Recognition of Modified Forms of Ribonuclease A by the Ubiquitin System

The substrate specificity of the ubiquitin (Ub) conjugation system was explored with regard to recognition of unfolded conformation and/or oxidized methionine residues in six derivatives of bovine RNase A. Based on the following observations, ubiquitination of RNase A substrates by the enzymes in a rabbit reticulocyte extract appears to correlate with unfolded conformation rather than with methionine oxidation. 1) Methionine oxidation in already unfolded forms of RNase A does not enhance ubiquitination. 2) Fluorescence measurements and iodoacetate trapping of free sulfhydryls show that the disulfide bonds of MetSO-RNase A, in which the 4 methionine residues are oxidized to the sulfoxide, are reduced by 2 mM dithiothreitol (DTT) in standard Ub conjugation assays so that this derivative also is unfolded. 3) Although MetSO-RNase A is ubiquitinated in the absence of DTT, its intrinsic fluorescence, cation-exchange properties, and susceptibility to reduction indicate a non-native conformation. 4) Methionine sulfoxide-containing peptides that mimic regions of RNase A fail to inhibit conjugation of 125I-Ub to MetSO-RNase A. Ub adducts to two of the six derivatives (MetSO- and reduced/carboxamidomethylated MetSO-RNase A) increase when DTT is omitted from the reactions. Ubaldehyde, an inhibitor of isopeptidases that disassemble Ub-protein conjugates, increased product yields and reduced or abolished the DTT effect, suggesting that an isopeptidase specific for these two RNase A derivatives may be inactivated by oxidation. Ub conjugates of the other RNase A derivatives also increase with Ub-aldehyde but are unaffected by DTT.