Elongation Factor 2 (eEF2) is a protein found within eukaryotic cells, including human cells. It plays a fundamental role in protein synthesis, the process by which cells build proteins for their structure and function. eEF2 facilitates a specific step in creating these proteins.
Function in Protein Translation
Protein synthesis occurs on ribosomes, complex molecular machines that read genetic instructions from messenger RNA (mRNA) and assemble amino acids into protein chains. The ribosome contains distinct functional sites: the A (aminoacyl), P (peptidyl), and E (exit) sites. During the elongation phase, the ribosome moves along the mRNA template, reading each three-nucleotide segment, known as a codon, to determine the next amino acid to add.
eEF2 has a specific role in this process, helping to move the ribosome along the mRNA. After a new amino acid is added to the growing protein chain at the A-site, the transfer RNA (tRNA) molecules are in a specific configuration. eEF2 then catalyzes the movement of these tRNA molecules and the mRNA molecule through the ribosome. This movement is called translocation.
Translocation shifts the tRNA with the growing protein chain from the A-site to the P-site, and the now empty tRNA from the P-site to the E-site, where it exits the ribosome. This precise shift ensures that the next codon on the mRNA is correctly positioned in the A-site, ready for the arrival of a new aminoacyl-tRNA. Without eEF2, this crucial step of ribosomal movement would not occur efficiently, halting protein synthesis.
Mechanism of Ribosomal Translocation
eEF2 drives ribosomal translocation through precise molecular interactions powered by Guanosine Triphosphate (GTP). eEF2 is a type of protein known as a GTPase, meaning it binds and hydrolyzes GTP to release energy. This energy drives conformational changes within eEF2 and the ribosome, enabling physical movement.
The process begins with eEF2, bound to a GTP molecule, associating with the ribosome after a new peptide bond has formed. Upon binding, eEF2 induces a conformational change within the ribosome, causing a “ratcheting” motion of the small ribosomal subunit. This prepares the ribosome for the subsequent shift of tRNAs and mRNA.
Following this initial binding, the GTP molecule bound to eEF2 undergoes hydrolysis, breaking down into Guanosine Diphosphate (GDP) and an inorganic phosphate. This hydrolysis provides the necessary energy for further conformational changes within eEF2, which then physically pushes the mRNA-tRNA complex. This moves the complex by exactly one codon, aligning the next codon in the A-site.
Once translocation is complete, eEF2, bound to GDP, has a lower affinity for the ribosome and releases. This dissociation allows the ribosome to reset for the next round of amino acid delivery and peptide bond formation. The structure of eEF2 mimics a tRNA molecule, enabling it to fit into the ribosome’s factor-binding site and facilitate this movement.
Cellular Regulation of eEF2
Cells carefully manage their resources, and protein synthesis is a highly energy-intensive process. Cells possess intricate mechanisms to control the activity of eEF2, adjusting protein production based on cellular needs and environmental conditions. A primary method for regulating eEF2 is through a reversible chemical modification called phosphorylation.
Phosphorylation involves the attachment of a phosphate group to a specific amino acid residue on the eEF2 protein. This modification is carried out by eEF2 kinase (eEF2K). When eEF2K adds a phosphate group to eEF2, specifically at threonine 56 (T56), it prevents eEF2 from properly interacting with the ribosome.
This phosphorylation effectively “turns off” eEF2, pausing or slowing down the elongation phase of protein synthesis. This mechanism allows the cell to conserve energy and amino acids, particularly under stressful conditions such as nutrient deprivation or low energy availability. The activity of eEF2K is tightly regulated by various cellular signaling pathways, including those sensitive to calcium levels and nutrient status.
Medical and Disease Implications
The proper functioning of eEF2 is important for cellular health, and disruptions to its activity can have serious medical consequences. A well-known example of eEF2’s involvement in disease is its targeting by diphtheria toxin. This toxin, produced by the bacterium Corynebacterium diphtheriae, inactivates eEF2.
Diphtheria toxin achieves this by attaching an ADP-ribose molecule to a unique modified histidine residue on eEF2 called diphthamide. This modification, ADP-ribosylation, irreversibly inactivates eEF2, causing a complete shutdown of protein synthesis in affected cells. The cessation of protein production leads to cell death, which underlies the symptoms of diphtheria, including tissue damage in the throat and heart issues.
Beyond diphtheria, eEF2 and its kinase, eEF2K, are also areas of interest in cancer research. Cancer cells exhibit rapid growth and division, which necessitates high rates of protein synthesis. In some cancers, eEF2K is found to be upregulated, suggesting its role in promoting tumor cell survival under challenging conditions like nutrient scarcity. Inhibiting eEF2K, and thus eEF2 activity, is being explored as a potential therapeutic strategy to slow tumor growth by reducing protein production in cancer cells.
Emerging research links the regulation of eEF2, particularly through eEF2K, to neurological function. The eEF2K/eEF2 pathway has a role in processes like learning and memory. Abnormal regulation of eEF2 phosphorylation has been observed in some neurological disorders, including Alzheimer’s disease models, indicating that targeting this pathway could be a potential avenue for therapeutic intervention.