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 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