Functional-structural analysis of threonine 25, a residue coordinating the nucleotide-bound magnesium in elongation factor Tu.
Elongation factor (EF) Tu Thr-25 is a key residue binding the essential magnesium complexed to nucleotide. We have characterized mutations at this position to the related Ser and to Ala, which abolishes the bond to Mg2+, and a double mutation, H22Y/T25S. Nucleotide interaction was moderately destabilized in EF-Tu(T25S) but strongly in EF-Tu(T25A) and EF-Tu(H22Y/T25S). Binding Phe-tRNAPhe to poly(U).ribosome needed a higher magnesium concentration for the latter two mutants but was comparable at 10 mM MgCl2. Whereas EF-Tu(T25S) synthesized poly(Phe), as effectively as wild type, the rate was reduced to 50% for EF-Tu(H22Y/T25S) and was, surprisingly, still 10% for EF-Tu(T25A). In contrast, protection of Phe-tRNAPhe against spontaneous hydrolysis by the latter two mutants was very low. The intrinsic GTPase in EF-Tu(H22Y/T25S) and (T25A) was reduced, and the different responses to ribosomes and kirromycin suggest that stimulation by these two agents follows different mechanisms. Of the mutants, only EF-Tu(T25A) forms a more stable complex with EF-Ts than wild type. This implies that stabilization of the EF-Tu.EF-Ts complex is related to the inability to bind Mg2+, rather than to a decreased nucleotide affinity. These results are discussed in the light of the three-dimensional structure. They emphasize the importance of the Thr-25-Mg2+ bond, although its absence is compatible with protein synthesis and thus with an active overall conformation of EF-Tu.
Krab
I M
IM
Groupe de Biophysique-Equipe 2, Ecole Polytechnique, F-91128 Palaiseau Cedex, France.
Parmeggiani
A
A
D020963:GTP Phosphohydrolase-Linked Elongation Factors / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D008958:Models, Molecular; D010444:Peptide Elongation Factor Tu / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D011728:Pyridones / Q000494:pharmacology; D012360:RNA, Transfer, Phe / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism; D013329:Structure-Activity Relationship; D013912:Threonine / Q000378:metabolism*
D011728:Pyridones; D012360:RNA, Transfer, Phe; D011993:Recombinant Fusion Proteins; D006153:Guanosine Diphosphate; D013912:Threonine; D006160:Guanosine Triphosphate; D020963:GTP Phosphohydrolase-Linked Elongation Factors; D010444:Peptide Elongation Factor Tu; D008274:Magnesium; C008536:mocimycin