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
H. Gonen et al., 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, Proceedings of the National Academy of Sciences of the United Statesof America, 91(16), 1994, pp. 7648-7652
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.