Kinetics of Folding of the All-β Sheet Protein Interleukin-1β

The folding of the all-β sheet protein, interleukin-1β, was studied with nuclear magnetic resonance (NMR) spectroscopy, circular dichroism, and fluorescence. Ninety percent of the β structure present in the native protein, as monitored by far-ultraviolet circular dichroism, was attained within 25 milliseconds, correlating with the first kinetic phase determined by tryptophan and 1-anilinonaphthalene-8-sulfonate fluorescence. In contrast, formation of stable native secondary structure, as measured by quenched-flow deuterium-hydrogen exchange experiments, began after only 1 second. Results from the NMR experiments indicated the formation of at least two intermediates with half-lives of 0.7 to 1.5 and 15 to 25 seconds. The final stabilization of the secondary structure, however, occurs on a time scale much greater than 25 seconds. These results differ from previous results on mixed α helix-β sheet proteins in which both the α helices and β sheets were stabilized very rapidly (less than 10 to 20 milliseconds).