Lesson 10: Ribosomes

Ribosomes

function as molecular machines that synthesize proteins (ribosomal proteins + ribosomal RNA)

ribosome importance

• Found in all living cells (bacteria, archaea, eukaryotes)

• Ribozyme (catalytic RNA) - evolutionary ancient 

• Most abundant RNA-protein complex in cells

• Energetically expensive (up to 40% of cellular energy in growing cells)

location 

• Attached to rough ER (including outer nuclear membrane) 

• Free in cytosol

structure

• Two subunits (large and small)

large subunit

• (60S) includes sequences 5S + 28S + 5.8S of rRNA

  • 49 proteins and 3 molecules of RNA

  • protein production

small subunit

• (40S) includes 18S of rRNA 

  • 33 proteins and 1 molecule of RNA

  • decoding

Polyribosomes/polysomes

• multiple ribosomes attached to the same mRNA

  • More efficient → make more proteins in the same amount of time

“Christmas tree” structure

•  trunk is DNA, branches are mRNA molecules being made and beads of the branches are ribosomes  

• visual appearance of transcribing DNA  

prokaryotic DNA transcription

Transcription and translation happen simultaneously → efficient but leads to more errors and mutations (eukaryotes make sure only a mature mRNA make it out of the nucleus, this doesn't occur here)

• In prokaryotes → genetic information is in cytosol (occurs here)

• DNA is being transcribed into mRNA, mRNA is attached to ribosomes that aid in protein synthesis

• Christmas tree structure

eukaryotic DNA transcription

• polyribosomes → only occurs on cytosol (where mRNA is)

  • Can decide when to translate something

transcription steps

• strand of mRNA complementary to DNA strand (gene)

• initiation, elongation and termination 

transcription initiation

• enzyme RNA polymerase binds to promotor of gene

• signals DNA to unwind

transcription elongation

• enzyme makes complementary mRNA strand 

• addition of nucleotides to the mRNA strand

transcription termination 

• RNA polymerase transcribes the terminator and detaches from the DNA

mRNA processing

• splicing removes introns from mRNA (don’t code for protein)

• addition of poly-A tail

• 5’ cap protects mRNA in cytoplasm 

free polyribosomes

• Proteins of cytosol and cytoskeleton  

• Specific proteins are imported to mitochondria, peroxisomes and nucleus

ER-bound polyribosomes

• Proteins go from ER to golgi apparatus for processing and sorting 

• Then go to secretory vesicles (proteins secreted from cell or proteins of cell membrane) or lysosomes (degradation)

misfolded and denatured proteins

• conjugated to ubiquitin → protein degradation by proteasome 

protein synthesis: translation steps

• initiation

• elongation

• termination 

translation initiation

• small subunit of ribosomes binds to 5’ side on start codon of mRNA

• large subunit attaches and initiator tRNA (w/Met) binds to P site

translation elongation 

• ribosome shifts one codon at a time

• a charged tRNA enters complex, polypeptide becomes one amino acid longer and uncharged tRNA leaves 

• tRNAs start in P site, move to A, new one enters E (P, PA, EP)

translation termination

• STOP codon is encountered 

• growing polypeptide is released

• ribosome dissociates 

differences in translation between prokaryotes and eukaryotes 

prokaryotes: 

• occurs in cytoplasm

• simultaneous

• minimal mRNA processing (none)

• fast (seconds)

• mRNA stability is short (2-5 min)

• simple and efficient 

eukaryotes:

• separate locations; nucleus + cytoplasm

• sequential timing 

• extensive mRNA processing (capping, splicing poly-A)

• slow (minutes to hours)

• long mRNA stability (hours to days)

• multiple checkpoints

• complex + versatile 

protein trafficking

route is determined by signal sequences

2 kinds

• post-translational import

• co-translational import

co-translational import 

•  a protein is transported into an organelle while it is still being synthesized by the ribosome 

Steps

1. As the ribosome translates mRNA, an N-terminal signal sequence emerges from the ribosome

2. The signal recognition particle (SRP) recognizes and binds to this signal sequence → temporarily pauses translation

3. The SRP-ribosome complex docks at the SRP receptor on the ER membrane

4. The ribosome attaches to a translocon (protein channel) in the ER membrane

5. Translation resumes and the growing polypeptide chain is threaded directly through the translocon into the ER lumen as it’s being made

6. The signal sequence is typically cleaved off by signal peptidase once inside

post-translational import

when a protein is fully synthesized in the cytosol then transported into an organelle afterward

Steps

1. The complete protein is made by free ribosomes in the cytosol

2. Chaperone proteins (ex: Hsp70) keep the protein unfolded to allow it to pass through membrane channels

3. Specific targeting sequences direct the protein to the correct organelle

4. Specialized translocons in the organelle membrane facilitate import 

5. Process requires energy (ATP or membrane potential)

6. Targeting sequences may be cleaved after import