What is the signal for the posttranslational arginylation of proteins?

The N-terminal, posttranslational arginylation of proteins is ubiquitous in eukaryotic cells. Previous experiments, using purified components of the r eaction incubated in the presence of exogenous substrates, have shown that only those proteins containing acidic residues at their N-terminals are arg inylation substrates. However, data from experiments that used crude extrac ts of brain and nerve as the source of the arginylating molecules, suggest that the in vivo targets for arginylation are more complex than those demon strated using purified components. One of the proposed functions for arginy lation is as a signal for protein degradation and proteins that have underg one oxidative damage have been shown to be rapidly degraded, In the present experiments we have tested the hypothesis that the presence of an oxidativ ely damaged residue in a protein is a signal for its arginylation. These ex periments have been performed by adding synthetic oxidized peptides to crud e extracts of rat brain, incubating them with [H-3]Arg and ATP and assaying for arginylated peptides using RP-HPLC. Results showed that while the oxid ized A-chain of insulin was arginylated in this system, confirming previous experiments, other peptides containing oxidized residues were not. When a peptide containing Glu in the N-terminus was incubated under the same condi tions it too was not a substrate for arginylation. These findings show that neither the presence of an N-terminal acidic residue nor an oxidized resid ue alone are sufficient to signal arginylation. Thus, another feature of th e oxidized A-chain of insulin is required for arginylation. That feature re mains to be identified.