Untitled Quiz

Cell Biology Lecture 3: Protein Translocation and the Secretory Pathway

• Lecture covered protein translocation and the secretory pathway
• Topics included protein targeting, eukaryotic secretory pathway, rough/smooth ER, protein translocation, secretory pathway modifications, quality control, and misfolded proteins.
• Misfolded membrane proteins were also discussed, including their pathogenicity.
• Learning objectives included understanding differences between rough and smooth ER, co- and post-translational translocation, ER protein modification and quality control; impact of misfolded proteins and cellular pathogenesis.

Protein Translocation and the Secretory Pathway

• Lecture covered the structure of a protein and targeting signals
• Proteins are made in the cytosol
• They are directed to the correct cellular compartment
• The lecture focused on the co-translational and post-translational translocation of proteins into the ER.
• The pathway includes the ER, Golgi, and other organelles.
• Protein targeting is crucial for protein function

Protein Targeting

• Most proteins are made in the cytosol.
• Proteins need to be directed to their correct cellular compartment for proper function.
• This lecture covered the mechanisms for directing proteins to specific locations, such as the nucleus, mitochondria, peroxisomes and the ER.
• Most protein translocation in humans is co-translational.

Eukaryotic Secretory Pathway

• Proteins destined for secretion or insertion into membranes follow a pathway beginning in the ER.
• The pathway continues through the Golgi apparatus and other organelles before reaching the cell exterior.
• Most secretory proteins are co-translationally targeted in humans to avoid potential aggregation issues.

Rough/Smooth ER Recap

• Rough ER is studded with ribosomes, essential for protein production
• Smooth ER is involved in lipid processing and secretion
• Both are critical parts of the secretory pathway

Protein Translocation

• Protein translocation is how proteins are moved across membranes.
• Co-translational translocation involves ribosomes translating mRNA into protein while simultaneously moving the protein into the ER membrane as it's being synthesized.
• Post-translational translocation involves already synthesized proteins being transported into the ER membrane.

Secretory Pathway Modifications

• Proteins undergo modifications like glycosylation, proteolytic cleavage, oligomerization, and disulfide bond formation.
• These modifications are vital for protein function and quality control

Quality Control & Misfolded Proteins

• Cells monitor protein folding.
• If misfolded, chaperones may help refolding or target the protein for degradation by the proteasome.
• Chaperone proteins assist in proper folding and quality control.
• They facilitate proper folding and help prevent aggregation of misfolded proteins.
• Calnexin recognizes misfolded proteins

Misfolded Membrane Proteins – Pathogenicity

• Misfolded membrane proteins can lead to diseases like cystic fibrosis (CFTR) - related to protein dysfunction affecting ion transport.
• Other potential disorders include Insulin-resistant syndrome and Autosomal Dominant Deafness type 2A (KCNQ4 mutations).

Protein Translocation in Summary

• Protein targeting signals direct proteins to the appropriate location.
• Co-translational translocation happens in the ER
• Post-translational translocation also happens
• Misfolded proteins are recognised by chaperones like calnexin and are tagged for degradation, if necessary
• Improper folding or trafficking can lead to disease

The Secretory Pathway

• The pathway transports various molecules within and outside the cell
• Exocytosis describes how substances are actively transported out of the cell.
• Endocytosis describes how substances are transported into the cell.
• Different types exist including pinocytosis, clathrin independent and dependent; and phagocytosis.

Exocytosis - Types of Secretion

• This section outlined two types of exocytosis
• Constitutive secretion is a continuous process where proteins are exported.
• Regulated secretion involves storing proteins in vesicles and releasing them in response to signals.

Neuronal Synapse Vesicle Delivery

• This section focused on how vesicles carrying neurotransmitters are targeted and released to the synaptic cleft for communication between neurons.
• The SNARE proteins play a critical role in vesicle fusion to the target membrane.

Toxins on Vesicle Fusion

• Specific toxins interfere with the process of vesicle fusion and potentially with disease.

Endocytosis: Transport Pathways

• Several endocytic pathways exist
• Endocytic pathways are involved in bringing materials into the cell.
• The processes involved in endocytosis include pinocytosis, macropinocytosis, receptor-mediated endocytosis, and phagocytosis.

Endocytosis - Types

• This section detailed the various mechanisms underpinning endocytosis
• Pinocytosis is a method for consuming fluid or small molecules through invagination of the plasma membrane.
• Receptor-mediated endocytosis is a highly specific process where receptors on the cell surface bind to specific ligands in the extracellular fluid, causing the formation of a clathrin-coated pit that eventually pinches off as a coated vesicle to deliver its cargo.
• Macropinocytosis involves large, non-selective intake of extracellular fluid and material.
• Phagocytosis is when the cell engulfs large particles like bacteria.

Golgi Trafficking - The Golgi

• The Golgi apparatus is critical for modifying, sorting, and transporting proteins and other molecules throughout the secretory pathways.
• The Golgi comprises different compartments like cis, medial, and trans cisternae.
• Different proteins are modified and sorted across these compartments to their targeted destinations.

Golgi to ER Movement

• Proteins within the ER may need to be retrieved
• The process uses COPI-coated vesicles to carry cargo
• KDEL receptors recognise specific retrieval signals on cargo proteins in the cis and medial Golgi.
• The vesicles then return the proteins to the ER.

ER/Golgi Trafficking - Vesicle Formation

• COPII proteins direct vesicles carrying cargo from the ER to the Golgi.
• Multiple steps are involved including GTP binding, signal exposure, and insertion into the ER membrane for transport to the Golgi.
• The mechanisms used for vesicle formation and maintenance of cell size are critical for cellular function.

The Secretory Pathway Summary

• Lecture highlights the various parts of the secretory pathway, including transport vesicle formation, delivery, and modifications.
• A combination of endocytosis and exocytosis allow for efficient movement of materials into and out of the cell.
• The pathways' regulation is essential to maintain healthy cellular function.