Roles of Protein Synthesis Elongation Factor EF-Tu in Heat Tolerance in Plants

EF-Tu proteins of plastids, mitochondria, and the cytosolic counterpart EF-1α in plants, as well as EF-Tu proteins of bacteria, are highly conserved and multifunctional. The functions of EF-Tu include transporting the aminoacyl-tRNA complex to the A site of the ribosome during protein biosynthesis; chaperone activity in protecting other proteins from aggregation caused by environmental stresses, facilitating renaturation of proteins when conditions return to normal; displaying a protein disulfide isomerase activity; participating in the degradation of N-terminally blocked proteins by the proteasome; eliciting innate immunity and triggering resistance to pathogenic bacteria in plants; participating in transcription when an E. coli host is infected with phages. EF-Tu genes are upregulated by abiotic stresses in plants, and EF-Tu plays important role in stress responses. Expression of a plant EF-Tu gene confers heat tolerance in E. coli, maize knock-out EF-Tu null mutants are heat susceptible, and over-expression of an EF-Tu gene improves heat tolerance in crop plants. This review paper summarizes the current knowledge of EF-Tu proteins in stress responses in plants and progress on application of EF-Tu for developing crop varieties tolerant to abiotic stresses, such as high temperatures. 1. Introduction Protein synthesis elongation factor Tu (EF-Tu) is a protein that plays a central role in the elongation phase of protein synthesis in bacteria and organelles including mitochondria and plastids in plants (Figure 1). The cytosolic homolog of EF-Tu in plants is EF-1α. The polypeptide elongation cycle proceeds in three steps: (1) EF-Tu binds GTP and aminoacyl-tRNA, which leads to the codon-dependent placement of this aminoacyl-tRNA at the A site of the ribosome, GTP hydrolysis, and release of EF-Tu-GDP from the ribosome; (2) EF-Ts (elongation factor Ts) facilitates the exchange of EF-Tu-bound GDP for GTP; (3) upon the peptide bond formation, EF-G (elongation factor G) translocates the mRNA one codon to allow for the arrival of the new aminoacyl-tRNA in the A site [1]. EF-Tu and EF-Ts were first isolated as components of so-called factor T (transfer), and labeled as thermounstable (Tu) and thermostable (Ts) fractions [2], respectively. This comparison of thermostability is questionable because later on EF-Tu has been proved to endure high-temperature treatments [3, 4], especially when complexed with nucleotide factors, GTP or GDP [5]. Figure 1: The role of EF-Tu in the elongation phase of protein synthesis. EF-Tu ·GTP (binary complex) binds aminoacyl-tRNA to