Study Notes on Intermediate Filaments, Cytoskeletal Dynamics, and Intracellular Transport

Stability of Intermediate Filaments
- Intermediate filaments are stable structures within the cell.
- Unlike other cytoskeletal elements (like actin or microtubules), they do not frequently grow or shrink.
- Serve primarily to provide structural support rather than participate in cellular processes.
Lack of Polarity
- Intermediate filaments do not have distinct plus or minus ends.
- Their symmetrical nature prevents directional growth like that seen in actin and microtubules.
Non-renewable Assembly
- Intermediate filaments do not bind nucleotides (GTP or ATP).
- Their assembly or disassembly is not regulated by enzymes as it is for actin or microtubules.
Keratin Filaments Example
- Staining shows keratin filaments in cells; these filaments fill the cytoplasmic space, providing structural support while the nucleus remains as a central, dark area.
- Once a network of keratin is established, its configuration remains mostly unchanged, contrasting with the dynamic nature of other filaments.

Main Roles of the Cytoskeleton
- Establishing cell polarity, where one side of the cell differs from the other.
- Essential for complex cells to function and perform diverse tasks in different areas of the cell.
- Actin and microtubules provide this polarity due to their inherent structural polarity (plus and minus ends).
Regulation of Cytoskeleton Dynamics
- Growth and regulation of cytoskeletal elements are mainly controlled by enzymes known as GTPases.
- GTPase: an enzyme that hydrolyzes GTP to GDP, releasing an inorganic phosphate.
- Example: Hydrolyzing GTP bound to tubulin enhances microtubule disassembly by reducing the affinity of GDP-tubulin for microtubule structures.
Other Regulators
- Includes various proteins that regulate GTP binding to effect cytoskeletal dynamics.
- Notable GTPases that impact actin polymerization include RO and RAC.

Interpreting Diagrams
- Arrows: indicative of a positive influence of one enzyme on another.
- T Symbols: show inhibitory interactions.
- Understanding these diagrams is crucial for grasping signaling pathways and how various enzymes and their activities connect.
Example of Signaling Cascade
- RAC GTP activates WAVE, which positively influences actin polymerization.
- PAC inhibits MLCK, which positively regulates myosin activity; thus, inhibiting PAC leads to decreased myosin activity.
- Two inhibitory steps combined can result in a positive outcome due to the sequential inhibition.

Principles of Chemotaxis
- Chemotaxis refers to the directed movement of a cell toward a chemical attractant, often linked with immune response!
- Local concentration gradients of signaling molecules dictate cell behavior; higher concentrations near the source result in a disproportionately higher signaling response.
Neutrophil Movement
- Neutrophils utilize chemotaxis to pursue bacteria by responding to gradients of chemical signals (chemoattractants).
- The directional sensing and migration towards stimuli occur without neural input; it’s purely biochemical.

Transport Mechanisms
- Intracellular cargo (like organelles and proteins) is moved via the cytoskeletal network using specialized motor proteins such as kinesin.
Signal Hypothesis
- Proteins contain specific signal sequences determining their localization within the cell.
- E.g., Lysine-rich sequences serve as nuclear import signals.

Defining Signal Sequences
- Short amino acid segments that interact with specific receptors to target proteins appropriately to their destinations.
- Disruption of these sequences results in mislocalization (e.g., mutated lysine to threonine leads to cytoplasmic fluorescent signals).
Cellular Compartmentalization
- Proteins destined for membranes, organelles, or secretion first enter the endoplasmic reticulum (ER) during translation.
- Ribosomes that synthesize membrane-bound or organelle proteins attach to ER membranes, leading to the formation of Rough ER.

Mitochondrial Transport Complexity
- Mitochondrial protein transport is complex due to the double membrane structure.
- Proteins require specific signals for correct localization across membranes.
- Proteins interact with chaperones to maintain their unfolded state prior to import.
Unique Features of Mitochondrial Import
- All mitochondrial protein import occurs post-translationally (after protein synthesis).
- Chaperone proteins assist in maintaining an unfolded state, allowing translocation across mitochondrial membranes.

Nuclear Pore Complexes
- The nucleus has a double membrane with nuclear pore complexes acting as gates for molecular transport.
- Small molecules can diffuse freely, but larger proteins require specific nuclear localization signals (NLS) to cross.
Transport Mechanism
- Import Receptors bind cargo proteins with NLS and navigate through the nuclear porin meshwork facilitated by interactions.
- Affinities of NLS and bound GTP drive the movement of import receptors between the nucleus and cytoplasm.
Key Proteins for Nuclear Transport
- RAM GTPase regulates directionality by establishing concentration gradients of GTP in the nucleus that promote binding and release of import receptors.
- GTP hydrolysis is key in disengaging cargo from import receptors upon reaching the nucleus.
Differential Affinity
- Import proteins have higher affinity for nuclear localization signals inside the nucleus and lower affinity outside, ensuring efficient transport.
This differential affinity allows for selective retention of proteins within the nucleus, thereby facilitating proper cellular function and regulation of gene expression.
Microtubules serve as a highway for vesicles and organelles
- Motor proteins in kinesin and dynein transport over long distances
Kinesin: usually move towards plus end of microtubules — looks like Myosin
Dynein: generally moves towards the minus end, much larger

A theoretical motor proteins with conformations A, B and C, will switch between them randomly
- Random switching leads to no net movement
Real motor proteins need to make sure one of the steps is irreversible — by coupling to ATP hydrolysis

***A kinesin head in solution (not bound to microtubule) has higher affinity for? — ATP

***ATP hydrolysis in a Kinesin head causes? — The head to detach from the microtubule

***ATP hydrolysis in a kinesin head causes?

Understanding structure, function, and transport mechanisms of cytoskeletal elements, organelle localization, and protein synthesis pathways is fundamental in cellular biology.
Each component and mechanism functions in a highly regulated manner to maintain cellular integrity, functionality, and response to environmental signals.