Protein Sorting part 5

Summary

This video script explains intracellular transport, focusing on exocytosis and endocytosis. Exocytosis involves the secretion of proteins, carbohydrates, and lipids after modifications like disulfide bond formation and glycosylation in the endoplasmic reticulum (ER). Endocytosis includes phagocytosis (engulfing large particles) and pinocytosis (ingesting liquids), with vesicles fusing with endosomes and lysosomes for degradation. The Golgi apparatus further modifies proteins after they leave the ER. The script also highlights protein quality control processes, where misfolded proteins are marked for degradation, ensuring proper cellular function through precise molecular mechanisms.

Takeaways

• 😀 Exocytosis involves the process of molecules (like proteins, carbohydrates, or lipids) being expelled from the cell after their synthesis and modification in the endoplasmic reticulum.
• 😀 Endocytosis is the process where particles or fluids from the extracellular environment are taken into the cell.
• 😀 Proteins involved in exocytosis undergo a modification process in the endoplasmic reticulum, including the formation of disulfide bonds and glycosylation.
• 😀 Disulfide bonds between cysteine residues play a crucial role in stabilizing the 3D structure of proteins, such as antibodies.
• 😀 Glycosylation of proteins occurs in the lumen of the endoplasmic reticulum, where oligosaccharides are added to asparagine residues on proteins.
• 😀 Dollycose, a lipid that plays a role in glycosylation, is involved in transferring oligosaccharides to proteins for glycosylation with the help of oligosaccharyltransferase enzymes.
• 😀 Proteins that need to remain in the endoplasmic reticulum are retained by specific signals, such as the endoplasmic reticulum retention signal.
• 😀 Misfolded proteins are detected by proteins like saveron and are marked for destruction by ubiquitination, after which they are degraded by the proteasome.
• 😀 Protein modification continues in the Golgi apparatus, where additional glycosylation and sugar modifications may occur.
• 😀 Endocytosis has two types: phagocytosis (where large particles are engulfed by cells like macrophages) and pinocytosis (where smaller molecules are engulfed by various cell types using clathrin-coated vesicles).

Q & A

What is the main difference between exocytosis and endocytosis?

Exocytosis is the process of expelling molecules like proteins, carbohydrates, or lipids from a cell, while endocytosis involves the intake of particles or liquids from the extracellular environment into the cell.

How are proteins modified before being secreted via exocytosis?

Before proteins are secreted via exocytosis, they undergo modifications such as the formation of disulfide bonds between cysteine amino acid residues and the addition of oligosaccharide chains in the endoplasmic reticulum.

What role do disulfide bonds play in protein structure?

Disulfide bonds help stabilize the three-dimensional structure of proteins by forming covalent linkages between cysteine residues, which is crucial for maintaining the protein's functional shape.

What is glycosylation and where does it occur?

Glycosylation is the addition of carbohydrate chains (oligosaccharides) to proteins. This process occurs in the lumen of the endoplasmic reticulum and is catalyzed by enzymes like oligosaccharyltransferase.

What is the role of dolichol in glycosylation?

Dolichol is a lipid carrier in the endoplasmic reticulum that holds the oligosaccharide chain. The oligosaccharide is then transferred to the protein, which is undergoing glycosylation.

What happens to proteins that fail to fold properly in the endoplasmic reticulum?

Proteins that fail to fold properly are retained in the endoplasmic reticulum and are not secreted. They are marked for degradation through ubiquitination and sent to the proteasome for destruction.

What is the function of the Golgi apparatus in protein modification?

The Golgi apparatus further modifies proteins by adding or removing sugar units from oligosaccharide chains. This modification takes place in the cis, medial, and trans regions of the Golgi.

What is the difference between phagocytosis and pinocytosis in endocytosis?

Phagocytosis involves the engulfing of large particles like bacteria by specialized cells (e.g., macrophages), while pinocytosis involves the intake of smaller molecules or fluids through the formation of small vesicles.

How are vesicles formed during pinocytosis?

In pinocytosis, vesicles are formed using clathrin, which aids in the formation of the vesicle that encapsulates the small molecules for transport into the cell.

What is the role of the lysosome in the endocytosis process?

The lysosome fuses with early and late endosomes, where its enzymes break down the contents of the vesicle, such as proteins or pathogens, through acidic conditions created by proton pumps.