Microfilaments

Microfilaments

Thinnest cytoskeletal element diameter of 7-8nm. Made up of globular (G-actin), monomeric form of actin. And filamentous (F-actin), G-actin molecules polymerize to form long thin microfilaments. F-actin is the polymerised form of actin. Polar, one end differs chemically and structurally from the other end. Double helix.

G-actin

Protein consisting of 375 amino acids, forming a compact folded structure. Single globular protein with central cavity, has a nucleotide binding site within the central cavity. ATP/ADP can bind to this site, which is crucial for the polymerisation process. ATP binding promotes polymerisation, while ADP binding typically occurs after polymerisation leading to filament instability

Actin

one of the most highly conserved proteins across eukaryotic species. While highly conserved it differs between organisms and among different tissues within the same organism. Two major groups: Muscle specific (alpha actin) and non-muscle (beta and gamma actin)

Muscle specific

alpha actin. Found primarily in muscle cells contributing to muscle contraction.

Nonmuscle

Beta and gamma actin. Present in various nonmuscle cells and tissues, involved in cell shape, motility and division.

Microfilament functions

1. Mechanical support: provide structural support helping to maintain cell shape and integrity

2. Muscle contraction: interact with myosin to facilitate muscle contraction

3. Cell migration/motility: enable the movement of cells essential for processes such as wound healing and immune response

4. Endocytosis/exocytosis: play a role in vesicle movement

5. Cell division: cytokinesis, where they form the contractile ring that pinches the daughter cells apart

Binding proteins

More than 60.

1. Proteins that regulate monomer polymerization, Thymosin B4, inhibits polymerisation.

2. Proteins that cap actin filaments Cap 2 (plus ends) Tropomodulin (minus end)

3. Proteins that bundle together bundle actin filaments. alpha actin, Fascin.

4. Proteins that crosslink actin filaments. Filamin