Cilia and Flagella
o
Use core of microtubules from basal body- generate the axoneme
Cilium beats by performing repetitive rounds of a “power stroke”
Actin filaments
o
Actin filaments are thin and flexible
o
Many proteins bind to actin and modify its properties
o
An actin rich cortex underlies the plasma membrane of most eukaryotic cells cell
crawling depends on actin
o
Actin associates with myosin to form contractile structures
o
IMPORTANT- extracellular signals control arrangement of actin filaments
allowing cell to respond to environment
o
5% total protein in a typical animal cell is actin
o
½ of actin is assembled into filaments, and other in actin monomer in cytosol
Actin filaments allow cells to adopt a variety of shapes
o
Actin fibers like microtubules are unstable
o
Made stable by interactions with other proteins “actin binding proteins”
o
Differential interactions determine “stiffness” of structure
o
Can for stuff microvilli or flexible cellular muscles
o
Can create temporary protrusions or a contractile ring which pinches off
Actin binding proteins control behavior of actin filaments in vertebrate cells- formins and
actin related proteins (ARP’s) control actin assembly
The cortex- actin rich network under cytoplasm, and gives cell its shape and allows it to
move
Cell movement
o
Cell first pushes out protrusions at its front or leading edge
o
The protrusion adheres to surface over which cell is crawling
o
The rest of the cell drags itself forward
Cell crawling relies on actin
o
Integrins- transmembrane proteins which bind to extra cellular matrix
A web of actin filaments pushes the leading edge forward
Actin associates with myosin motor proteins to form contractile structures
o
Carry vesicle along actin filament tracks
o
Bind plasma membrane and move relative to cortical actin = change shape
Extracellular signals control the arrangement of actin filaments- allows the cell to
respond to its environments
How do cells contract?
o
Muscle contraction depends on bundles of actin and myosin
o
During muscle contraction, actin filaments slide against myosin filaments
o
Muscle contraction is triggered by a sudden rise in Ca2+
o
Muscle cells perform highly specialized functions in the body
Myosin II molecules can associate to form myosin filaments
o
Tails associate to form bipolar myosin filament- heads project outward from the
middle in opposite directions; globular heads- bind ATP
Actin filaments slide along myosin filaments
o
Slide actin filaments over each other = shortening of actin bundle
o
The myosin head walks towards the actin plus end of the actin, causing
contraction
Skeletal muscle is packed with myofibrils
o
In skeletal muscles, cells fuse to form long fibers, the cytoplasms contain
myofibrils
o
Myofibrils- contractile elements of the muscle cell
o
Sarcomeres- contractile units of the myofibrils
Sacromeres are the contractile unit of the muscle
o
Stacks of actin and myosin filaments
o
Contraction of a muscle is due to simultaneous shortening of all the sarcomeres
o
2 types- thin and thick. Thick filament (myosin)- centrally located; thin filament
