Actin Cytoskeleton

What is G-actin?

G-actin is the globular monomeric form of actin, and is present in all eukaryotes. It’s structure consists of an ATP-ADP binding cleft and ATPase activity, and it polymerizes into filamentous F-actin. One actin filament consists of two strands of subunits wound around each other, where aach monomer jas tje same orientation and the filament has polarity. The end with the exposed ATP-binding cleft is the minus end.

How do the plus and minus ends of actin filaments differ?

The end with the exposed ATP-binding cleft is the minus end. Both ends of an actin filament can add or loose G-actin, but addition is much faster at the plus end. After addition of ATP-G-actin, ATP is rapidly hydrolyzed, and loosing phosphate changes the conformation of G-actin. ATP-actin is only present at the plus end, while ADP-actin is only present at the minus end. Loss of G-actin is similar at both ends, and is independent of the G-actin concentration.

What is threadmilling?

At steady state, the length of an actin filament doesn’t change, as ATP-G-actin is added at the plus end at the same rate as ADP-G-actin is lost at the minus end. This also leads to treadmilling, where G-actin subunits change their position along the filament. This is powered by ATPase activity.

How do formin and Arp2/3 nucleate actin filament?

Formins nucleate actin filaments, protect their plus end and promote elongation. Formin has different domains, where the FH2 domains bind to the plus end of actin monomers to stablize and allow addition of new actin monomers. The ARP2/3 complex promotes formation of branched actin-networks, activated by NPFs. Activation allows the ARP2/3 complex to bind to the side of F-actin, and actin polymerization starts.

Regulation of formins occurs by binding to an active GTPase, while regulation of the ARP2/3 complex occurs by binding of NPF to specific membrane lipids.

How does myosins move along actin filaments? How is myosin movement related to the ATPase activity of myosin?

Myosin are actin-based motorproteins. The head domain contains an actin and an ATP binding site, and ATPase activity provides the energy for mechanical work.

Contraction is caused by an action potential that travels along transverse tubules. During contraction, the myosin heads pull the actin filaments towards the center of sarcomere, whom’s structure is maintained by several other proteins.Titin connects the Z disc with the M band, and keeps the myosin filaments in the center of the sarcomere, while nebulin binds the actin filaments and regulate their length. The AP triggers release of Ca²⁺ from the sarcoplasmic retikulum, which binds to Troponin and changes its conformation. Troponin affects the position of Tropomyosin along actin filament, and as long as Ca²⁺ is present, the myosin binding site on actin is accessible.

How is the cytoskeleton regulated during cell migration?

Cell migration requires the regulated polymerization of actin, and adhesion + deadhesion to the substrate / ECM.