Protein synthesis is a fundamental biological process, enabling cells to create the diverse proteins necessary for their structure and function. These proteins act as cellular machinery, performing roles from catalyzing reactions to providing structural support. This process, known as translation, converts genetic information encoded in messenger RNA (mRNA) into a specific sequence of amino acids, the building blocks of proteins. This conversion is important for the cell’s ability to develop, maintain, and respond to its environment.
The Broader Context of Translation
Translation is the second major step in gene expression, following transcription, where DNA’s genetic instructions are copied into an mRNA molecule. This mRNA then travels to ribosomes, cellular structures composed of ribosomal RNA (rRNA) and proteins. The overall process of translation unfolds in three distinct phases: initiation, elongation, and termination.
Initiation marks the beginning, where the ribosome assembles around the mRNA and the first transfer RNA (tRNA) carrying an amino acid binds to the start codon. Following initiation, the elongation phase commences, during which amino acids are sequentially added to the growing protein chain. This involves reading mRNA codons and delivering corresponding amino acids by tRNAs.
The final phase, termination, occurs when the ribosome encounters a stop codon on the mRNA, signaling the end of protein synthesis and the release of the newly formed protein. Translocation is a key movement that occurs repeatedly during the elongation phase, advancing the process one codon at a time.
What Translocation Is
Translocation is a precise, coordinated movement of the ribosome along the mRNA molecule during protein synthesis. After a peptide bond forms between the amino acid on the tRNA in the A site and the growing polypeptide chain in the P site, the ribosome is in a pre-translocational state. To allow for the addition of the next amino acid, the tRNAs and the mRNA must shift their positions within the ribosome.
During translocation, the peptidyl-tRNA, carrying the elongated protein chain, moves from the A (aminoacyl) site to the P (peptidyl) site. Simultaneously, the deacylated tRNA moves from the P site to the E (exit) site. As these tRNAs shift, the mRNA molecule also advances by exactly one codon. This synchronized movement ensures that the next codon on the mRNA is correctly positioned in the empty A site, ready to receive the next incoming aminoacyl-tRNA.
Components Driving Translocation
The ribosome, composed of a large and small subunit, features three distinct tRNA binding sites: the A site, P site, and E site. The A site serves as the entry point for incoming tRNAs carrying new amino acids, while the P site holds the tRNA attached to the growing polypeptide chain. The E site is where deacylated tRNAs briefly reside before exiting the ribosome.
Elongation factors are proteins that drive translocation. In bacteria, elongation factor G (EF-G) is the primary factor, while in eukaryotes, eukaryotic elongation factor 2 (eEF2) performs a similar function. These factors bind to the ribosome and, through the hydrolysis of guanosine triphosphate (GTP), provide the energy required for the ribosome to advance along the mRNA.
Why Translocation Matters
Accurate and efficient translocation is important for the proper functioning of cells, impacting the quality and integrity of synthesized proteins. This process ensures the ribosome maintains the correct “reading frame” of the mRNA molecule. The reading frame dictates how the mRNA sequence is divided into codons, which are then translated into amino acids.
Errors in translocation can have consequences for protein synthesis and cellular health. If the ribosome shifts incorrectly, it can lead to a “frameshift mutation,” where the reading frame is altered. This results in the ribosome reading the wrong codons, leading to a different and often non-functional protein sequence. Such errors can also cause the production of truncated proteins or misfolded proteins that cannot achieve their correct three-dimensional structure, potentially impairing cellular function or becoming toxic.