iron–sulfur cluster) pathway, and most of the proteins from the isc operon have had their structures solved, several of them by structural genomics centers.
Such studies have contributed towards the two-step model for their biogenesis. In the first step, the iron–sulfur cluster is assembled on the scaffold protein IscU, and in the second, the cluster is transferred to an acceptor apoprotein. This transfer is assisted by a molecular chaperone system consisting of the Hsp70 family chaperone, HscA, and a J-type co-chaperone, HscB. However, the molecular mechanism for this iron–sulfur transfer has not been completed resolved.
Bitto et al. studied HscB, a protein from the isc pathway, and produced a 3.0 Å crystal structure of human HscB. This structure can be divided in to three different domains: the N-domain (residues 39–71), the J-domain (residues 72–145) and the C-domain (residues 156–235). Overall, it forms an L-shaped protein similar to that of its Escherichia coli homolog, with which it shares 29% identity, but there are important differences.
The N-domain does not closely resemble that of any previously described proteins. Compared with the E. coli structure, it has an additional metal-binding domain at the N terminus that coordinates a metal through a tetracysteine consensus motif (CWXC-X 9–13-FCXXCXXXQ). The metal is coordinated by Cys41, Cys44, Cys58 and Cys61, which are located on two apposed β-hairpins. The metal was tentatively modeled as Zinc(II), but the true identity of the metal is yet to be determined.
The loops within the N-domain show some similarity to the 'knuckles' of rubredoxin, a tetracysteine iron-binding protein involved in many redox reactions including the oxidative stress response pathways, although the HscB domain itself lacks any recognizable secondary structure elements.
The second important difference from the E. coli HscB is the relative orientations of the J- and C-domains. These domains are connected to each other through a flexible linker and meet at roughly a right angle to create an L-shaped molecule. The C-domain is composed of a helical bundle, a fairly common motif, but the arrangement of the C- and J-domain together is new, with the only other protein with this domain being the E. coli HscB.
Bitto et al. used normal mode analysis—a computational method for predicting the preferred directions in which a protein will move—to look at the relative alignment of the J- and C-domains. This technique predicted several movements, including a scissors-like motion with one blade formed by the N- and J-domain and the other by the C-domain, suggesting that the L-shaped structure is dynamic. This flexibility might be important in facilitating the iron–sulfur cluster transfer process.
These structural insights hint at new functions for HscB. The authors speculate that the metal-binding domain might mediate the interaction with specialized transport proteins. Another possibility is that the N-domain binds to other metals, perhaps iron, and could be involved in one-electron redox reactions.
Another observation supports this possibility: although this domain is found in many organisms, the only bacteria that have it have unusual iron or heavy metal dependent metabolic pathways. Identification of the metal bound by the four cysteines will be key to understanding its function.
Eduard Bitto, Craig A. Bingman, Lenka Bittova, Dmitry A. Kondrashov, Ryan M. Bannen et al. Structure of a human J-type co-chaperone HscB reveals a tetracysteine metal binding domain.
J. Biol. Chem. (2008). doi:10.1074/jbc.M804746200