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

We have used variations in the trypsin sensitivity of eukaryotic protein synthesis elongation factor 2 (eEF-2) to probe for structural alterations induced by phosphorylation, ribosomal binding, or guanosine nucleotides. We could not detect any nucleotide-related effect on the tryptic cleavage rate of Arg66. However, eEF-2 was protected from trypsin after ribosomal binding. Also, phosphorylation of eEF-2 led to a protection of Arg66. This indicates that phosphorylation leads to a structural rearrangement that could explain the reduced affinity of the phosphorylated factor for ribosomes (Carlberg, U., Nilsson, A., and Nyg?rd, O. (1990) Eur. J. Biochem. 191, 639-645). Cleavage of Arg66 led to a complete loss of the ability of the factor to be phosphorylated. Furthermore, ribosome-bound eEF-2 was found to be inaccessible for phosphorylation. Based on these findings and previously published data, we suggest that the region around the sites of phosphorylation and trypsin cleavage is vitally important for the factor function and ribosomal binding.