Proteins are an integral part of a living body; they give structure to the cells and are involved in various biochemical reactions. Protein biosynthesis is carried out through a process called translation. Following the rule of the central dogma, DNA undergoes the process of transcription to produce RNA and mRNA, with the help of transfer RNA (tRNA) and ribosomes, produces proteins.
Protein Translation. Image Credit: Designua/Shutterstock.com
A protein comprises 20 different amino acids, which are linked together by peptide bonds. A particular protein consists of a specific combination and sequence of amino acids. As mentioned earlier, in the process of biosynthesis of protein, ribosomal RNA (rRNA), tRNA and mRNA play a vital role. Additionally, in the case of eukaryotic organisms, two other types of RNA, namely, heterogeneous nuclear RNA (hnRNA) and small nuclear RNA (snRNA), are also involved.
Role of Ribosomes in Protein Biosynthesis
Ribosomes are specialized cellular structures where protein synthesis takes place. Each cell contains many ribosomes and their number depends upon the amount of protein synthesis performed by the cell. An electron micrograph of cells shows that they exist either in a cluster (polyribosomes) or are scattered. They consist of two subunits, namely, a large subunit and a small subunit.
In a ribosome, only a few rRNA molecules are present and are directly involved in the catalytic steps of protein synthesis. Owing to their function, i.e., binding of amino acids for the development of protein molecules, rRNA is also known as ribozyme or catalytic RNA. They make up about half of the ribosomal mass and the remaining mass consists of several proteins.
Messenger RNA (mRNA)
The mRNA is a single-stranded sequence and the order of the bases are- A, U, C, and G. This order is complementary to a specific section of cellular DNA. mRNA contains the information that dictates the sequence of amino acids to be linked to form a protein. It consists of introns and exons, i.e., nucleotide sequences within a gene.
The structure of mRNA shows the presence of poly-A tail, i.e., consisting of several adenine bases, at one end, and the other end comprises a guanosine triphosphate cap. Translation occurs in the cytoplasm. mRNA leaves the nucleus and undergoes modifications before being translated.
In other words, the introns that do not code for amino acids are excised by RNA splicing. Thus, the mature mRNA consists of the exons that code for specific proteins.
Transfer RNA (tRNA)
tRNA plays a huge role in protein synthesis. It is shaped like a cloverleaf with three loops. tRNA comprises an amino acid binding site and an anticodon site. The anticodon site recognizes a specific codon present in mRNA. The main function of tRNA is to translate the message within the nucleotide sequence of mRNA to a specific amino acid sequence. During translation, the tRNAs carry amino acids to the ribosome and join with their complementary codons. These amino acid sequences are joined together to form a protein.
Basic Mechanism of Protein Biosynthesis-Translation
After the completion of the mRNA modifications, the process of translation takes place. The modified mRNA gets attached to a specific site on a ribosome. Each ribosomal subunit has three binding sites for tRNA. These are as follows:
- An (aminoacyl) site - receives the aminoacylated-tRNA
- P (peptidyl) site - holds the peptidyl-tRNA with the nascent peptide chain and allows the growth of the polypeptide chain
- E (exit) site - holds the deacylated tRNA before it leaves the ribosome.
Sometimes, several ribosomes (polysome) may simultaneously translate a single mRNA sequence. All these ribosomes must start at the first codon and move along the mRNA strand one at a time till they reach the stop codon. Polysome results in the formation of multiple amino acids from a single mRNA. The process of translation can be divided into three phases, a) initiation, b) elongation and c) termination. These are discussed below.
Initiation
Initiation is dependent on several factors known as initiation factors IF1, IF2, and IF3. This phase of the translation process involves the attachment of a small ribosomal subunit to an mRNA molecule. This is instantly recognized by an initiator tRNA molecule that leads to the binding to a specific codon sequence (AUG) on the same mRNA molecule. Subsequently, a large ribosomal subunit attaches to the newly formed complex consisting of Met-tRNA, mRNA, and the small subunit. This step completes the initiation phase.
Elongation
This phase begins when the ribosome starts moving to the next codon on the mRNA. At this stage, the corresponding tRNA binds to this codon and, hence, for a short time, two tRNA molecules remain attached to the mRNA strand. The amino acids carried by these tRNA molecules are then bound together. In the next step, the ribosome shifts, and the first tRNA gets released.
Likewise, the third codon in the mRNA strand gets ready to be bound with the specific tRNA. Thereby, the tRNA binds to the mRNA strand, the third amino acid is added to the series, the ribosome shifts, and the second tRNA (which no longer carries an amino acid) is released. This process is repeated throughout the length of the mRNA which leads to the elongation of the polypeptide chain. The newly formed polypeptide emerges out from the top of the ribosome.
Termination
The process of elongation continues till it reaches a termination codon on the mRNA. After the ribosome reaches the termination codon, it detaches from the mRNA and the growing polypeptide chain gets released.
Post-translational Modification
Many a time, the newly formed polypeptide chain undergoes moderate to extensive post-translational modifications before becoming a fully functioning protein. The process is known as proteolysis where a section of amino acid gets removes to make the protein fully functional.
The efficiency of protein biosynthesis is determined by several factors such as tRNA abundance, amino acid sequence, mRNA sequence, and structure. Further, the codon distribution at the N-terminus of mRNA also plays a role in the efficiency of protein biosynthesis.
Sources
- Verma, M., Choi, J., Cottrell, K.A. et al. (2019). A short translational ramp determines the efficiency of protein synthesis. Nature Communication, 10, pp. 5774. https://doi.org/10.1038/s41467-019-13810-1
- Riba, A., et al. (2019). Protein synthesis rates and ribosome occupancies reveal determinants of translation elongation rates. Proceedings of the National Academy of Sciences, 116 (30), pp. 15023-15032; DOI: 10.1073/pnas.1817299116
- Griffiths AJF, et al. (2000). An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman. Protein synthesis. Available from: https://www.ncbi.nlm.nih.gov/
Further Reading