INTERACTION OF PHOSPHORYLATED ELONGATION-FACTOR EF-2 WITH NUCLEOTIDESAND RIBOSOMES

The intrinsic fluorescence emission spectrum of elongation factor EF-2 due to the 7 Trp residues was not modified after complete phosphoryla tion of the factor by the specific Ca2+/Calmodulin-dependent kinase II I. The effect of nucleotide binding on this fluorescence revealed diff erences between phosphorylated and unmodified EF-2. Low concentrations of GTP had a smaller quenching effect on the fluorescence of phosphor ylated EF-2 than on the fluorescence of unmodified EF-2, whereas GDP h ad exactly the same quenching effect on the fluorescence of both sampl es. These results suggest that phosphorylation of EF-2 decreased its a ffinity for GTP but not for GDP. Ability of phosphorylated EF-2 to for m a ternary complex with ribosomes in the presence of a non-hydrolysab le GTP analog and its ability to protect ribosomes against ricin-inact ivation were both decreased to the same extent. The lower affinity of phosphorylated EF-2 for GTP could be responsible for a weaker and/or i ncorrect interaction of the factor with the ribosome, in particular wi th the ricin-site of the 28-S rRNA assumed to be involved in transloca tion initiation.