IDENTIFICATION OF THE GENE ENCODING THE MITOCHONDRIAL ELONGATION FACTOR-G IN MAMMALS

Protein synthesis in cytosolic and rough endoplasmic reticulum associa ted ribosomes is directed by factors, many of which have been well cha racterized. Although these factors have been the subject of intense st udy, most of the corresponding factors regulating protein synthesis in the mitochondrial ribosomes remain unknown. in this report we present the cloning and initial characterization of the gene encoding the rat mitochondrial elongation factor-G (rEF-G(mt)) The rat gene encoding E F-G(mt) (rMef-g) maps to rat chromosome 2 and it is expressed in all t issues with highest levels in liver, thymus and brain. Its DNA sequenc e predicts a 752 amino acid protein exhibiting 72% homology to the yea st Saccharomyces cerevisiae mitochondrial elongation factor-G (YMEF-G) , 62% and 61% homology to the Thermus thermophilus and E. coli elongat ion factor-G (EF-G) respectively and 52% homology to the rat elongatio n factor-2 (EF-2). The deduced amino acid sequence of EF-G contains ch aracteristic motifs shared by all GTP binding proteins. Therefore, sim ilarly to other elongation factors, the enzymatic function of EF-G(mt) is predicted to depend on GTP binding and hydrolysis. EF-G(mt) differ s from its cytoplasmic homolog, EF-2, in that it contains an aspartic acid residue at amino acid position 621 which corresponds to the EF-2 histidine residue at position 715. Since this histidine residue, follo wing posttranslational modification into diphthamide, appears to be th e sole cellular target of diphtheria toxin and Pseudomonas aeruginose endotoxin A, we conclude that EF-G(mt) will not be inactivated by thes e toxins. The severe effects of these toxins on protein elongation in tissues expressing EF-G(mt) suggest that EF-G(mt) and EF-2 exhibit non overlapping functions. The cloning and characterization of the mammali an mitochondrial elongation factor G will permit us to address its rol e in the regulation of normal mitochondrial function and in disease st ates attributed to mitochondrial dysfunction.