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. (Arp2/3 complex)– Monomer-sequestering proteins – bind to

actin-ATP monomers and prevent them from polymerizing. (thymosin ß4)

– 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 three-dimensional 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 lamellipodial-based 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 developmentof the nervous system

Early stages in the developmentof the nervous system