Lecture 13 - Endomembrane Trafficking, Cytoskeletal System, ECM Structure, Transport and Communication

Definition: The endomembrane system connects (not literally) all the internal membranes of the eukaryotic cell.
Components: Includes structures like the nucleus, endoplasmic reticulum (ER), and Golgi apparatus.
Function: Enables cellular compartments to communicate and move components around effectively.

Definition: The cytoskeleton is composed of protein fibers that provide the cell with shape and structural stability.
Function:
- Organizes organelles and other cellular structures into a cohesive whole.
- Aids in cell movement and transport of materials within the cell.
- Helps anchor cells into tissues.

Definition: The ECM integrates the cell into tissue and gives it strength and resiliency.
Function: Assists in cell signaling and interactions with the surrounding environment.

Overview: Components need to efficiently communicate and transport materials.
- Nucleus: Acts as the command center of the cell.
- Endoplasmic Reticulum (ER): Functions like a distribution center, labeling materials appropriately for transport (using signal sequences).
- Golgi Apparatus: Processes and sorts materials for export or for internal use, akin to Amazon delivery services.

Gene Expression: mRNA production occurs in the nucleus.
Transport: mRNAs are exported from the nucleus through nuclear pore complexes (NPCs) into the cytoplasm.
- Export Proteins: Responsible for transporting mRNAs out of the nucleus.
- Ribosomes: Translate mRNA into polypeptides within the cytoplasm.

Process: mRNAs link with ribosomes to synthesize polypeptides, which are then translocated into the ER:
1. Ribosomes dock on the ER with mRNA.
2. Polypeptides are either folded by chaperones or inserted into the ER membrane for assembly.

Definition: Suggests that proteins intended for secretion undergo a defined series of steps:
1. Synthesized and processed in the RER.
2. Packaged into vesicles for transport to the Golgi apparatus.
3. Processed in the Golgi, then transported to the cell surface for secretion.

Transport Vesicles: Have coats indicating their destination and carry specific molecular tags that ensure proteins are delivered to the correct compartment.
Processes: Vesicles move from the cis face (receiving) to the trans face (shipping) of the Golgi apparatus.

Mechanism: Proteins destined for peroxisomes, mitochondria, and chloroplasts are actively imported from the cytosol and contain signal sequences for targeting.
Process:
1. Chaperone proteins help keep proteins unfolded as they are transported.
2. Proteins bind to receptors at the organelle's outer membrane before being transferred through channels into the inner membrane.
3. Final folding occurs in the matrix with the help of chaperones.

Exocytosis: Process where large molecules such as proteins are transported to the surface in vesicles that fuse with the plasma membrane to release contents.
Endocytosis: Process by which cells take in molecules by forming new vesicles from the plasma membrane, essentially reversing exocytosis.
- Types: Includes phagocytosis (solid particles), pinocytosis (liquid), and receptor-mediated endocytosis (specific binding).

Functions: The cytoskeleton provides structure and enables cellular functions:
- Composed of protein fibers including microtubules, intermediate filaments, and actin filaments.
Microtubules: Form tube-like structures providing stability and serving as tracks for vesicle transport, powered by kinesin motor proteins.
Intermediate Filaments: Provide structural support and include keratin (found in skin, hair, etc.), vimentin (connective tissue), neurofilaments (nerve cells), and nuclear lamin (nuclear support).
Actin Filaments: Help define the cell shape and work with myosin to facilitate muscle movement, cell division, and movement.

Functions of Transport Proteins: Facilitate hydrophilic substances passage across membranes. Types include:
- Channel Proteins: Allow molecules to pass through via a hydrophilic channel, including aquaporins for water transport.
- Carrier Proteins: Bind to substances and change shape to transport them across the membrane. They can perform facilitated diffusion without energy and active transport requiring energy.

Diffusion: Random movement of molecules leading to spreading across available spaces until equilibrium is reached.
Facilitated Diffusion: Transport proteins speed the passive movement across the membrane using channel and carrier proteins without energy expenditure. Includes:
- Ion Channels: Open/close in response to stimuli (gated channels).

Definition: Moves substances against their concentration gradients, requiring ATP energy, allowing maintenance of concentration gradients. Examples include:
- Sodium-Potassium Pump: Exchanges Na+ for K+, maintaining essential concentration differences across the membrane.
- Electrogenic Pump: Generates voltage across membranes, critical for energy storage in cellular processes.

Definition: Active transport of one solute that drives the transport of another solute against its own gradient. Includes:
- Symporter: Moves solutes in the same direction.
- Antiporter: Moves solutes in opposite directions, allowing for nutrient uptake, such as sugars through proton-driven transport in plants.

Types of Signaling: Local signaling (e.g., neurotransmitters) and long-distance signaling (e.g., hormones).
Cell Processes in Signaling:
- Reception: Ligands bind specifically to receptors, causing a change in shape.
- Transduction: Signals are transmitted through relay molecules.
- Response: Activation of proteins or genetic changes happens as results of signaling pathways.

G Protein-Coupled Receptors (GPCRs): Interact with G proteins to activate signaling pathways.
Ligand-Gated Ion Channels: Open in response to ligand binding, allowing ions to flow through, critical in nervous system signaling, activating cellular responses based on ion diffusion.