Protein production is fundamental to life, translating instructions encoded in DNA into functional proteins. This process, known as gene expression, involves transcribing DNA into messenger RNA (mRNA) and then translating the mRNA into amino acid chains. In human cells, this translation requires the ribosome and accessory proteins called eukaryotic initiation factors (eIFs). These eIFs ensure that protein synthesis begins at the correct location and time, maintaining cellular function.
Defining Eukaryotic Initiation Factor 5
Eukaryotic initiation factor 5, or eIF5, is a single-unit protein encoded by the EIF5 gene and plays a specialized role in the initial assembly of the protein-making machinery. In mammals, this factor is a monomeric protein weighing approximately 49 kilodaltons (kDa). It is structurally organized into two major parts: an N-terminal domain and a C-terminal domain, which are linked by a flexible region.
The N-terminal domain is responsible for the factor’s primary mechanical action, while the C-terminal domain serves as a binding site that coordinates interactions with several other initiation factors. Its contextual placement is within the 43S pre-initiation complex, which is composed of the small 40S ribosomal subunit, the mRNA, and several other eIFs.
eIF5’s Central Role in Starting Protein Synthesis
The main function of eIF5 is to act as a GTPase-activating protein (GAP), which provides the irreversible signal that commits the cell to begin protein synthesis. This action targets a different factor, eIF2, which is bound to a high-energy molecule called Guanosine Triphosphate (GTP) and carries the first amino acid, methionine, to the ribosome. The complete assembly, known as the eIF2-GTP-methionyl-tRNA complex, must dock at the correct start codon (AUG) on the mRNA before translation can proceed.
Once the correct start codon is located, eIF5 physically interacts with the complex and stimulates eIF2 to break down its bound GTP into Guanosine Diphosphate (GDP) and an inorganic phosphate. This hydrolysis causes a major conformational change in the complex.
The resulting hydrolysis of GTP to GDP is the signal that triggers the release of all the initiation factors, including eIF5 itself, from the small ribosomal subunit. This factor release is a prerequisite for the large 60S ribosomal subunit to join the complex, forming the complete and active 80S ribosome. In addition to its GAP activity, eIF5 also plays a significant part in the accuracy of start codon selection, promoting the stringent recognition of the AUG codon over similar, incorrect sequences.
How eIF5 Activity is Controlled by the Cell
eIF5 activity is tightly managed to match the cell’s current needs and environmental conditions. Beyond its GAP function, eIF5 possesses a second, distinct activity: it acts as a GDP dissociation inhibitor (GDI) for eIF2. This GDI function involves binding to eIF2 when it is in its inactive, GDP-bound state, thereby preventing its recycling back to the active, GTP-bound form.
This dual functionality allows eIF5 to exert fine-tuned control over the initiation process, especially during times of cellular stress, such as nutrient deprivation or viral infection. Under these conditions, specific kinases phosphorylate the alpha subunit of eIF2, which strongly enhances the inhibitory GDI activity of eIF5.
Moreover, other factors like the protein kinase CK2 have been shown to phosphorylate eIF5, a modification linked to promoting cell proliferation and cell-cycle progression. This phosphorylation suggests that eIF5 acts as a regulated checkpoint, allowing the cell to quickly adjust global protein production rates in response to both internal and external signals.
eIF5’s Link to Human Health and Disease
Dysregulation of eIF5 is implicated in various human diseases due to its central role in protein synthesis initiation. Uncontrolled or excessive protein synthesis is a hallmark of many cancers, and initiation factors like eIF5 are frequently found to be overactive or mutated in these malignancies. For instance, a somatic mutation in the EIF5 gene has been documented in a breast cancer sample, pointing to a direct pathological role.
The regulatory link between eIF5 and eIF2 phosphorylation is also significant in diseases beyond cancer. The stress-responsive kinases that phosphorylate eIF2 alpha are involved in conditions like diabetes, viral defense, and neurodegeneration, including Alzheimer’s and Parkinson’s diseases. By modulating the state of eIF2, eIF5 indirectly influences the selective translation of specific mRNAs that encode proteins involved in survival or stress response pathways.
The importance of eIF5 in disease has made it an active subject of research as a potential therapeutic target. Inhibiting or restoring the balance of eIF5 activity could offer a strategy to selectively block the uncontrolled growth of cancer cells or mitigate the pathological effects of chronic cellular stress seen in neurological disorders.