Altered sensitivity of eukaryotic elongation factor 2 for trypsin after phosphorylation and ribosomal binding

Conformational changes in switch I of EF-G drive its directional cycling on and off the ribosome

We have trapped elongation factor G (EF-G) from Escherichia coli in six, functionally defined states, representing intermediates in its unidirectional catalytic cycle, which couples GTP hydrolysis to tRNA-mRNA translocation in the ribosome. By probing EF-G with trypsin in each state, we identified a substantial conformational change involving its conserved switch I (sw1) element, which contacts the GTP substrate. By attaching FeBABE (a hydroxyl radical generating probe) to sw1, we could monitor sw1 movement (by approximately 20 A), relative to the 70S ribosome, during the EF-G cycle. In free EF-G, sw1 is disordered, particularly in GDP-bound and nucleotide-free states. On EF-G*GTP binding to the ribosome, sw1 becomes structured and tucked inside the ribosome, thereby locking GTP onto EF-G. After hydrolysis and translocation, sw1 flips out from the ribosome, greatly accelerating release of GDP and EF-G from the ribosome. Collectively, our results support a central role of sw1 in driving the EF-G cycle during protein synthesis.

Effect of oxidizing agents and haemin on the phosphorylation of eukaryotic elongation factor 2 in rabbit reticulocyte lysates

Incubation of rabbit reticulocyte lysates in the absence of added haemin resulted in the phosphorylation of a 95 kDa protein. This protein was suggested to be elongation factor 2 (eEF-2) based on the following observations, (i) phosphorylation of the 95 kDa protein was Ca2+ and CaM-dependent. (ii) eEF-2 supplemented to the lysates became phosphorylated and co-migrated with the endogenous 95 kDa phosphoprotein upon electrophoresis in SDS gels. (iii) The tryptophane specific cleavage pattern obtained from the isolated 95 kDa phosphoprotein was identical to that of phosphorylated eEF-2. Phosphorylation of the 95 kDa protein was stimulated by oxidizing agents such as oxidized glutathione and NAD+ and inhibited by addition of haemin. The haemin concentration needed for 50% inhibition (IC50) was 2.5 microM. Haemin also had an inhibitory effect on eEF-2 phosphorylation in a system containing highly purified components (IC50 = 2 microM). In this system haemin inhibited phosphorylation of eEF-2 even in the presence of a 100-fold excess of beta-mercaptoethanol. Oxidizing agents had no effect on the kinase activity in the purified system.

Reduced puromycin sensitivity of translocated polysomes after the addition of elongation factor 2 and non-hydrolysable GTP analogues

Treatment of reticulocyte polysomes with elongation factor eEF-2 and GTP led to an increased sensitivity of peptidyl-tRNA for puromycin as a result of the translocation from the ribosomal A-site to the P-site. Upon addition of an excess of the non-hydrolysable GTP analogue, GuoPP[CH2]P, the puromycin sensitivity decreased rapidly. The decrease in sensitivity required high concentrations of eEF-2 with half maximal effect at an eEF-2 concentration of around 1 microM. The data suggest either that peptidyl-tRNA had re-translocated back to the A-site due to the higher affinity of eEF-2 for the pre-translocation than for the post-translocation ribosome, or that the eEF-2-GuoPP[CH2]P complex blocks the peptidyl-transferase activity.

Saccharomyces cerevisiae elongation factor 2 is phosphorylated by an endogenous kinase

Mammalian cells contain a Ca2+/calmodulin-dependent protein kinase that specifically phosphorylates and inactivates elongation factor 2 (EF-2) in response to hormones and other agents which increase intracellular Ca2+ concentrations. Therefore, it has been proposed that the rate of translation in mammals is regulated by EF-2 phosphorylation. In the present study, EF-2 purified from the yeast Saccharomyces cerevisiae is shown to be a substrate for the mammalian EF-2 kinase. Furthermore, evidence was obtained using two-dimensional gel electrophoresis and peptide mapping which suggests that yeast EF-2 is a substrate for an endogenous kinase which phosphorylates the same site as the mammalian EF-2 kinase. Based on these findings, we propose that in yeast as in higher eukaryotes, the protein synthesis elongation cycle is regulated by phosphorylation of EF-2.