The joining of ribosomal subunits in eukaryotes requires eIF5B

Initiation of eukaryotic protein synthesis begins with the ribosome separat ed into its 40S and 60S subunits(1). The 40S subunit first binds eukaryotic initiation factor (eIF) 3 and an eIF2-GTP-initiator transfer RNA ternary c omplex. The resulting complex requires eIF1, eIF1A, eIF4A, eIF4B and eIF4F to bind to a messenger RNA and to scan to the initiation codon(2). eIF5 sti mulates hydrolysis of eIF2-bound GTP and eIF2 is released from the 48S comp lex formed at the initiation codon before it is joined by a 60S subunit to form an active 80S ribosome(3-8). Here we show that hydrolysis of eIF2-boun d GTP induced by eIF5 in 48S complexes is necessary but not sufficient for the subunits to join. A second factor termed eIF5B (relative molecular mass 175,000) is essential for this process. It is a homologue of the prokaryot ic initiation factor IF2 (refs 6, 7) and, like it(8-12), mediates joining o f subunits and has a ribosome-dependent GTPase activity that is essential f or its function.