The role of tropmyosin in muscle contraction 9

The role of tropmyosin in muscle contraction

9. 7 Nonmuscle motility (1) • Actin-binding proteins affect the localized assembly or disassembly of the actin filaments.

The roles of actin-binding proteins

Nonmuscle motility (2) • Actin-binding proteins (continued) – Nucleating proteins – provide a template for adding actin monomers. (Arp 2/3 complex) – Monomer-sequestering proteins – bind to actin-ATP monomers and prevent them from polymerizing. (thymosin ß 4) – End-blocking (capping) proteins – regulate the length of actin filaments. – Monomer-polymerizing proteins –promote the growth of actin filaments. (profilin)

Nonmuscle motility (3) • Actin-binding proteins (continued) – Actin filament depolymerizing proteins – bind actin-ADP subunits for rapid turnover of actin filaments. Example: cofilin – Cross-linking proteins – alter the threedimensional organization of actin filaments. Examples: vilin, fimbrin

Nonmuscle motility (4) • Filament-severing proteins – shorten filaments and decrease cytoplasmic viscosity. Example: gelsolin • Membrane-binding proteins – link contractile proteins to plasma membrane.

Nonmuscle motility (5) • Examples of Nonmuscle Motility and Contractility – Actin polymerization as a force-generating mechanism • Responsible for some types of motility such as cytoplasmic streaming in Listeria

Nonmuscle motility (6) • Examples of nonmuscle motility and contractility – Cell Locomotion • Cells lacking cilia or flagella move by crawling over a substrate.

Nonmuscle motility (7) • Cell locomotion (continued) – Cells that crawl over a substratum display a repetitive sequence of events.

Nonmuscle motility (8) • Cells that Crawl over the Substratum – Cultured cells crawl by forming a protrusion called a lamellipodium. – Force generation in lamellipodia occurs by adding actin monomers to filaments, prividing temporary anchorage for the cell.

Motility using a lamellipodium

Major steps in formation of a lamellipodium

Lamellipodial extension

Lamellipodial extension

Roles of actin and myosin in lamellipodialbased movement

Nonmuscle motility (9) • Axonal Outgrowth – The bulk of the axon shows little evidence of motile activity. – The tip of the axon (growth cone) shows several types of locomotor protrusions: • Microspikes – point outward to the edge of the lamellipodium. • Filopodia – elongations that extend and retract during motile activity.

The structure of a growth cone

Nonmuscle motility (10) • Axonal outgrowth (continued) – The growth cone explores its environment and elongates its axon. – Lamellipodia and filopodia of growth cone respond to the presence of physical and chemical stimuli.

Nonmuscle motility (11) • Changes in Cell Shape during Embryonic Development – Ectodermal cells elongate and for a neural plate as microtubules become oriented parallel to the cell’s axis. – Change in cell shape produced by contraction of microfilaments. – Curvature of the neural tube causes outer edges to contact one another forming a tube which gives rise to nervous system.

Early stages in the development of the nervous system

Early stages in the development of the nervous system