Ssq1, a Mitochondrial Hsp70 Involved in Iron-Sulfur (Fe/S) Center Biogenesis

Abstract

The results of in vivo and in organellar experiments indicate that the Hsp70 Ssq1 and the J-protein Jac1 function together to assist in the biogenesis of ironsulfur (Fe/S) centers in the mitochondrial matrix. Here we present biochemical evidence supporting this idea. Isu, the proposed scaffold on which Fe/S centers are assembled, is a substrate for both Jac1 and Ssq1. Jac1 and Isu1 cooperatively stimulate the ATPase activity of Ssq1. In addition, Jac1 facilitates the interaction of Ssq1 with Isu1 in the presence of ATP. These findings are consistent with the role in Fe/S biogenesis previously proposed for the bacterial Hsp70 Hsc66 and J-protein Hsc20 that interact with the bacterial Isu homologue IscU. However, unlike the bacterial Hsp70, we found that Ssq1 has a high affinity for nucleotide, and shares a nucleotide exchange factor, Mge1, with a second mitochondrial Hsp70, Ssc1. Thus, whereas the bacterial and mitochondrial chaperone systems share critical features, they possess significant biochemical differences as well.

Sequence-specific Interaction between Mitochondrial Fe-S Scaffold Protein Isu and Hsp70 Ssq1 Is Essential for Their in Vivo Function

“…Both Ssq1 and Hsc66 and their respective co-chaperones Jac1 and Hsc20 interact independently with Isu/IscU. In addition, Isu/IscU and Jac1/Hsc20 cooperatively stimulate Ssq1/Hsc66 ATPase activity, a strong indication that Isu/IscU is a substrate for the chaperone systems (7,16).…”

“…As expected of an Hsp70-J-protein-substrate interaction, Jac1 interacts with Isu independently of adenine nucleotide, although the interaction of Isu with Ssq1 is nucleotide-dependent (7). Typical of an Hsp70-substrate interaction, Isu binds stably to Ssq1 in the presence of ADP, whereas no direct interaction between Isu and Ssq1 has been detected in vitro in the presence of ATP.…”

“…Although IscU/Isu proteins are clearly able to interact with their respective chaperone systems (6,7,16), the biological importance of these interactions has not been tested. In E. coli, in which these interactions were first observed, redundant systems of Fe-S cluster assembly exist (24).…”

“…Ssq1/Hsc66 interacts with a substrate protein, Isu/ IscU (6, 7), the scaffold on which an Fe-S cluster is built prior to transferring to a recipient apoprotein (8,9). Like other Hsp70s, Ssq1 and Hsc66 do not work alone but in collaboration with a J-type co-chaperone, Jac1 and Hsc20, respectively (7,10). Results of in vivo and in organelle studies support the idea that Ssq1 and Jac1 function in Fe-S cluster biogenesis.…”

Reactivation of protein aggregates by mortalin and Tid1—the human mitochondrial Hsp70 chaperone system

“…Using purified proteins, it was shown that Ssc1 can mediate ATP-dependent disaggregation and proper refolding of stably misfolded reporter proteins, when supplemented with bacterial DnaJ and GrpE (De Los Rios et al 2006). Ssq1 is known to play a role in the biogenesis of iron sulfur clusters and the function of Ecm10 is still unclear (Baumann et al 2000;Dutkiewicz et al 2003).…”