Iron-sulfur cluster biosynthesis: biochemical characterization of the conformational dynamics of Thermotoga maritima IscU and the relevance for cellular cluster assembly.

Important for the understanding of the functional properties of the iron-sulfur scaffold IscU is knowledge of the structure and dynamics of this protein class. Structural characterization of Thermotoga maritima IscU by CD (Mansy, S. S., Wu, G., Surerus, K. K., and Cowan, J. A. (2002) J. Biol. Chem. 277, 21397-21404) and high resolution NMR (Bertini, I., Cowan, J. A., Del Bianco, C., Luchinat, C., and Mansy, S. S. (2003) J. Mol. Biol. 331, 907-924) yielded data indicating a high degree of secondary structure. However, the latter also revealed IscU to exist in a dynamic equilibrium between two or more distinct conformations, possibly existing in a molten globule state. Herein, we further characterize the molten globule characteristics of T. maritima IscU by near-ultraviolet circular dichroism, 1-anilino-8-naphthalenesulfonic acid binding, free energy of unfolding, hydrodynamic radius measurements, and limited tryptic digestion. The data suggest unusual dynamic behavior that is not fully consistent with typical protein states such as fully folded, fully unfolded, or molten globule. For instance, the existence of a stable tertiary fold is supported by near-UV CD spectra and hydrodynamic radius measurements, whereas other data are less clearly interpretable and may be viewed as consistent with either a molten globule or fully folded state. However, all of the data are consistent with our previous hypothesis of a protein sampling multiple discrete tertiary conformations in which these structural transitions occur on a "slow" time scale. To describe such proteins, we introduce the term multiple discrete conformers.