PROTEIN-SYNTHESIS ELONGATION-FACTOR EF-1-ALPHA IS ESSENTIAL FOR UBIQUITIN-DEPENDENT DEGRADATION OF CERTAIN N-ALPHA-ACETYLATED PROTEINS AND MAY BE SUBSTITUTED FOR BY THE BACTERIAL ELONGATION-FACTOR EF-TU

Targeting of different cellular proteins for conjugation and subsequen t degradation via the ubiquitin pathway involves diverse recognition s ignals and distinct enzymatic factors. A few proteins are recognized v ia their N-terminal amino acid residue and conjugated by a ubiquitin-p rotein ligase that recognizes this residue. Most substrates, including the N-alpha-acetylated proteins that constitute the vast majority of cellular proteins, are targeted by different signals and are recognize d by yet unknown ligases. We have previously shown that degradation of N-terminally blocked proteins requires a specific factor, designated FH, and that the factor acts along with the 26S protease complex to de grade ubiquitin-conjugated proteins. Here, we demonstrate that FH is t he protein synthesis elongation factor EF-1 alpha. (a) Partial sequenc e analysis reveals 100% identity to EF-1 alpha. (b) Like EF-1 alpha, F H binds to immobilized GTP (or GDP) and can be purified in one step us ing the corresponding nucleotide for elution. (c) Guanine nucleotides that bind to EF-1 alpha protect the ubiquitin system-related activity of FH from heat inactivation, and nucleotides that do not bind do not exert this effect. (6) EF-Tu, the homologous bacterial elongation fact or, can substitute for FH/EF-1 alpha in the proteolytic system. This l ast finding is of particular interest since the ubiquitin system has n ot been identified in prokaryotes. The activities of both EF-1 alpha a nd EF-Tu are strongly and specifically inhibited by ubiquitin-aldehyde , a specific inhibitor of ubiquitin isopeptidases. It appears, therefo re, that EF-1 alpha may be involved in releasing ubiquitin from multiu biquitin chains, thus rendering the conjugates susceptible to the acti on of the 26S protease complex.