TranslationStep 2 of Protein Synthesis

Transfer RNA: takes amino acids from the cytoplasm to a ribosome

Each tRNA carries a specific amino acid at one end and has an anticodon on the other end.

Anticodon: a nucleotide triplet which base-pairs with a complementary codon on mRNA.

tRNA is a translator because it reads a nucleic acid word (mRNA codon) and interprets it as a protein word (amino acid).

Molecular Components

The structure and function of tRNA:◦ Job – to take a specific amino acid to the ribosome,

then go pick up another from the cytosol◦ Looks like a cloverleaf if flattened into a 2D structure

due to the H-bonding between bases◦ Its 3D shape is roughly an “L.”◦ An enzyme called aminoacyl-tRNA synthetase

attaches a given amino acid to the appropriate tRNA. There is a different synthetase for each amino acid.

◦ tRNAs can sometimes match multiple codons because of the third base pair not needing to match exactly (“wobble”)

Molecular Components

Ribosomes◦ Facilitate the coupling of tRNA anticodons with

mRNA codons during protein synthesis◦ 2 subunits: large and small◦ Made of proteins and rRNA (ribosomal RNA)◦ Cells contain thousands of ribosomes, so rRNA is

the most abundant type of RNA.◦ Since prokaryotic ribosomes are different from

eukaryotic ribosomes, antibiotic drugs are able to inactivate them without inhibiting the ability of eukaryotic cells to make proteins.

Molecular Components

Ribosomes◦ Each has 3 binding sites:◦ P site: (peptidyl-tRNA site) holds the tRNA

carrying the growing polypeptide chain◦ A site: (aminoacyl-tRNA site) holds the tRNA

carrying the next amino acid to be added to the chain

◦ E site: (exit site) where discharged tRNAs leave the ribosome

◦ The ribosome forms peptide bonds between the amino acids.

Molecular Components

3 stages: Initiation – The small ribosomal unit binds to an

initiator tRNA (carrying Methionine) and mRNA, and then finds the start codon (AUG);

Next, proteins called initiation factors bring this complex and the large ribosomal unit together;

Then, the initiator tRNA is in the P site.

Building a Polypeptide

Elongation◦ Involves proteins called elongation factors◦ Uses energy from GTP (like ATP)◦ mRNA is moved through the ribosome in the 5’ to

3’ direction only◦ Many ribosomes may trail along one mRNA at a

time (polyribosomes)

Building a Polypeptide

Termination◦ Stop codon reaches the A site (triplets do not

code for amino acids)◦ A protein called a release factor binds to the stop

codon and causes the addition of a water molecule instead of an amino acid

◦ The polypeptide is released◦ Translation assembly comes apart

Building a Polypeptide

Protein Folding and Post-translational modifications:◦ A protein may begin to coil and fold

spontaneously.◦ Chaperone proteins (“chaperonins”) usually

help the polypeptide fold correctly.◦ Examples of post-translational modifications

before the protein can do its job… attachment of sugars, lipids, phosphate groups; removal of amino acids from the leading end; enzymatic cleaving into 2 or more pieces

Completing and Targeting the Functional Protein

Targeting Polypeptides to Specific Locations:◦ 2 kinds of ribosomes –◦ Free: suspended in cytosol and mostly synthesize

proteins that dissolve in the cytosol and function there

◦ Bound: attached to the cytosolic side of the endoplasmic reticulum or nuclear envelope; make proteins of the endomembrane system as well as proteins secreted from the cell

Completing and Targeting the Functional Protein

Targeting Polypeptides to Specific Locations:◦ The above ribosomes are identical and can switch

their status from free to bound.◦ Polypeptide synthesis always starts in the cytosol

with a free ribosome.◦ If a protein that are destined for the

endomembrane system are marked by a signal peptide.

◦ As it emerges from the ribosome, a signal-recognition particle (SRP) recognizes it and brings the ribosome to a receptor protein built into the ER.

Completing and Targeting the Functional Protein