Proteins Present in the Endoplasmic Reticulum

Proteins Present in the ER

• Resident ER proteins are key for proper protein synthesis and folding.
• Translocons and Receptors are involved in protein translocation into the ER lumen.
• Signal Peptidase removes the signal peptide, which directs proteins to the ER.
• Chaperones are essential for proper protein folding.
• BiP, Calnexin, and Calreticulin are some important chaperones found in the ER.
• Proteins of the Unfolded Protein Response (UPR) are crucial for managing cellular stress and preventing misfolded proteins from accumulating.
• Proteins catalyzing disulfide bonds help maintain proper protein structure.
• Glycosylation is the process of adding carbohydrates to proteins.
• Oligosaccharyl Transferase mediates the transfer of the precursor oligosaccharide to the N.
• **The ER provides an environment and processes such as disulfide bond formation and glycosylation to facilitate proper protein folding. **
• Chaperone proteins play a crucial role in ensuring correct protein folding and only properly folded proteins leave the ER.
• Oligosaccharides act as tags indicating the folding state of a protein.
• Improperly folded proteins are exported and degraded in the cytosol.
• Misfolded proteins in the ER trigger the Unfolded Protein Response (UPR).

Endoplasmic Reticulum

• The ER is important for protein modification and lipid biosynthesis.
• The ER provides an environment that promotes the correct folding of proteins.
• The ER functions as a quality control center for proteins.

Intracellular Membrane Traffic

• Vesicle and compartment identity is achieved by a combination of factors:

  • Coat proteins in the vesicle
  • Phosphoinositides (PIPs) in the membrane
  • Rab proteins present in various organelles
  • v-SNARES and t-SNARES in various vesicles and organelles

• Rab proteins are GTP-binding proteins that guide transport vesicles to their target membranes.

• Different Rab proteins are located in different organelles.

• Rab proteins interact with specific effector proteins, ensuring that a vesicle fuses with the correct compartment.

• SNAREs mediate membrane fusion.

  • v-SNARES are located on the vesicle.
  • t-SNARES are located on the target compartment.
  • SNAREs form a trans-SNARE complex to facilitate vesicle fusion.

Endocytic Pathway

• The endocytic pathway is responsible for transporting materials from the plasma membrane into the cell.
• Early endosomes mature into late endosomes.
• ESCRT protein complexes are involved in forming intralumenal vesicles in multivesicular bodies.

Cholesterol Transport

• The majority of cholesterol is transported in the bloodstream as cholesterol esters within Low-Density Lipoproteins (LDL).
• LDL receptors mediate the binding of LDL to cell surfaces.
• The low pH within the endosome causes the dissociation of LDL from the receptor.
• LDL particles are then transported to lysosomes for degradation.

Lysosomes

• Lysosomes are specialized organelles containing hydrolytic enzymes that degrade various materials.
• The low pH and hydrolytic enzymes within lysosomes are responsible for the destruction of materials.
• The lysosomal membrane is highly glycosylated to protect it from the acidic environment.
• Lysosomes are essential for recycling cellular components and the degradation of foreign materials.

Material Delivery to Lysosomes

• Multiple pathways deliver materials to lysosomes:
  • Directly from the cytosol.
  • Via the ER-Golgi pathway.
  • Through the endocytic pathway.
  • Via macropinocytosis.
  • Through phagocytosis.