Structure of Human J-type Co-chaperone HscB Reveals a Tetracysteine Metal-binding Domain

Abstract

Iron-sulfur proteins play indispensable roles in a broad range of biochemical processes. The biogenesis of iron-sulfur proteins is a complex process that has become a subject of extensive research. The final step of iron-sulfur protein assembly involves transfer of an iron-sulfur cluster from a cluster-donor to a cluster-acceptor protein. This process is facilitated by a specialized chaperone system, which consists of a molecular chaperone from the Hsc70 family and a co-chaperone of the J-domain family. The 3.0 Å crystal structure of a human mitochondrial J-type co-chaperone HscB revealed an L-shaped protein that resembles Escherichia coli HscB. The important difference between the two homologs is the presence of an auxiliary metal-binding domain at the N terminus of human HscB that coordinates a metal via the tetracysteine consensus motif CWXCX 9 -13 FCXXCXXXQ. The domain is found in HscB homologs from animals and plants as well as in magnetotactic bacteria. The metal-binding site of the domain is structurally similar to that of rubredoxin and several zinc finger proteins containing rubredoxin-like knuckles. The normal mode analysis of HscB revealed that this L-shaped protein preferentially undergoes a scissors-like motion that correlates well with the conformational changes of human HscB observed in the crystals.Iron-sulfur proteins are a ubiquitous class of proteins that play essential roles in a wide range of biochemical processes in the cell (1). The unique characteristic of these proteins is that they harbor a distinctive class of prosthetic groups: the ironsulfur clusters formed from multiple iron and sulfide ions. The most common iron-sulfur clusters are tetranuclear 2ϩ,ϩ and binuclear [2Fe-2S] 2ϩ,ϩ . Usually, the clusters are attached to proteins by four cysteine residues, although in several cases other residue types have been as ligands (2). Many iron-sulfur proteins are considered to be evolutionarily ancient because they play important roles in some of the most fundamental biological pathways and processes (1). These processes include electron transfers in respiratory and photosynthetic electron transport chains, nitrogen fixation, and biosynthesis of coenzymes. In addition, iron-sulfur proteins can function as initiators of radical chemistry, as iron and/or iron-sulfur cluster storage proteins, as sensors of cellular iron levels, as sensors of oxygen levels involved in gene regulation of the aerobic and anaerobic respiratory pathways, or as sulfur donors (1, 3, 4). The functions of iron-sulfur proteins have been studied extensively for decades, but the mechanism and control of their biogenesis has started to be elucidated only recently (5, 6). Three types of systems involved in the assembly of iron-sulfur clusters and maturation of iron-sulfur cluster proteins have been identified so far. These include the nif system of nitrogen fixing bacteria involved in maturation of nitrogenase (7), the suf (sulfur utilization factor) system that plays a significant role during iron and/or sulfur starvation an...

Iron –sulfur cluster biogenesis in mammalian cells: New insights into the molecular mechanisms of cluster delivery

“…Both in the bacterial and mitochondrial systems, the C-terminal domain of the cochaperone is directly responsible for binding the scaffold protein ISCU, with three highly conserved hydrophobic residues being of crucial importance for the HSC20- ISCU interaction [37, 73–75]. The crystal structures of HscB from E. coli [76], of Jac1 from S. cerevisiae [74], and of human HSC20 [77] revealed common features among cochaperones dedicated to Fe-S cluster biogenesis (Figure 2). They have a conserved structural core that consists of two domains arranged in a L-shaped fold.…”

“…The N-terminus of human HSC20 (Figure 2A) is clearly different from the specialized DnaJ type III cochaperones of bacteria and fungi, in that it contains, adjacent to the mitochondrial targeting sequence (residues 1–26 [70]), an extra-domain, which harbors two CxxC modules (C41/C44 and C58/C61) of unknown function (Figure 2A). Their ability to coordinate a zinc ion in vitro results in a zinc finger- like structure [77]. …”

An Unexpected Duo: Rubredoxin Binds Nine TPR Motifs to Form LapB, an Essential Regulator of Lipopolysaccharide Synthesis

“…Examples of rubredoxin domains within larger proteins have also been identified. The key features of these rubredoxin-like domains are the extended loops or ''knuckles'' and the tetracysteine mode of iron binding (Bitto et al, 2008). Rubredoxins are usually implicated in redox reactions and electron transfer.…”

Biogenesis and functions of mammalian iron-sulfur proteins in the regulation of iron homeostasis and pivotal metabolic pathways

“…genesis in yeast strains that encode Ssq1 has a truncated N terminus (79) compared with human HSC20, in which an extended N-terminal domain contains two CXXC modules (Cys-41/Cys-44 and Cys-58/Cys-61) that coordinate a zinc ion in vitro (80). The physiological relevance of the zinc finger domain of HSC20 remains to be elucidated, although it may facilitate dimerization of the co-chaperone.…”