Abstract
The x-ray structure of the gamma -subunit of the heterotrimeric translation initiation factor eIF2 has been determined to 2.4-Aa resolution. eIF2 is a GTPase that delivers the initiator Met-tRNA to the P site on the small ribosomal subunit during a rate-limiting initiation step in translation. The structure of eIF2 gamma closely resembles that of EF1A super(.)GTP, consisting of an N-terminal G domain followed by two beta -barrels arranged in a closed configuration with domain II packed against the G domain in the vicinity of the Switch regions. The G domain of eIF2 gamma has an unusual zinc ribbon motif, not previously found in other GTPases. Structure-based site-directed mutagenesis was used to identify two adjacent features on the surface of eIF2 gamma that bind the alpha - subunit and , respectively. These structural, biochemical, and genetic results provide new insights into eIF2 ternary complex assembly.