Endomembrane System Study Notes

The endoplasmic reticulum is directly continuous with the nuclear envelope, which allows for efficient communication between the nucleus and the cytoplasmic processes.
Understanding the function of these organelles is vital to comprehend how proteins are synthesized and transported within the cell.

The process of protein synthesis can lead to a protein being made in either the cytoplasm or the rough ER. The location of synthesis depends on:
- Signal sequences that direct ribosomes to the ER
A protein's destination is encoded in its gene, influencing the presence of a signal polypeptide that defines its trajectory in the cell.

Comprises two major parts:
- Rough ER: Studded with ribosomes, where protein synthesis occurs.
- Smooth ER: Lacks ribosomes, involved in lipid synthesis and metabolism.
Functions of the ER:
- Synthesis of proteins for export or incorporation into membranes.
- Lipid synthesis for membrane formation or cellular export.
- Contains chaperone proteins to assist in the correct folding of polypeptides.

Proteins destined for the rough ER start synthesis in the cytoplasm.
- mRNA is exported from the nucleus; ribosomes assemble on mRNA for translation.
- The first 16 to 30 amino acids produce the signal polypeptide, which is crucial for targeting.
Signal Recognition Particle (SRP):
- Binds to the signal polypeptide and pauses translation, guiding the ribosomal complex to the rough ER.
Upon docking to the rough ER, translation resumes, and the growing polypeptide is fed into the ER lumen.
The signal peptide is cleaved off, allowing the protein to fold correctly with help from chaperone proteins.

The smooth ER varies in function depending on cell types. For example:
- Muscle cells have sarcoplasmic reticulum specialized for calcium ion storage and signaling.
- Smooth ER's universal role: Lipid synthesis, including hormones and membrane lipids.

Vesicles bud off from the ER carrying proteins and lipids, facilitated by cargo receptors and COPII proteins which target vesicles to their next destination (Golgi).
Steps of vesicle transport:
- Cargo (proteins/lipids) attaches to cargo receptors on the ER.
- Membrane buds off, forming vesicles containing the cargo and marking proteins (e.g., COPII).
- Vesicles merge with the Golgi apparatus, transferring contents into the Golgi's cis region.

Golgi is structured into multiple regions (cis to trans) that facilitate processing, modification, and sorting of proteins and lipids.
Functions:
- Modifications such as glycosylation (addition of sugars) occur here.
- Sorting of proteins and lipids for their final destination: extracellular export, membrane insertion, or lysosomal targeting.
Two models for movement through the Golgi:
- Entire compartments shift forward (cisternal maturation model).
- Vesicles carry modified products from cis to trans regions.

Special vesicles containing hydrolytic enzymes for degradation of cellular waste and recycling of components.
Proteins destined for lysosomes follow similar trafficking as ER and Golgi products.
Critical for cellular metabolism and maintenance, breaking down old or damaged organelles and macromolecules.

SNARE Proteins: Facilitate specific fusion between vesicles and target membranes.
- v-SNAREs: Found on vesicles; specific for target membranes.
- t-SNAREs: Found on target membranes, enabling specificity in vesicle delivery.

The endomembrane system plays a crucial role in maintaining cellular organization and function through a series of interconnected dynamic processes involving various organelles. Understanding these pathways is essential for grasping how cells produce, modify, and sort biomolecules.