Cell Biology: Protein Synthesis and Cytoskeleton

Cell Biophysiology - Learning Objectives

• Describe protein synthesis.
• State the fate of proteins synthesized in the rough endoplasmic reticulum (rER) and those synthesized by free ribosomes.
• Explain the signal hypothesis in protein synthesis.
• List the factors that determine the fate of newly synthesized proteins.
• Explain the function of the unfolded protein response and its value to the cell.
• Compare the mechanisms by which proteins are targeted for degradation by proteasomes or lysosomes.

Cytoskeleton

• Explain the role of the cytoskeleton in determining cell shape, positioning of organelles, intracellular trafficking, and cell movement.
• Compare the characteristics and functions of microfilaments, microtubules, and intermediate filaments.
• Explain treadmilling of actin filaments and dynamic instability of microtubules.
• Explain how cytoskeletal dynamics control cellular movement.

Chromatin and Nucleolus

• Chromatin is DNA bound to histones and other proteins.
• Chromatin exists in two forms:
  • Heterochromatin: Condensed chromatin, transcriptionally inactive (intensely stained).
  • Euchromatin: Dispersed, unpacked chromatin, often actively transcribed (lightly stained).
• Nucleolus is the site of ribosomal RNA transcription and ribosome production (intensely stained).

Constitutive and Facultative Heterochromatin

• Constitutive heterochromatin: Repetitive, highly condensed throughout the cell cycle.
• Facultative heterochromatin: Not repetitive, may lose condensed state and become transcriptionally active.

Centrosome and Centriole

• Centrosome: The main microtubule organizing center (MOTC) of animal cells.
  • Consists of a pair of orthogonal centrioles surrounded by pericentriolar material (PCM).
  • PCM is responsible for microtubule anchoring.
• Centriole: A cylindrical structure (~200 nm in diameter) composed of a ring of nine triplets of microtubules.

Polyribosomes (Polysomes)

• Polyribosomes (polysomes): Multiple ribosomes bound to a single mRNA molecule.
• Simultaneously translate mRNA into many copies of a protein.
• Free polyribosomes synthesize cytosolic and cytoskeletal proteins.
• Polyribosomes attached to ER membranes synthesize proteins for incorporation into membranes, storage in lysosomes, or secretion from the cell.

Protein Synthesis - Signal Hypothesis

• mRNAs for proteins destined for the ER have an N-terminal signal sequence (signal peptide).
• Signal recognition particle (SRP) binds to the signal peptide.
• SRP binds to the SRP receptor and protein translocator complex in the ER membrane.
• Ribosome-SRP-nascent polypeptide complex attaches to the ER membrane.
• Signal peptidase removes the signal peptide in the lumen of the ER.
• Nascent polypeptide is translocated into the ER lumen. or becomes embedded in the ER membrane through the translocation complex.
• Other proteins (chaperones) "pull" the polypeptide through the translocator complex.
• The polypeptide folds properly in the ER cisterna.

ERAD (ER-Associated Degradation)

• New proteins that cannot be folded properly are targeted for degradation.
• Conjugated to ubiquitin.
• Degraded by proteasomes.
• This process is called the unfolded protein response (UPR).

UPR (Unfolded Protein Response)

• Helps maintain the balance of protein folding in the ER.
• Activated when unfolded proteins accumulate in the ER.
• Works to lower the number of unfolded proteins.
• Multiple mechanisms to achieve this.
• Unsuccessful UPR can lead to cell death.

Ubiquitin-Proteasome Pathway

• Ubiquitin-protein ligation: Ubiquitin attaches to the protein.
• Recognition by the 26S proteasome.
• Degradation of the ubiquitin-conjugated protein by the 26S proteasome.
• Release and recycling of ubiquitin.