Final answer:
ATP-dependent scanning of the initiation site in eukaryotic cells is mediated by various eukaryotic initiation factors (eIFs). The 40S ribosomal subunit, in coordination with eIFs like eIF1, eIF1A, and a GTP-bound eIF2 ternary complex, scans the mRNA to locate the AUG start codon. Additional eIFs, including the eIF4F complex, assist in the regulation of the scanning process.
Step-by-step explanation:
The initiation factors responsible for ATP-dependent scanning of the initiation site in eukaryotic cells are mainly comprised of the eukaryotic initiation factors (eIFs). The key sequence of events in forming an initiation complex involves multiple initiation factors. Initially, the 40S ribosomal subunit associates with eukaryotic initiation factor 1 (eIF1), eIF1A, and the ternary complex comprised of eukaryotic initiation factor 2 (eIF2) in its GTP-bound form, Met-tRNAi, and GTP. This complex scans the mRNA to find the start codon AUG.
Once the start codon is located, additional eIFs (eIF3, eIF4F complex, and eIF5) play roles in installing the scanning process, hydrolyzing GTP, and facilitating the joining of the large 60S ribosomal subunit, thereby completing the assembly of the 80S initiation complex ready for protein synthesis. The scanning mechanism is crucial for the correct identification of the start codon and is tightly regulated. For example, in the ATP-dependent mechanism, eIF4F complex (containing eIF4A, eIF4E, and eIF4G) uses ATP hydrolysis to unwind secondary structures in the 5' untranslated region (5'-UTR), allowing the 40S subunit to move forward.