Figure 2.

Catalysis of amide proton exchange of barnase (2.4 mM) by the fragments GroEL191-345 (a) and sht-GroEL191-376 (b). The results are very similar to those described in refs. 10 and 11 for intact GroEL. The rate constants (in units of min⁻¹) for the exchange of individual NH protons in barnase in the presence of fragment [ (+G)] are plotted against those in the absence (). Amide protons that exchange by global, mixed, and local unfolding mechanisms are displayed by circles, triangles, and squares, respectively. The plot for 90 μM sht-GroEL191-345 at pD 7.1 (not shown) is virtually superimposable on that for sht-GroEL191-376 (b). (pD = pH measured in ²H₂O.) It is clearly seen that those protons that require global unfolding for exchange have significantly increased rates, thus showing that the fragments bind to the unfolded state of barnase and catalyze its unfolding. We could only estimate the final concentration of GroEL fragment in a since GroEL191-345 tended to crystallize during the exchange experiment at the high initial protein concentration.