Stability and Folding of Precursor and Mature Tryptophan-Substituted Ribose Binding Protein ofEscherichia coli

A mutant ribose binding protein (RBP) ofEscherichia coliwas obtained by site-directed mutagenesis, replacing Phe-187 in the wild-type RBP (WT-RBP) with a Trp residue, in order to compare its stability and folding behavior with those of the WT-RBP. The equilibrium unfolding properties and the folding kinetics of these proteins were monitored by fluorescence and circular dichroism (CD). For both WT-RBP and the Trp-substituted RBP (Trp-RBP), the conformational stabilities of the precursor proteins and the mature proteins were the same, indicating that the signal peptide had no influence on the property of the mature domain. The Phe/Trp substitution in the mature domain, however, brought about a significant decrease in the conformational stability. The signal peptide had an appreciable retarding effect on the folding of the precursor Trp-RBP as was reported for the WT-RBP. Refolding kinetics of the WT-RBP and Trp-RBP showed a two-step reaction when monitored by fluorescence and by CD.