Ribosomes and Protein Export
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You should understand the basic structure of the endomembrane system, including the rough and smooth endoplasmic reticulum, and be familiar with the central dogma of molecular biology, specifically how DNA is transcribed into mRNA.
Why this matters
Protein synthesis is the primary mechanism by which cells execute their genetic instructions to maintain structure and function.
Ribosomes are the molecular machines responsible for translating messenger RNA (mRNA) into polypeptide chains. However, the location of this translation is not random; it is tightly regulated to ensure that proteins are directed to their correct cellular destinations. Whether a protein is destined for the cytosol, the nucleus, or the secretory pathway, the cell employs specific targeting signals and membrane-bound machinery to sort these molecules efficiently. This lecture explores the structural duality of ribosomes and the precise mechanisms governing protein export from the cell.
Ribosomal Architecture and Translation
Ribosomes are complex ribonucleoprotein particles composed of ribosomal RNA (rRNA) and numerous ribosomal proteins. They exist in two primary states: free in the cytosol or bound to the cytosolic face of the endoplasmic reticulum (ER). Despite their different locations, both types of ribosomes are structurally identical and perform the same fundamental task: decoding the genetic information carried by mRNA into a sequence of amino acids.
The ribosome consists of two subunits: a large subunit and a small subunit. In eukaryotic cells, these are designated as the 60S and 40S subunits, respectively, which assemble to form the complete 80S ribosome. The "S" stands for Svedberg units, a measure of sedimentation rate during ultracentrifugation, which is not additive; thus, 60S plus 40S does not equal 100S. The small subunit binds to the mRNA and ensures the correct pairing of codons with the anticodons of transfer RNA (tRNA). The large subunit contains the peptidyl transferase centre, which catalyses the formation of peptide bonds between adjacent amino acids. This process is universal across all domains of life, highlighting the evolutionary conservation of the translation machinery.
Translation proceeds in three stages: initiation, elongation, and termination. During initiation, the small ribosomal subunit binds to the 5' end of the mRNA and scans for the start codon, typically AUG. Once the start codon is recognised, the large subunit joins to form the complete ribosome. During elongation, tRNAs deliver specific amino acids to the ribosome in a sequence dictated by the mRNA codons. The ribosome moves along the mRNA in a 5' to 3' direction, adding amino acids to the growing polypeptide chain. Termination occurs when a stop codon (UAA, UAG, or UGA) enters the A site, causing the release of the completed polypeptide and the dissociation of the ribosomal subunits.
Targeting to the Endoplasmic Reticulum
Proteins destined for the secretory pathway, including membrane proteins and proteins for lysosomes, the plasma membrane, or secretion, are synthesised on ribosomes bound to the ER. This targeting is initiated by a specific sequence of amino acids at the N-terminus of the nascent polypeptide, known as the signal sequence or signal peptide. This hydrophobic sequence is recognised by a cytosolic complex called the signal recognition particle (SRP).
As the ribosome synthesises the first 15 to 20 amino acids of the signal sequence, the SRP binds to the emerging polypeptide chain. This binding temporarily pauses translation, preventing the ribosome from completing the protein in the cytosol. The SRP then directs the ribosome-nascent chain complex to the ER membrane by binding to the SRP receptor. Once docked, the ribosome is transferred to a protein channel called the translocon (or Sec61 complex) in the ER membrane.
Translation resumes, and the growing polypeptide chain is threaded through the translocon into the lumen of the ER. The signal sequence is typically cleaved off by a signal peptidase once the protein has entered the lumen. This co-translational translocation ensures that the protein is folded in the correct environment, which contains chaperone proteins and enzymes for post-translational modifications such as glycosylation.
Post-Translational Translocation
Not all proteins are targeted to the ER during synthesis. Some proteins, such as certain mitochondrial proteins and some nuclear proteins, are synthesised entirely on free ribosomes in the cytosol and then translocated into their target organelles after translation is complete. This process is known as post-translational translocation.
In this mechanism, the target organelle possesses specific receptors on its outer membrane that recognise targeting signals on the completed polypeptide. The signals differ by destination: an amphipathic, positively charged presequence for mitochondria, and a short nuclear localisation signal rich in lysine and arginine for the nucleus. The translocation machinery uses energy derived from ATP hydrolysis to unfold the protein and pull it through a membrane channel. For mitochondria and chloroplasts the finished protein must be unfolded to pass through the channel (nuclear pores, by contrast, import fully folded proteins), whereas co-translational translocation allows the protein to fold as it is synthesised.
The distinction between these two mechanisms is crucial for understanding cellular organisation. Co-translational translocation is primarily associated with the ER and the secretory pathway, while post-translational translocation is common for proteins destined for mitochondria, chloroplasts, and the nucleus. Both mechanisms rely on specific signal sequences and receptor recognition, underscoring the importance of molecular specificity in cellular trafficking.
Protein Folding and Quality Control
Once a protein enters the lumen of the ER, it must fold into its correct three-dimensional conformation to become functional. This process is assisted by chaperone proteins, such as BiP (Binding immunoglobulin Protein), which bind to exposed hydrophobic regions of the nascent polypeptide and prevent incorrect aggregation. Chaperones use ATP hydrolysis to cycle between binding and releasing the protein, allowing it to fold progressively.
The ER also contains enzymes that catalyse the formation of disulfide bonds between cysteine residues, which stabilise the protein structure. Additionally, glycosylation occurs in the ER, where oligosaccharides are added to specific asparagine residues (N-linked glycosylation). These modifications are critical for protein stability, folding, and recognition by the Golgi apparatus.
Quality control mechanisms in the ER ensure that only correctly folded proteins are released into the Golgi apparatus. Misfolded proteins are retained in the ER and targeted for degradation through a process called ER-associated degradation (ERAD). This quality control is essential for maintaining cellular homeostasis and preventing the accumulation of toxic protein aggregates.
The concept
Organelles and the structure-function principle
Each organelle's form is matched to its role.
A common misconception is that ribosomes bound to the ER are structurally different from free ribosomes. In reality, they are identical; the difference lies in the presence of the signal sequence on the nascent polypeptide, which recruits the SRP and directs the ribosome to the ER membrane. C
Worked example
Problem. A eukaryotic cell synthesises a protein with a signal sequence of 20 amino acids. The total length of the mature protein is 200 amino acids. If the ribosome adds amino acids at a rate of 6 amino acids per second (typical of eukaryotic ribosomes; bacterial ones manage about 20), calculate the time required to synthesise the mature protein, assuming the signal sequence is synthesised before translocation begins.
- Determine the total number of amino acids synthesised: The mature protein has 200 amino acids, and the signal sequence has 20 amino acids. However, the signal sequence is part of the initial synthesis. The question states the mature protein is 200 amino acids long, which implies the final product after cleavage. Therefore, the total number of amino acids synthesised by the ribosome is 200 (mature) + 20 (signal) = 220 amino acids.
- Calculate the time required: The rate of synthesis is 6 amino acids per second. Time = Total amino acids / Rate = 220 / 6 ≈ 36.7 seconds.
Units check. The units of amino acids cancel out, leaving seconds, which is the correct unit for time.
Three quick tests
- What is the primary function of the signal recognition particle (SRP)?
Answer
It recognises the signal sequence on the nascent polypeptide and directs the ribosome to the ER membrane. - How does co-translational translocation differ from post-translational translocation?
Answer
Co-translational translocation occurs while the protein is being synthesised, whereas post-translational translocation occurs after synthesis is complete. - What is the role of chaperone proteins in the ER lumen?
Answer
They assist in the proper folding of nascent polypeptides and prevent incorrect aggregation.
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Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes drafted with a local model and checked line by line by the editor; every number in the worked example was recomputed.