Which initiation factor blocks A-site in eukaryotes?
During eukaryotic initiation, preventing premature tRNA entry into the A-site is essential for fidelity and maintaining P-site initiation accuracy. eIF1A, the conserved ortholog of bacterial IF1 sharing OB-fold domain and binding site, binds near the ribosomal A-site on the small 40S subunit, stabilizing an open, scanning-competent conformation and physically blocking the A-site pocket from non-initiator tRNAs. This obstruction ensures only initiator Met-tRNAi occupies the P-site during scanning phases. Upon AUG recognition, eIF1 and eIF1A rearrange and dissociate, vacating A-site for elongation factors like eEF1A to deliver the next aminoacyl-tRNA productively.
Ref: NCBI Structure 1HR0 IF1 blocks A-site; Alberts Fig 6-72 - eIF1A occupies A-site to prevent premature tRNA binding