Protein Translation

Author: Shenzhou
Reviewed by: Shenzhou and Myelin

  Proteins are the principal functional molecules responsible for the activities of life, and their biosynthesis is known as translation. Translation is the process in which mRNA serves as a template and, with the aid of a ribosome, tRNA converts the nucleotide sequence on the mRNA into an amino acid sequence to synthesize a polypeptide chain. The main participants are tRNA, rRNA, and mRNA. tRNA carries and transfers amino acids. rRNA combines with several ribosomal proteins to form the ribosome, where polypeptides are translated. mRNA acts as the messenger carrying codons and provides the template for translation.

mRNA

  Jacob and Monod proposed the concept of messenger RNA in 1961. They reasoned that because proteins are synthesized in the cytoplasm rather than the nucleus, some substance must carry genetic information out of the nucleus. They designed several experiments to demonstrate that such a substance existed, including experiments on the regulation of lactose metabolism in E. coli, bacterial conjugation experiments, and density-gradient centrifugation.
  Their experiments on the regulation of lactose metabolism in E. coli first showed that β-galactosidase is an “inducible enzyme” expressed only in the presence of lactose. Bacterial conjugation experiments then showed that the information directing synthesis of this enzyme was not directly associated with rRNA but was conveyed by another, short-lived substance. Finally, density-gradient centrifugation demonstrated the existence of mRNA while distinguishing it from rRNA and showing that rRNA is a stable nucleic-acid molecule. In other words, rRNA reads the code during translation, while mRNA controls which protein is synthesized.
  In prokaryotes, mRNA is transcribed directly from DNA in the cytoplasm and translated into protein at the same time. Prokaryotic mRNA also has a short lifespan of only a few minutes. Eukaryotic mRNA is synthesized in the nucleus and spliced into mature mRNA before entering the cytoplasm as a template for protein translation. Unlike prokaryotic mRNA, eukaryotic mRNA is more stable and can persist for several hours.
1
  In prokaryotes, every cistron (p1) has its own ribosome-binding site, the -AGGAGGU- sequence known as the Shine–Dalgarno sequence (SD sequence), upstream of its start codon. A single prokaryotic mRNA can therefore be used to translate several proteins.
2 3
  In eukaryotes (p2), by contrast, only the first AUG on an mRNA has an upstream CCACC signal sequence that can be recognized during ribosomal scanning. A eukaryotic transcript can therefore contain only one cistron.

tRNA

  During protein translation, tRNA serves as the bridge between mRNA and amino acids. It ensures that amino acids in the polypeptide are assembled in the order specified by the codons on the mRNA.
  tRNA structure differs little among organisms and shows a high degree of similarity and homology. With a few exceptions, such as tRNAs in vertebrate mitochondria, all tRNAs have a cloverleaf secondary structure that underpins their function. tRNA has the following structures for carrying out its related functions:
  • A conserved CCU sequence at the 3’ end that serves as the amino-acid acceptor arm
  • The D loop, which binds the corresponding aminoacyl-tRNA synthetase to load the matching amino acid
  • The anticodon loop, which contains the anticodon at positions 34, 35, and 36 and pairs with a codon during translation, enabling codon recognition
  • The TΨC loop, the main site that binds the ribosome to form a stable translation apparatus

The secondary structure is shown below:

4
  The cloverleaf is only tRNA's secondary structure; its higher-order spatial structure takes the form of an inverted “L.” As this inverted-L structure forms, two tRNA loops—the D loop and TΨC loop—pair their bases as the molecule folds in space. The two diagrams below show the way it folds.
5 6
  Continuous base pairing in tRNA creates a degree of base-stacking force, which helps keep its spatial structure relatively stable.

rRNA

  The ribosome is the principal site of protein synthesis. It is an abundant and structurally complex RNA–protein complex within the cell. E. coli contains roughly 20,000 ribosomes, and the average eukaryotic cell contains still more. Ribosomes can occur freely in the cytoplasm; within eukaryotic cells, they can also bind to the rough endoplasmic reticulum.
  Prokaryotic ribosomes are smaller and eukaryotic ribosomes slightly larger, but both consist of two subunits of different sizes, each made up of several rRNAs and proteins. The E. coli ribosome is a 70S ribosome composed of a 30S small subunit, containing 16S rRNA and 21 proteins, and a 50S large subunit, containing 23S and 5S rRNA and 34 proteins. The eukaryotic 80S ribosome consists of a 40S small subunit, containing 33 proteins and 18S rRNA, and a 60S large subunit, containing 50 proteins and 28S, 5.8S, and 5S rRNA.
  The large ribosomal subunits of both eukaryotes and prokaryotes contain 5S rRNA. Through hydrogen-bond base pairing with the TΨC loop in tRNA, this component stabilizes the binding of aminoacyl-tRNA to the ribosome.
  Prokaryotic 16S rRNA and eukaryotic 18S rRNA are highly similar, but their structures differ to some extent because the two recognize start codons through different mechanisms.
  A complete ribosome has eight important structural domains and functional sites (p7). In general, however, it performs two main functions. One is to coordinate tRNA, mRNA, and the protein being synthesized, ensuring that all three occupy the correct positions relative to one another during synthesis. The other is to catalyze several key chemical reactions in translation, allowing the process to proceed smoothly until termination.
  5
  A complete ribosome is assembled temporarily from the large and small subunits inside the cell when translation begins. Once synthesis is complete, it dissociates again and the subunits return to the cytoplasm for use in the next round of synthesis.