Eukaryotic initiation factor (eIF) 1 is a small protein (12 kDa) governing fidelity in translation initiation. It is recruited to the 40 S subunit in a multifactor complex with Met-tRNA(i)(Met), eIF2, eIF3, and eIF5 and binds near the P-site. eIF1 release in response to start codon recognition is an important signal to produce an 80 S initiation complex. Although the ribosome-binding face of eIF1 was identified, interfaces to other preinitiation complex components and their relevance to eIF1 function have not been determined. Exploiting the solution structure of yeast eIF1, here we locate the binding site for eIF5 in its N-terminal tail and at a basic/hydrophobic surface area termed KH, distinct from the ribosome-binding face. Genetic and biochemical studies indicate that the eIF1 N-terminal tail plays a stimulatory role in cooperative multifactor assembly. A mutation altering the basic part of eIF1-KH is lethal and shows a dominant phenotype indicating relaxed start codon selection. Cheung et al. recently demonstrated that the alteration of hydrophobic residues of eIF1 disrupts a critical link to the preinitiation complex that suppresses eIF1 release before start codon selection (Cheung, Y.-N., Maag, D., Mitchell, S. F., Fekete, C. A., Algire, M. A., Takacs, J. E., Shirokikh, N., Pestova, T., Lorsch, J. R., and Hinnebusch, A. (2007) Genes Dev. 21, 1217-1230 ). Interestingly, eIF1-KH includes the altered hydrophobic residues. Thus, eIF5 is an excellent candidate for the direct partner of eIF1-KH that mediates the critical link. The direct interaction at eIF1-KH also places eIF5 near the decoding site of the 40 S subunit.