The role of the initiation surveillance complex in promoting efficient protein synthesis

Initiation is most often the rate-limiting step of translation. Translation initiation requires the involvement of numerous factors that assist binding of the 40 S ribosomal subunit to an mRNA and the assembly of the 80 S ribosome at the correct initiation codon. Recruitment of an initiation surveillance complex is required for translation and serves to identify mRNAs that are structurally and functionally competent for translation. For most cellular mRNAs, recruitment of the surveillance complex requires the 5'-cap and 3'-poly(A) tail. However, some cellular and viral mRNAs that naturally lack either of these have evolved alternatives that serve to recruit the complex. The initiation surveillance complex functions to stabilize eIF4F (where eIF stands for eukaryotic initiation factor), the cap-binding complex, to the cap; promote eIF4A helicase activity to remove secondary structure in the 5'-leader that might otherwise reduce 40 S ribosomal subunit scanning; promote eIF4B binding to increase eIF4A/eIF4F function and stabilize binding of the poly(A)-binding protein to the poly(A) tail. The surveillance complex is regulated through changes in phosphorylation in response to environmental conditions or by developmental signals as a means to regulate globally protein synthesis. Thus the initiation surveillance complex ensures that only intact mRNAs are recruited for translation and serves to regulate protein synthesis.