1. Protein Transport and Signal Sequences
1.1. Nuclear Localization Signals (NLS)
1.1.1. Function: Direct proteins to the nucleus
1.1.2. Mechanism: Proteins with NLS are recognized by importin proteins
1.1.2.1. The importin-protein complex is transported through the nuclear pore complex into the nucleus
1.1.2.1.1. Once inside, Ran GTPase helps release the protein from importin
1.1.3. Key Proteins: Importin, Ran GTPase
1.2. Mitochondrial Signal Sequences
1.2.1. Function: Direct proteins to mitochondria
1.2.2. Mechanism: Proteins with mitochondrial targeting sequences are recognized by receptors on the mitochondrial surface
1.2.2.1. Transported through the translocase of the outer membrane (TOM) and inner membrane (TIM) complexes
1.2.2.1.1. Chaperone proteins like Hsp70 assist in folding and translocation
1.2.3. Key Proteins: TOM complex, TIM complex, Hsp70 chaperones
2. Protein Transport Across the ER Membrane
2.1. Signal Recognition Particle (SRP)
2.1.1. Function: Directs ribosome to the ER membrane
2.1.2. Mechanism: SRP binds to the signal sequences of the nascent protein
2.1.2.1. The SRP-ribosome complex is directed to the SRP receptor on the ER membrane
2.1.2.1.1. The protein is translocated into the ER lumen through the Sec61 thranslocon
2.1.3. Key Proteins: SRP, SRP receptor, Sec61 translocon
2.2. Embedding in the Membrane
2.2.1. Mechanism: Transmembrane domains of proteins are integrated into the ER membrane via the Sec61 complex
2.2.1.1. Hydrophobic regions of the protein interact with the lipid bilayer, embedding the protein
2.2.1.1.1. Signal peptidase cleaves the signal sequence if necessary
2.2.2. Key Proteins: Sec61 complex, signal peptidase
3. Vesicle Formation and Transport
3.1. Endocytosis
3.1.1. Types:
3.1.1.1. Phagocytosis: Engulfing large particles
3.1.1.2. Pinocytosis: Engulfing fluids and dissolved substances
3.1.1.3. Receptor-Mediated Endocytosis: Specific molecules bind to receptors and are internalized
3.1.1.3.1. Mechanism:
3.1.1.3.2. Key Proteins: Clathrin, adaptins, dynamin, receptors
3.1.2. Mechanism:
3.1.2.1. Clathrin-coated pits form and internalize the ligand-receptor complex
3.1.2.2. Vesicles uncoat and fuse with early endosomes for sorting
3.1.3. Key Proteins: Clathrin, adaptins, dynamin
3.2. Exocytosis
3.2.1. Function: Expels materials from the cell
3.2.2. Mechanism:
3.2.2.1. Vesicles containing cargo fuse with the plasma membrane
3.2.2.2. Contents are released outside the cell
3.2.3. Key Proteins: SNAREs, Rab GTPases, tethering proteins
3.3. Vesicle Sorting and Docking
3.3.1. Proteins Involved:
3.3.1.1. Rab GTPase: Direct vesicles to specific locations
3.3.1.2. SNAREs: Facilitate vesicle fusion with target membranes
3.3.1.3. Coat Proteins: Clathrin, COPI, COPII for vesicle formation
3.3.2. Mechanism:
3.3.2.1. Vesicles bud from donor membranes with the help of coat proteins
3.3.2.2. Transported along cytoskeletal tracks by motor proteins
3.3.2.3. Dock and fuse with target membranes via SNARE complexes
3.3.3. Key Proteins: Rab GTPases, SNAREs, tethering proteins, motor proteins