Featured Article - April 2012
Short description: The structure of a yeast co-chaperone reveals interactions essential for Fe-S cluster biogenesis.
Iron-sulfur (Fe-S) cluster assemblies play essential roles in catalysis, electron transport, redox reactions and other regulatory functions. Biogenesis of Fe-S clusters requires assembly on specialized scaffold proteins before transfer to recipient apo-proteins. The mitochondria of Saccharomyces cerevisiae contain two functionally redundant scaffold proteins (Isu1 and Isu2) that are the only known clients for Ssq1, a dedicated Hsp70-type chaperone required for the Fe-S cluster transfer. The ATPase activity of Ssq1 is stimulated by the co-chaperone Jac1, a J-protein that binds and targets the Isu1/2 scaffold clients to the chaperone complex.
Craig, Marszalek and colleagues (PSI MCSG) have solved the crystal structure of the co-chaperone Jac1 at a resolution of 1.85 Å (PDB 3UO3). The structure contains two distinct α-helical domains; the J-domain comprising helices H1 to H3 is connected via the linker helix H4 to the C-domain composed of helices H5 to H8. The J-domain has unique structural features, including an unstructured loop between helices H2 and H3 containing the invariant sequence required for stimulation of Ssq1 ATPase activity. The authors suggest that due to its specialized mitochondrial function, the Jac1 J-domain has evolved a structure adapted for interaction with only Ssq1.
The authors next examined the specificity determinants of the interaction between the C-domain and Isu1. In contrast to its J-domain, the C-domain of Jac1 is structurally conserved, allowing the authors to focus on a binding surface also found in a bacterial ortholog HscB (PDB 1FPO). They first mutated eight surface residues to alanine, which abolished Jac1 stimulation of Ssq1 ATPase activity in vitro and resulted in a null phenotype in vivo. Alanine substitution of four conserved charged residues revealed that the residues contribute to activity, but are inessential as no growth defect was observed. The authors finally replaced three leucine residues and one tyrosine residue in an adjacent hydrophobic patch and found that two of the leucines and the tyrosine are important for the Isu1 interaction both in vitro and in vivo. Overall, the study elucidates the role of a composite charged and hydrophobic binding surface on Jac1 for Isu1 interaction.
S. J. Ciesielski et al. Interaction of J-protein co-chaperone Jac1 with Fe-S scaffold Isu is indispensable in vivo and conserved in evolution.
J. Mol. Biol. 417, 1-12 (2012).