20 Cytoskeleton II- Cell Bio

What are microfilaments (actin filaments) composed of?

globular actin proteins (monomers) aligned into a twisted two-stranded helix (37 nm repeat).

What’s the diameter of microfilaments?

7 nm

What side does actin add to in microfilaments?

more likely to add to the plus end

What is the structure of actin monomers?

Fold into two lobes with a deep cleft in between them with either ADP or ATP

What do ATP-bound monomers in microfilaments do?

Tend to polymerize into a stable actin filament

Does actin use GTP or ATP?

ATPase

How does actin polymerize (grow and disassemble)?

Actin-ATP added to the microfilaments plus end which is then hydrolyzed to ADP causing a conformation change, locking it in.

What happens at the minus end when Actin-ATP is added to the microfilament?

Actin-ADP dissociates at the minus end

T or F: The binding of ATP to actin and tubulin promotes the polymerization of these cytoskeletal structures.

False, only actin, tubulin is GTPase

What drugs affect filaments?

Phalloidin- stabilizes by binding

Cytochalasin- prevents polymerization by capping the plus end

Latrunculin- prevents polymerization by binding to actin monomers

What can modify microfilament behaviors?

actin-binding proteins

What are the two main types of actin-binding proteins, and what do they do?

Nucleating protein = promote polymerization

Monomer sequestering protein = binds monomers; reduces the chance of polymerization

T or F: Actin-binding proteins regulate when and where actin filaments will form and grow.

True

How many protofilaments are found in a typical actin filament?

2

2 parallel protofilaments twist around each other to form a right-handed helic

How many protofilaments do microtubules have?

13

What is the role of actin filaments in microvilli form?

Lots, secretion, absorption, etc. (stable)

What is the role of actin filaments as contractile bundles?

Drive contraction through ATP-driven movements of myosin II, which facilitate cell division, tissue integrity, and mechanotransduction (stable).

What is the role of actin filaments as protrusions?

needed for cell movement (temporary)

What is the role of actin filaments as a cytokinetic contractile ring?

Divides cells (temporary)

What does cell movement/ crawling depend on?

cortical actin

What is the actin cortex?

helps with cell movement, thin layer of actin meshwork that uniformly underlies the plasma membrane of the entire cell

What is Lamellipodium?

helps with cell movement, thin (0.1-0.3 um) and usually long (1-5 μm) projection that adheres to the underlying substrate

What is the primary energy source for cell crawling?

ATP (most actin polymerization and movement requires ATP)

What does actin polymerization result in the protrusion of?

at the edge of the cell, lamellipodia (branched actin network) and filopodia (unbranched actin network), both motile structures that form and retract at great speed

What are Lamellipodia nucleated by?

Actin-Related Protein

What do integrin transmembrane proteins do?

a part of cell movement, binds to extracellular proteins and internal microfilaments, and interact with the crawling cell’s environment

What is collagen?

main structural protein in the extracellular matrix

What is fibronectin?

important ECM protein that mediates cell-ECM interaction

T or F: The polysaccharide chains on the extracellular side of the transmembrane proteins are added by specific enzymes in the ECM.

False, the sugar chain is made in the ER and Golgi apparatus and added inside the cell, not by enzymes outside

What are the extracellular signals that can alter arrangement of actin filaments from?

a family of Rho GTP binding proteins (GTPase)

Rho, Rac, and Cdc42

What does Rho do?

Promotes contractile bundles

What does Rac do?

promote lamellipodia

What does Cdc42 do?

promote filopodia

How are contractile structures formed?

actin associates with myosin I and mysoin II

How does myosin I interact with actin?

Its head attaches to the actin filament, and its tail attaches to another molecule of an organelle in the cell, involved in vesicle movement, moves toward + end of the actin, and uses ATP hydrolysis to move along the actin

How does myosin II interact with actin?

they associate with other myosin II molecules to form bipolar myosin filaments and then move opposite as actin filaments, creating a contractile force, move toward + end of actin filament

T or F: Myosin II is responsible for muscle contraction by forming cross-bridges with actin filaments and moving toward the + end of the actin filaments

True

What are myofibrils?

repeating sarcomeres (contractile units of muscles) packed in skeletal muscle cells

T or F: The - end of actin filaments attaches to the Z disc in sarcomere.

False, the plus end attaches

How do muscles contract by the sliding-filament mechanism?

Plus end of the actin filaments attached to the Z disc of sarcomeres, myosin attached to actin filaments slide towards the plus end, contracting the muscles.

What part of myosin II walks along an actin filament?

the head with ATP and conformational changes

Which structure shortens during muscle contraction?

sarcomeres

What triggers muscle contraction?

sudden ride in cytosolic Ca2+

What do transverse tubules do?

convey the excitatory signal from an activating neuron to the sarcoplasmic reticulum when there is a rise in Ca2+

What is the sarcoplasmic reticulum?

specialized region of the ER that stores Ca2+

What stops myosin from binding to actin and starting muscle contractions?

tropomyosin that binds to microfilaments until Ca2+ is released from the sarcoplasmic reticulum, which binds to the troponin complex, changing the conformation of everything, allowing myosin to interact with actin

When a muscle is stimulated to contract, myosin heads walk along actin filaments in repeated cycles of attachment and detachment. What is a correct description of the events in this cycle?

Myosin attaches to actin; ATP binding reduces the affinity of myosin for actin; myosin is cocked as its head is displaced along the actin filament; the power stroke puts myosin in a rigor configuration