Abstract
IscX (or YfhJ) is a protein of unknown function which takes part in the iron-sulfur cluster assembly machinery, a highly specialized and essential metabolic pathway. IscX binds to iron with low affinity and interacts with IscS, the desulfurase central to cluster assembly. Previous studies have suggested a competition between IscX and CyaY, the bacterial ortholog of frataxin, for the same binding surface of IscS. This competition could suggest a link between the two proteins with a functional significance. Using a hybrid approach based on nuclear magnetic resonance, small angle scattering and biochemical methods, we show here that IscX is a modulator of the inhibitory properties of CyaY: by competing for the same site on IscS, the presence of IscX rescues the rates of enzymatic cluster formation which are inhibited by CyaY. The effect is stronger at low iron concentrations, whereas it becomes negligible at high iron concentrations. These results strongly suggest the mechanism of the dual regulation of iron sulfur cluster assembly under the control of iron as the effector.
Highlights
Iron and sulfur are elements essential for life thanks to their unique redox properties
Using a hybrid approach based on nuclear magnetic resonance, small angle scattering and biochemical methods, we show here that IscX is a modulator of the inhibitory properties of CyaY: by competing for the same site on IscS, the presence of IscX rescues the rates of enzymatic cluster formation which are inhibited by CyaY
The effect we observed at high IscX/IscS ratios could be explained by considering that IscX binds to iron and at high iron:IscX molar ratios (i.e., 80:1) aggregates (Pastore et al, 2006; Prischi et al, 2010a)
Summary
Iron and sulfur are elements essential for life thanks to their unique redox properties. Of Fe-S clusters is carried out by highly conserved machines which, in prokaryotes, are encoded by the suf, nif, and isc operons. Isc is the most general machine with highly conserved orthologs in eukaryotes. The last component of the machine according to the order of genes in the operon is IscX ( known as YfhJ), a small acidic protein about which very little is known (Tokumoto and Takahashi, 2001; Tokumoto et al, 2002).
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