Universal conservation in translation initiation revealed by human and archaeal homologs of bacterial translation initiation factor IF2

Binding of initiator methionyl-tRNA to ribosomes is catalyzed in prokaryote s by initiation factor (IF) IF2 and in eukaryotes by eIF2. The discovery of both IF2 and eIF2 homologs in yeast and archaea suggested that these micro bes possess an evolutionarily intermediate protein synthesis apparatus. We describe the identification of a human IF2 homolog, and we demonstrate by u sing in vivo and in vitro assays that human IF2 functions as a translation factor. In addition, we show that archaea IF2 can substitute for its yeast homolog both in vivo and in vitro. We propose a universally conserved funct ion for IF2 in facilitating the proper binding of initiator methionyl-tRNA to the ribosomal P site.