Study Notes on Actin Filaments, Cell Migration, Protein Transport, and Glycosylation.

Microfilaments, specifically actin filaments, possess polarity.
- Minus End: One side of the actin protein (designated as -).
- Plus End: The other side of the actin protein (designated as +).
The protein that nucleates the formation of actin is called ARP.
- ARP stands for Actin-Related Protein, specifically ARP2/3 complex.

For actin microfilament formation, nucleotide binding is crucial.
Actin proteins require ATP for binding, especially at the plus end.
- At the minus end, ATP binding is not favored physiologically; it requires a high critical concentration to bind, which is not achieved under normal cellular conditions.
- In laboratory settings, different conditions may allow for the manipulation of concentrations.

Critical Concentration: Refers to the specific concentration of actin monomers needed for polymerization at the filament ends.
- Plus End: Has a low critical concentration, allowing for the addition of ATP-bound actin.
- Minus End: Has a high critical concentration, making it physiologically unfavorable for actin binding, hence it remains stable.

The process of treadmilling refers to maintaining a constant filament length while actively adding and removing actin monomers one at a time.
- Growth occurs primarily at the plus end and disassembly takes place mainly at the minus end.
Unlike microtubules, actin filaments do not exhibit catastrophic disassembly.
- Microtubules experience dynamic instability, often losing mass rapidly, while actin maintains its structure more steadily.

Various regulatory proteins interact with actin microfilaments to promote stability or disassembly.
  - Key proteins discussed:
    - ARP: Essential for initiating nucleation and polymerization of actin.
    - Sequestering Proteins: Bind actin monomers to promote rapid polymerization.
      - Example: Cofilin binds to actin, promoting disassembly and maintaining a pool of ADP-actin that cannot readily polymerize with ATP.

Actin filaments play a significant role in cell migration, particularly involving the extension of the cell's leading edge and retraction of the trailing edge.
- This dynamic movement applies to cells like macrophages which utilize actin for locomotion.
As ATP hydrolysis occurs, actin polymerization contributes to the physical movement of cells.

Interaction of actin with myosin is critical for muscle contractions.
  - Myosin: The motor protein that interacts with actin filaments.
  - Requires energy in the form of ATP:
    - Hydrolysis of ATP facilitates movement along the actin filament.
The binding dynamics involve conditions where myosin is transiently