Re-examination of the post-translational arginylated protein of 125-kD initially identified as N-STOP.

Post-translational modification of proteins is a complex mechanism by which cells regulate protein activities. One post-translational modification is the incorporation of arginine into the NH2-terminus of proteins. It has been hypothesized that in rat brain extracts, one of the proteins modified by this reaction is the microtubule-associated protein Neuronal Stable Tubule Only Polypeptide (N-STOP). This was inferred from its electrophoretic mobility (125 kD) and because it was immunoprecipitated with a monoclonal antibody against the N-STOP. However, this hypothesis is not supported by our recent results. Herein, we found that rat N-STOP interacts with Ca(2+)-calmodulin, whereas the 125-kD [14C]-arginylated protein does not. The 125-kD [14C]-arginylated protein from rat brain is separated from the N-STOP by two-dimensional electrophoresis, and it is not recognized by a STOP monoclonal antibody. Mouse brain contains N-STOP, which migrates as a protein of 116 kD and could not be labeled by the post-translational incorporation of [14C]-arginine. The 125-kD [14C]-arginylated protein appears in wild-type as well as in STOP knock out mice. Based on these results, we conclude that the 125-kD arginylated protein is different from N-STOP.