Effects of replacement of Lys25 with Gln on the conformation of ribonuclease T1: sequence-specific 1H NMR resonance assignments of Gln25 ribonuclease T1 by two-dimensional NMR spectroscopy.

Abstract

Two isozymes of ribonuclease (RNase) T1 exist in nature, i.e. Gln25 RNase T1 and Lys25 RNase T1. Gln25 RNase T1 is less stable than Lys25 RNase T1, although the enzymatic activity is not distinguishable between these two isozymes. To elucidate the effects of the replacement of Lys25 with Gln on the conformation and microenvironments of RNase T1 in detail, two-dimensional NMR spectra were measured, sequence-specific 1H NMR resonance assignments of Gln25 RNase T1 were performed, and then the determined parameters and microenvironments of Gln25 RNase T1 were compared with those of Lys25 isozyme [Hoffmann, E. and Rüterjans, H. (1988) Eur. J. Biochem. 177, 539-560]. The main chain protons were assigned for 101 out of the total of 104 amino acid residues. Secondary structure elements were identified from analysis of characteristic NOE patterns, interstrand NOE connectivities, and hydrogen-deuterium exchange rates of main chain amide protons. The results indicated that Gln25 RNase T1 contains a single alpha-helix and seven beta-strands. The secondary structure of Gln25 RNase T1 is, thus, essentially the same as that of Lys25 RNase T1. On the other hand, comparison of the conformation-dependent shifts of Gln25 RNase T1 with these of Lys25 RNase T1 showed that the replacement of Lys25 with Gln has significant effects on the C-terminal part of the alpha-helix region and the base-binding site. These results may indicate that the base-binding site is relatively flexible in the RNase T1 molecule. Among the residues of the C-terminal part of the alpha-helix region, the protons of Asp29 were most affected in terms of their chemical shifts, which may indicate that the side chain carboxylate anion of Asp29 is the counterpart of the electrostatic interaction of Lys25 in Lys25 RNase T1. The Gln25 of Gln25 RNase T1 may have little or no interaction with Asp29, and this may be the reason why Gln25 RNase T1 is less stable than the Lys25 isozyme.