How do messenger RNAs direct protein synthesis ? The cellular factory
responsible for synthesizing proteins is the ribosome. The
ribosome consists of structural RNA and about 80 different proteins.
In its inactive state, it exists as two subunits; a large
subunit and a small subunit. When the small subunit
encounters an mRNA, the process of translation of the mRNA to
protein begins. There are two sites in the large subunit, for
subsequent amino acids to bind to and thus be close enough to each
other for the formation of a peptide bond. The A site accepts a new
tRNA bearing an amino acid, and the P site bears the tRNA attached to
the growing chain. Here is a schematic diagram of the ribosome:
Charging the tRNA
As discussed previously, the adaptor molecule that acts as a
translator between mRNA and protein is a specific RNA molecule, tRNA
(transfer RNA). Each tRNA has a specific anticodon and acceptor site.
Each tRNA also has a specific charger protein; this protein can only
bind to that particular tRNA and attach the correct amino acid to the
acceptor site. The energy to make this bond comes from ATP. These charger proteins are called aminoacyl tRNA synthetases
Initiation of Translation
The start signal for translation is the codon ATG, which codes
for methionine. Not every protein necessarily starts with methionine,
however. Often this first amino acid will be removed in
post-translational processing of the protein. A tRNA charged with
methionine binds to the translation start signal. The large subunit
binds to the mRNA and the small subunit, and elongation begins.
Elongation of the New Protein
After the first charged tRNA appears in the A site, the ribosome
shifts so that the tRNA is in the P site. New charged tRNAs,
corresponding the codons of the mRNA, enter the A site, and a peptide
bond is formed between the two amino acids. The first tRNA is now
released and the ribosome shifts again so that a tRNA carrying two
amino acids is now in the P site, and a new charged tRNA can bind to
the A site. This process of elongation continues until the ribosome
reaches a stop codon.
Termination of the Protein
When the ribosome reaches a stop codon, no aminoacyl tRNA binds to the
empty A site. This is the ribosomes signal to break into its large and
small subunits, releasing the new protein and the mRNA.
This isn't always the end of the story for the new protein. Often it will undergo post-translational modifications. It might:
Be cleaved by a proteolytic (protein-cutting) enzyme at a specific place.
Have some of its amino acids altered. For example, a tyrosine residue might become phosphorylated.
Be glycosylated. Many proteins have carbohydrates covalently attached to asparagine residues.
Discussion about where and how these processes occur can be found in the Cell Biology chapter of the hypertext.