eIF2 is an essential factor for protein synthesis that forms a ternary complex (TC) with GTP and the initiator Met-tRNA. After its formation, the TC binds the 40S ribosomal subunit to form the 43S preinitiation complex (43S PIC). 43S PIC assembly is believed to be stimulated by the initiation factors eIF1, eIF1A, and the eIF3 complex according to in vitro experiments. The 43S PIC then binds mRNA that has previously been unwound by the eIF4F complex. The 43S PIC and the eIF4F proteins form a new 48S complex on the mRNA, which starts searching along the mRNA for the start codon (AUG). Upon base pairing of the AUG-codon with the Met-tRNA, eIF5 (which is a GTPase-activating protein , or GAP) is recruited to the complex and induces eIF2 to hydrolyse its GTP. This causes eIF2-GDP to be released from this 48S complex and translation begins after recruitment of the 60S ribosomal subunit and formation of the 80S initiation complex. Finally, with the help of the guanine nucleotide exchange factor (GEF) eIF2B, the GDP in eIF2 is exchanged for a GTP and the ternary complex reforms for a new round of translation initiation.