Organelles and Protein Trafficking Notes

Identify Organelles and Functions:
- Organelles include the nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and peroxisomes.
- Each organelle has distinct functions crucial for cellular processes.
Importance of Protein Sorting:
- Correct sorting of proteins is essential for organelle function and overall cell survival.

Definition: The endomembrane network consists of membranes that divide the cell into functional and structural compartments.
Evolutionary Origins:
- Nucleus and mitochondrial origins differ; their evolutionary history impacts vesicle trafficking and organelle interactions.

Function of Signal Sequences:
- Short peptide motifs that direct protein sorting to specific cellular compartments.
General Characteristics:
- Typically contain hydrophobic regions and are cleaved post-delivery.
Import Destinations:
- Required for the ER and nucleus, but not for cytosolic proteins.

Translocation Mechanism:
- Involves the Signal Recognition Particle (SRP), SRP receptor, translocator, and signal peptidase during translation.
ER Import and Membrane Insertion Signals:
- Different characteristics for soluble and membrane proteins, leading to various transmembrane protein structures.

Structure of Nuclear Envelope:
- Composed of an inner and outer membrane with nuclear pores for selective transport.
Nuclear Pores:
- Facilitate diffusion of small molecules and selective import of large proteins.
Nuclear Import Mechanism:
- Involves the nuclear localization signal, nuclear import receptor, and GTP-binding protein Ran, ensuring unidirectional transport.

Sorting Pathways:
- Proteins encoded by nuclear genes are tagged for import into organelles like mitochondria and peroxisomes.

Clathrin-Coated Vesicle Formation:
- Multi-step process involving cargo selection, clathrin assembly, adaptor proteins, and dynamin for vesicle uncoating.
Vesicle Targeting:
- Mechanisms ensure vesicles reach correct membrane destinations.
Tethering and Fusion Mechanisms:
- Involves Rab proteins, SNAREs, and tethering proteins to facilitate docking and fusion at target membranes.

Exocytic Route:
- Secreted proteins travel from transcription to secretion through the ER, Golgi, and eventually to the plasma membrane.
Glycosylation and Disulfide Bond Formation:
- Crucial for protein stability and function in secreted proteins, occurring within the ER.
Quality Control Mechanisms:
- Ensures only properly folded proteins exit the ER, while misfolded proteins are targeted for degradation.
Unfolded Protein Response (UPR):
- Activates in response to protein misfolding, altering cellular behavior to manage misfolded protein accumulation.
Constitutive vs. Regulated Exocytosis:
- Constitutive exocytosis occurs continuously, while regulated requires specific signals (e.g., neurotransmitter secretion).

Pinocytosis vs. Phagocytosis:
- Pinocytosis involves uptake of fluids and small particles, while phagocytosis targets large particles or cells.
Pathways for Materials:
- Materials can be processed in endosomes and directed to lysosomes for degradation.
Receptor-Mediated Endocytosis (RME):
- Specific uptake of molecules like LDL, involving receptor binding and internalization into clathrin-coated vesicles.
Structure and Function of Lysosomes:
- Lysosomes emerge from maturing endosomes; contain enzymes for degradation of biomolecules.
Lysosome Material Sources:
- Materials enter lysosomes via phagocytosis, autophagy, and pinocytosis, essential for cellular recycling and waste management.