Pro-interleukin (IL)-1β Shares a Core Region of Stability as Compared with Mature IL-1β While Maintaining a Distinctly Different Configurational Landscape A COMPARATIVE HYDROGEN/DEUTERIUM EXCHANGE MASS SPECTROMETRY STUDY

Interleukin-1 beta (IL-1 beta) is a master cytokine involved in initiating the innate immune response in vertebrates (Dinarello, C. A. (1994) FASEB J. 8, 1314-1325). It is first synthesized as an inactive 269-residue precursor (pro-interleukin-1 beta or pro-IL-1 beta). Pro-IL-1 beta requires processing by caspase-1 to generate the active, mature 153-residue cytokine. In this study, we combined hydrogen/deuterium exchange mass spectrometry, circular dichroism spectroscopy, and enzymatic digestion comparative studies to investigate the configurational landscape of pro-IL-1 beta and the role the N terminus plays in modulating the landscape. We find that the N terminus keeps pro-IL-1 beta in a protease-labile state while maintaining a core region of stability in the C-terminal region, the eventual mature protein. In mature IL-1 beta, this highly protected region maps back to the area protected earliest in the NMR studies characterizing an on-route kinetic refolding intermediate. This protected region also encompasses two important functional loops that participate in the IL-1 beta/receptor binding interface required for biological activity. We propose that the purpose of the N-terminal precursor region in pro-IL-1 beta is to suppress the function of the eventual mature region while keeping a structurally and also functionally important core region primed for the final folding into the native, active state of the mature protein. The presence of the self-inhibiting precursor region provides yet another layer of regulation in the life cycle of this important cytokine.