Amide proton exchange measurements as a probe of the stability and dynamics of the N-terminal domain of the ribosomal protein L9: Comparison with the intact protein

Amide H/D exchange rates have been measured for the N-terminal domain of the ribosomal protein L9, residues 1-56. The rates were measured at pD 3.91, 5.03, and 5.37. At pD 5.37, 18 amides exchange slowly enough to give reliable rate measurements. At pD 3.91, seven additional residues could be followed. The exchange is shown to occur by the EX2 mechanism for all conditions studied. The rates for the N-terminal domain are very similar to those previously measured for the corresponding region in the full-length protein (Lillemoen J et al., 1997, J Mol Biol 268:482-493). In particular, the rates for the residues that we have shown to exchange via global unfolding in the N-terminal domain agree within the experimental error with the rates measured by Hoffman and coworkers, suggesting that the structure of the domain is preserved in isolation and that the stability of the isolated domain is comparable to the stability of this domain in intact L9.