Regulatory phosphorylations on the eukaryotic elongation factor 1A mediated by RAF kinases

The elongation factor 1A has a crucial role in protein biosynthesis. EF1A belongs to the GTP-binding family of proteins and is able to deliver the aa-tRNA to the A-site of the ribosome because of its GTPase activity. Higher eukaryotes express two tissue specific isoforms of EF1A. In the last years, these isoforms were found to be involved in several different processes like senescence, apoptosis and transformation. In particular EF1A2 is overexpressed in several tumor cells and participates in drug resistence processes. Previous experiments in H1355 cells demonstrated that IFN? treatment results in an antiapoptotical answer during which EF1A2 is increasingly expressed due to its RAF kinase mediated phosphorylation. As this interaction establishes a new link between the mitogenic cascade and protein biosynthesis, in vitro kinase assays were performed in presence of B- and C-RAF kinase and recombinant EF1A1/2 to investigate this important interaction. Mass spectrometry analysis identified treonine 88 (exclusively mediated by B-RAF) and serine 21 on EF1A1 and serine 21 on EF1A2 as B-/ and C-RAF mediated phosphorylation sites. Interestingly, serine 21 belongs to the consensus sequence of the GTP/GDP binding domain of EF1A. Its phosphorylation prevents the binding of the nucleotide suggesting that this potentially RAF mediated modification has a regulatory role affecting the function of the elongation factor.