X-Ray Structures of the Universal Translation Initiation Factor IF2/eIF5B: Conformational Changes on GDP and GTP Binding

X-ray structures of the universal translation initiation factor IF2/eIF5B have been determined in three states: free enzyme, inactive IF2/eIF5B·GDP, and active IF2/eIF5B·GTP. The “chalice-shaped” enzyme is a GTPase that facilitates ribosomal subunit joining and Met-tRNAi binding to ribosomes in all three kingdoms of life. The conserved core of IF2/eIF5B consists of an N-terminal G domain (I) plus an EF-Tu-type β barrel (II), followed by a novel α/β/α-sandwich (III) connected via an α helix to a second EF-Tu-type β barrel (IV). Structural comparisons reveal a molecular lever, which amplifies a modest conformational change in the Switch 2 region of the G domain induced by Mg2+/GTP binding over a distance of 90 Å from the G domain active center to domain IV. Mechanisms of GTPase function and ribosome binding are discussed.