Molecular Chaperones HscA/Ssq1 and HscB/Jac1 and Their Roles in Iron-Sulfur Protein Maturation

Abstract

Genetic and biochemical studies have led to the identification of several cellular pathways for the biosynthesis of iron-sulfur proteins in different organisms. The most broadly distributed and highly conserved system involves an Hsp70 chaperone and J-protein co-chaperone system that interacts with a scaffold-like protein involved in [FeS]-cluster preassembly. Specialized forms of Hsp70 and their co-chaperones have evolved in bacteria (HscA, HscB) and in certain fungi (Ssq1, Jac1), whereas most eukaryotes employ a multifunctional mitochondrial Hsp70 (mtHsp70) together with a specialized co-chaperone homologous to HscB/Jac1. HscA and Ssq1 have been shown to specifically bind to a conserved sequence present in the [FeS]-scaffold protein designated IscU in bacteria and Isu in fungi, and the crystal structure of a complex of a peptide containing the IscU recognition region bound to the HscA substrate binding domain has been determined. The interaction of IscU/Isu with HscA/Ssq1 is regulated by HscB/Jac1 which bind the scaffold protein to assist delivery to the chaperone and stabilize the chaperone-scaffold complex by enhancing chaperone ATPase activity. The crystal structure of HscB reveals that the N-terminal J-domain involved in regulation of HscA ATPase activity is similar to other J-proteins, whereas the C-terminal domain is unique and appears to mediate specific interactions with IscU. At the present time the exact function(s) of chaperone-[FeS]-scaffold interactions in iron-sulfur protein biosynthesis remain(s) to be established. In vivo and in vitro studies of yeast Ssq1 and Jac1 indicate that the chaperones are not required for [FeS]-cluster assembly on Isu. Recent in vitro studies using bacterial HscA, HscB and IscU have shown that the chaperones destabilize the IscU[FeS] complex and facilitate cluster delivery to an acceptor apo-protein consistent with a role in regulating cluster release and transfer. Additional genetic and biochemical studies are needed to extend these findings to mtHsp70 activities in higher eukaryotes.

Conserved, Disordered C Terminus of DnaK Enhances Cellular Survival upon Stress and DnaK in Vitro Chaperone Activity

“…In the cell, multivalent molecular recognition by the DnaK C terminus may accommodate the chaperoning of diverse cellular substrates, as opposed to other E. coli Hsp70s lacking the C-terminal motif that have specialized functional roles. For example, E. coli HscA specifically assists in the maturation of iron-sulfur cluster assembly protein IscA (55) and has a natural C-terminal deletion. The chaperones SecB and DnaJ, which are functionally linked to the C-terminal role of DnaK, play related roles in the protection of misfolded states from aggregation and degradation as well in the maintenance of the unfolded state for delivery to the secretory pathway (38,39,56,57).…”

Specialized Function of Yeast Isa1 and Isa2 Proteins in the Maturation of Mitochondrial [4Fe-4S] Proteins

“…In addition, the desulfurase and the Isu proteins interact with frataxin (yeast Yfh1), which may serve as an iron donor and/or a regulator of desulfurase activity (17,18). The subsequent cluster transfer to recipient apoproteins is facilitated by the Hsp70 chaperone Ssq1 (bacterial HscA), its cognate J-type co-chaperone Jac1 (bacterial HscB), and the monothiol glutaredoxin Grx5 (7,19,20).…”

Analysis of iron–sulfur protein maturation in eukaryotes

“…This biosynthetic step requires the redox chain NADH-ferredoxin reductase (Arh1)-ferredoxin (Yah1) and the putative iron donor frataxin (Yfh1). The preassembled Fe/S cluster is then transferred to apoproteins involving a dedicated chaperone system 6 and the monothiol glutaredoxin Grx5. Some mitochondrial Fe/S proteins further depend on Isa1, Isa2 and Iba57 proteins 7 or GTP 8 for functional assembly.…”