Pulsed-alkylation mass spectrometry for the study of protein folding and dynamics: development and application to the study of a folding/unfolding intermediate of bacterial luciferase.
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A new method employing the classical techniques of chemical modification of proteins and the new technology of mass spectrometry, known as pulsed-alkylation mass spectrometry (PA/MS), has been developed to probe the dynamic structure of folding intermediates and folded complexes of proteins under a variety of conditions. This method is fast and simple, and the results are easily interpreted. PA/MS may provide an alternative to H/D exchange monitored either by NMR or by electrospray ionization mass spectrometry for some experiments; for others, it may provide access to questions not readily answered by available methods. The objective of PA/MS is to determine simultaneously the location and the extent of labeling of functional groups in a protein by measuring the reactivity of cysteines with N-ethylmaleimide, within the context of the conformation of the protein under specific conditions. The method can also be applied to chemical modification of other amino acid residues employing any of a vast array of reagents, depending upon the specifics of the protein under investigation. The enormous range of reactivity of the thiol groups of the cysteinyl residues in proteins and the change in reactivity upon denaturation or conformational rearrangement afford a large signal change that can be correlated with changes in accessibility of the thiol group. The information obtained from the correlation of observed thiol reactivity with the local environment of each cysteinyl residue in the structure of the folded protein can be supplemented by results obtained from fluorescence, circular dichroism, or other methods, to develop an understanding of the structure and dynamics of altered conformational states. With bacterial luciferase as a model system, we have applied PA/MS to investigate the structural differences between the native heterodimeric enzyme and a folding intermediate that is well-populated in 2 M urea. The thiol residues at positions 307, 324, and 325 of the alpha subunit were much more reactive with N-ethylmaleimide in the presence of 2 M urea than in the native enzyme, suggesting that the C-terminal region of the alpha subunit was less tightly packed in the folding intermediate. The apparent unfolding of the C-terminal region of the alpha subunit of the alphabeta structure in 2 M urea appears to mimic the unfolding of the C-terminal domain of the free alpha subunit, also in 2 M urea, described by Noland, B. W., Dangott, L. J., and Baldwin, T. O. (1999) Biochemistry 38, 16136-16145. The approach described here should be applicable to a wide array of problems that have in common the need to determine the locations of conformational changes in proteins. Application of PA/MS to the investigation of the relative thermodynamic stability of the coordination complexes of zinc within each of the six zinc-finger domains of MRE-binding transcription factor-1 (Zn(6) MTF-zf) in its free and DNA-bound forms is presented in the companion paper in this issue [Apuy, J. L., Chen, X., Russell, D. H., Baldwin, T. O., and Giedroc, D. P. (2001) Biochemistry 40, 15164-15175].