Reversible unfolding of ribosomal protein E-L30: an NMR study

Ribosomal protein E-L30 unfolds reversibly at pH values between 7.0 and 4.5. Unfolding of the protein involves a fast and a slow equilibrium, which depend on the degree of protonation of His19 and His33. Both the fast equilibrium between protonated and deprotonated histidines and the slow equilibrium between folded and unfolded protein could be monitored by means of 500-MHz 1H NMR spectroscopy. The degree of protonation of His19 and His33 appears to be determinant in the unfolding process of the protein. It is shown however that even when the histidines are uncharged, the protein has only limited stability, probably as a result of the presence of all four Glu's of E-L30 in its triple-stranded beta-sheet. At equimolar concentrations of the folded and unfolded form, the rate constant characterizing the transition between these forms is approximately 0.14 s-1. Making use of sequential resonance assignments of the 1H NMR spectrum [van de Ven, F.J.M., & Hilbers, C.W. (1986) J. Mol. Biol. 192, 419-441], the fast equilibrium could be interpreted in terms of alterations in the spatial structure of E-L30 in a specific domain of the molecule. This domain is also affected by temperature although not in exactly the same manner as by pH.