The Complex Energy Landscape of the Protein IscU

• Published in: Biophysical journal Vol. 109; no. 5; pp. 1019 - 1025
• Main Authors: Bothe, Jameson R., Tonelli, Marco, Ali, Ibrahim K., Dai, Ziqi, Frederick, Ronnie O., Westler, William M., Markley, John L.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.09.2015; Biophysical Society; The Biophysical Society
• Subjects: Biosynthesis; Cold Temperature; Energy; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - metabolism; Hot Temperature; Iron; Iron - metabolism; Iron-Sulfur Proteins - chemistry; Iron-Sulfur Proteins - metabolism; Models, Molecular; NMR; Nuclear magnetic resonance; Protein Conformation; Protein Stability; Proteins; Proteins and Nucleic Acids; Sulfur; Sulfur - metabolism; Thermodynamics
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1016/j.bpj.2015.07.045

IscU, the scaffold protein for iron-sulfur (Fe-S) cluster biosynthesis in Escherichia coli, traverses a complex energy landscape during Fe-S cluster synthesis and transfer. Our previous studies showed that IscU populates two interconverting conformational states: one structured (S) and one largely disordered (D). Both states appear to be functionally important because proteins involved in the assembly or transfer of Fe-S clusters have been shown to interact preferentially with either the S or D state of IscU. To characterize the complex structure-energy landscape of IscU, we employed NMR spectroscopy, small-angle x-ray scattering (SAXS), and differential scanning calorimetry. Results obtained for IscU at pH 8.0 show that its S state is maximally populated at 25°C and that heating or cooling converts the protein toward the D state. Results from NMR and DSC indicate that both the heat- and cold-induced S→D transitions are cooperative and two-state. Low-resolution structural information from NMR and SAXS suggests that the structures of the cold-induced and heat-induced D states are similar. Both states exhibit similar 1H-15N HSQC spectra and the same pattern of peptidyl-prolyl peptide bond configurations by NMR, and both appear to be similarly expanded compared with the S state based on analysis of SAXS data. Whereas in other proteins the cold-denatured states have been found to be slightly more compact than the heat-denatured states, these two states occupy similar volumes in IscU.