Translation is the process whereby genetic information is interpreted to assemble the protein molecules it codes for. Translation is fundamental to life and is largely controlled during the initiation phase. Eukaryotic initiation factor 3 (eIF3) is required for life, translation initiation, and the regulation of gene expression at the translation stage.

All organisms depend on the transformation of genetic information into molecules capable of performing the molecular task of life. The ribosome is an ancient macromolecular machine that translates messenger-RNA (mRNA) copies of individual genes and assembles the protein molecules for which each gene is a recipe and which are largely responsible for performing molecular actions within living cells.

Initiation is the rate-limiting and most regulated step of translation, and misregulation of translation initiation is tied to myriad human diseases, including cancer, neurodegenerative disease, and viral infections. Because initiation dedicates the ribosome to making a specific protein, it also provides a decision point that enables cells to respond to external stimuli or choose a specific developmental fate.

eIF3 is one of a host of protein initiation factors (eIFs) that orchestrate the process of translation initiation. eIF3 is the largest of these eIFs and is, in fact, not one protein but a multi-subunit complex of at least 5 different proteins in simpler eukaryotic organisms (such as budding yeast) and up to 12 different proteins in more complex eukaryotic organisms (such as humans). Cells cannot survive without eIF3 and it is required to deliver a specific mRNA molecule to the ribosome during initiation, a process known as mRNA recruitment.

Initiation sets the reading frame for messenger RNA (mRNA) translation and determines which mRNAs are translated into proteins at any given moment. Initiation is the rate-limiting and most highly regulated phase of translation and its misregulation is a causative factor in human cancers. Altered expression or regulation of a variety of eukaryotic initiation factors (eIFs) has been implicated in the development and progression of numerous human cancers, promoting proliferation, tumor survival, angiogenesis, and metastasis. In eukaryotes, initiation follows a multi-step pathway that begins with the formation of a ribosomal pre-initiation complex (PIC), in which the methionylated initiator tRNA (Met-tRNAi) and several eIFs are loaded onto the small ribosomal (40S) subunit. The PIC initially attaches to the 5’ end of the mRNA and then scans the mRNA in the 3’ direction until it locates the start (AUG) codon. The 5’-untranslated region (UTR) through which the PIC scans can be in excess of 1000 nucleotides in length and contain structured elements which the PIC must somehow resolve to allow for efficient scanning. Upstream open reading frames (uORFs) whose translation regulates the translation of the main open reading frame (mORF) are also found within the 5’UTR of many mRNAs. Point mutations in the 5′-UTRs of the c-Myc, BRCA1, and CDKN2A mRNAs have been tied to both tumor initiation and maintenance. Both initial mRNA attachment and scanning are thought to be facilitated by an ‘open’ PIC conformation in which the mRNA-entry channel (mEnC) of the 40S subunit is widened. Start-codon recognition elicits a conformational change to a ‘closed’ PIC, which constricts this channel and is thought to halt scanning. Collectively, the processes of initial attachment, scanning, and start-codon recognition constitute mRNA recruitment.

Consistent with the role translational misregulation plays in tumor development and progression, eIF3 plays a causative role in cancer. Altered expression of each of the five core subunits—eIF3a, eIF3b, eIF3c, eIF3i, and eIF3g—provokes development or progression of a variety of human cancers and eIF3a, eIF3b, and eIF3c have been identified as therapeutic targets. In addition, eIF3 appears to interact with or be targeted by a host of players in translational regulation and has been tied to the regulation of cell proliferation, stress response, and other pathways. These studies implicate eIF3 subunits within both the mExC and the mEnC arms, including eIF3a, which contributes to and connects the mEnC and mExC arms. In fact, eIF3 appears to bind directly to a number of specific mRNAs to drive their translation, including mRNAs involved in cellular proliferation, development, and other processes. Subunits of the eIF3 mEnC arm bind a structured element in the 5’-UTR of the proliferation regulator c-Jun. Genome-scale approaches have begun to uncover possible connections between the roles of eIF3 in translation, its regulation, and cancer. However, a framework connecting the mechanistic roles of eIF3 with its roles in regulation and human disease does not yet exist.

Given the role of the eIF3 mEnC and mExC arms during initiation and the involvement of their component subunits in translational regulation and cancer, how they contribute to the events of mRNA recruitment, independently and in concert, and how these mechanistic roles are connected to the translation of mRNAs across the transcriptome represent central questions in our understanding of translation initiation and cancer.