Contractile Proteins Flashcards
_______ and its binding proteins determine shape of the cell, cell locomotion and pinching of one cell into two.
Actin
Motor proteins, such as _______, are molecular machines that convert biochemical energy from ATP hydrolysis to mechanical energy that can move organelles along filaments or move filaments on proteins.
Myosin
Each actin subunit is called ________ actin. This is a 375 amino acid polypeptide carrying a tightly associated molecule of ATP or ADP.
Globular (G-actin)
G-actin subunits assemble head to tail to form a tight, right-handed helix called _________ actin.
Filamentous (F-actin)
F-actin has a slow growing (or degrading) ______ end and a faster growing ______ end (grows 5-10x faster).
Minus
Plus
Small oligomers of actin associate spontaneously but are unstable and disassemble readily. For a new actin filament to form, subunits must assemble into an initial aggregate (nucleus made of 3 actin monomers) that will elongate rapidly. This is called…
Filament nucleation
T/F. Actin monomers have bound ATP which is hydrolyzed to ADP and Pi following filament assembly. ATP is not required for polymerization, but actin will polymerize more rapidly with it.
True
What are the phases of assembly of actin polymers?
Nucleation (lag phase) Elongation (growth phase) Steady state (equilibrium phase)
The rate of subunit association is proportional to the concentration of free monomers. This is written as _________.
C x Kon
***C = concentration of free monomers
The rate of subunit dissociation is independent of monomer concentration. This is written as…
Koff
An apparent equilibrium is reached at the critical concentration of monomers (Cc), where ______ = ______.
Koff = Cc x Kon
Actin filaments organized into higher order structures, forming bundles or 3D networks within cells. Filaments are particularly abundant beneath the plasma membrane, where they form a network that does what?
- Provides mechanical support
- Determines cell shape
- Allows movement of cell surface – enabling cells to migrate, engulf particles, and divide
Actin filaments are assembled into two general types of structures, formed via interactions with accessory proteins. They are…
Actin bundles
Actin networks
Actin _______ are cross-linked into closely packed parallel arrays. Polarity of actin filaments is the same.
Bundles
Actin _______ are loosely cross-linked in orthogonal arrays that form 3D meshwork with more flexible gel-like properties. Makes cell flexible and polarity of actin filaments are different.
Networks
Actin filaments and _______ form a parallel bundle (actin bundle) that involves tight packing, preventing Myosin from entering the bundle. Holds two parallel filaments close together (i.e., Microvilli).
Fimbrin
***Has 2 actin binding domains
Actin filaments and _______ form a contractile bundle (actin bundle) that involves loose packing, allowing Myosin to enter the bundle (i.e., Contractile ring used in mitosis).
Alpha-actinin
***Binds as a dimer
Actin filaments in networks are held together by large actin-binding proteins. ________ binds actin as a dimer, with the actin-binding domains on opposite ends of the dimer. This creates a 3D meshwork and are present in cells that need to withstand forces.
Filamin
These are thin projections of the plasma membrane supported by actin bundles. The formation and retraction of these structures is based on the regulated assembly and disassembly of actin filaments.
Filopodia
These are broad, sheet-like extensions at the leading edge of cell containing a network of actin filaments.
Lamellipodia
These are based on actin filaments cross-linked into a 3D network, responsible for phagocytosis.
Pseudopodia
Erythrocytes contain an actin binding protein called ________. It is composed of tetramers which associate laterally and forms an actin network that creates a cortical cytoskeleton.
Spectrin
Spectrin-Actin network interacts with membrane proteins via interactions with ________ and ________.
Ankyrin
Protein 4.1
This disease is caused by mutations in erythrocyte cortical skeleton proteins (Spectrin, Nnkyrin, Protein 4.1). Causes impaired deformability and reduced stability of RBCs. Cells appear very spherical, membrane breaks down and cells lyse and die.
Hereditary Spherocytosis (HS)
This is a superfamily of motor proteins that move along actin filaments via ATP hydrolysis.
Myosin
This type of myosin are composed of bipolar filaments. They have tails that associated form the shaft of the filament and heads exposed at both ends that “walk” along actin.
Skeletal Muscle Myosin II
Myosin has 3 major domains, which are…
Head
Neck
Tail
This domain of myosin contains the actin binding and ATP binding sites, and has ATPase activity.
Head
This domain of myosin is a flexible region that binds myosin light chain peptides.
Neck
This domain of myosin intertwines to bring myosin head regions in close proximity. It binds to membranes/organelles.
Tail
This class of myosin has 1 heavy and 1 light chain. Its functions are membrane association and endocytosis.
Myosin I
This class of myosin has 2 heavy and 2 light chains, and its function is contraction. It is the only class that can assemble into bipolar filaments through tail interactions.
Myosin II
This class of myosin has 2 heavy and 6 light chains, and its function is organelle transport.
Myosin V
Put the following steps of myosin movement along F-actin in order from start to finish:
A. Hydrolysis of ATP to ADP + Pi, myosin head rotates into “cocked” state.
B. “Power stroke” – Release of P and i elastic energy straightens myosin; moves actin filaments left. Conformational change in neck.
C. ADP released, ATP bound; Head released from actin.
D. Myosin head binds actin filament.
E. Binds ATP, head released from actin.
1) E
2) A
3) D
4) B
5) C
The power stroke mechanism is proportional to what?
Length of neck domain
***Longer neck domain = increased rate of movement
Skeletal muscle (THIN/THICK) filaments are made of 6 myosin polypeptide chains. They have 1 pair of heavy chains which form the tail, and 2 pairs of light chains which form the head.
Thick
Skeletal muscle (THIN/THICK) filaments are made of 3 proteins: actin, tropomyosin, and troponin.
Thin
Actin has myosin binding sites, and when the muscle fiber is at rest then myosin binding sites are covered by _________ so that actin and myosin cannot interact. ________ is a calcium binding protein that helps initiate contraction.
Tropomyosin
Troponin
This protein is made of long, elastic molecules. One end attaches to Z disk and the other end attaches to the M band. It interacts with thick filaments (myosin) and prevents catastrophic overstretch of sarcomere. It give contractility to the thick filament.
Titin
_______ protein caps the (+) end of the thin filaments at the Z disk, while ________ caps the (-) end.
CapZ
Tropomodulin
This protein binds actin subunits and determines the length of the thin filament.
Nebulin
Calcium induces a conformational change in _________, which loosens its hold on _________, which loosens its hold on actin so that myosin head can bind to actin and power movement.
Troponin
Tropomyosin
When this enzyme is phosphorylated, it allows the interaction of myosin with actin to power movement. When it is dephosphorylated, it does NOT allow interaction.
Myosin light-chain kinase
T/F. Non-muscle cells contain several types of actin-myosin structures similar to skeletal muscle fibers, but they are much less stable or organized. They are formed in a transient manner as need by cells (for example, cytokinesis).
True
During this process, bundles of F-actin and myosin II form a contractile ring and the myosin moment along actin filaments creates a cleavage furrow.
Cytokinesis
This type of myosin can carry cargo along actin filaments.
Myosin V
This is part of a protein complex that links the cytoskeleton muscle fibers to the surrounding CT (basal lamina). It is a long protein with numerous redundant coils that provides a structural link between the cytoskeleton of the muscle cell and the ECM (acts like shock absorber during contraction).
Dystrophin
This protein stabilizes the sarcolemma and prevents contraction-induced injury (rupture). Without it being functional to support muscle strength and stability, muscle fibers are easily damaged.
Dystrophin
***Links cytoskeleton (actin filaments) to the cell membrane
This is a disease associated with loss of dystrophin.
Duchenne’s Muscular Dystrophy
This disease is X-linked recessive and occurs in 1 in 3500 boys. It presents as progressive muscle wasting and patients are confined to wheelchairs by age 12, and usually die of respiratory failure by age 22.
Duchenne’s Muscular Dystrophy (DMD)
Explain DMD gene carriers.
Females who have a normal dystrophin gene on one X chromosome and an abnormal dystrophin gene on the other X chromosome.
There is a milder form of muscular dystrophy with a later onset, which is called…
Becker Muscular Dystrophy (BMD)
DMD patients are ________ mutants, meaning they have little to no expression of dystrophin.
Out-of-frame
BMD patients are ________ mutants, meaning they have smaller proteins with partial function of dystrophin.
In-frame
What are treatments being developed to help with DMD or BMD?
Gene therapy
Dystrophin replacement
Drugs to preven exon skipping
