Chapter 14 B (Cell Bio)

Myosin I

Myosin II

Myosin V

What are the motor proteins of microfilament-based cell movements ?

At the N-terminal of myosin

Where are the two heads of myosin II located ?

ATPase activity

What activity do myosin II heads have ?

At the base of the heads

Where are the light chains and regulatory chains located ?

Connects to the tail through a hinge region

What does the neck of myosin II connected to ?

Bundles of extended myosin II heavy chains, with myosin heads positioned at the ends.

What is the myosin superstructure ?

Phosphorylation

What regulates myosin assembly into thick filaments ?

Ca²⁺ plays a critical role in complex regulatory events.

What role does calcium play in myosin assembly ?

Extension of myosin heads towards microfilaments.

What is the first step of the myosin cross-bridge cycle ?

Binding of ATP and ATP hydrolysis

What is the second step of the myosin cross-bridge cycle ?

Binding of myosin to actin on microfilaments

• Release of ATP breakdown products.

• Large free energy release.

• Push of myosin on microfilament moving head back to 90˚

• position. “Power-stroke”

What is the third step of the myosin cross-bridge cycle ?

Binding of another ATP molecules

• Facilitates disengagement.

• Cycle begins again.

What is the fourth step of the myosin cross-bridge cycle ?

Muscle fibers

What is skeletal muscle composed of ?

Bundles of myofibrils

What are muscle fibers composed of ?

Functional units called sarcomeres

What are myofibrils divided into ?

Has Lots of mitochondria

Networks of sarcoplasmic reticulum (SR)

Multiple nuclei

What features are listed for a sarcomere ?

Actin filaments or microfilaments

What are thin filaments made of ?

Bundles of 300–400 extended myosin II molecules

What are thick filaments made of ?

Z-lines

these form the boundaries of individual sarcomeres.

A Band

is a zone of overlap between the thin and thick filaments.

Myosin heads attach to and pull thin filaments, bringing Z-lines closer together

How do thick filaments cause sarcomere shortening ?

Coordinated shortening of sarcomeres, which shortens the muscle and causes movement of limbs or other structures.

What is the basis of muscle contraction ?

They can be controlled at will, unlike smooth and cardiac muscles

How are voluntary skeletal muscles controlled ?

Actin-linked regulation

Myosin-linked regulation

What are the two control mechanisms of muscle contraction ?

In striated muscles (skeletal and cardiac muscles)

Where does actin-linked regulation occur ?

Troponin (TN) and tropomyosin (TM).

What two proteins control actin-linked regulation ?

The ON and OFF states of the sarcomere

What do troponin and tropomyosin control ?

TN and TM move off the myosin-binding sites, allowing myosin heads to access them. The cross-bridge cycle causes contraction.

What happens in the ON state ?

TN and TM cover the myosin-binding sites. Myosin cannot bind to thin filaments, so there is no cross-bridge cycle or contraction.

What happens in the OFF state ?

Controls conformational changes that determine the ability of TN and TM to bind to actin filaments.

What is the role of calcium in actin-linked regulation ?

The ATPase activity of myosin heads

What do myosin light chains (MLC) regulate ?

Through phosphorylation and dephosphorylation events

How is myosin light-chain activity regulated ?

Acts together with calmodulin to activate and deactivate myosin light chain kinase (MLCK).

What does calcium do with calmodulin ?

Myosin to interact with actin filaments

What does phosphorylation of myosin light chains allow ?

Myosin light chain kinase is inactive

No phosphate groups are added to myosin light chains

Myosin does not interact with actin filaments

What happens at low calcium concentration ?

Calcium-calmodulin activates MLCK

Myosin light chains are phosphorylated

The cross-bridge cycle occurs

What happens at high calcium concentration ?

Sarcoplasmic reticulum

vesicles within the sarcoplasm that can sequester or release Ca²⁺.

Gated-calcium channels

Located in SR membranes; they open to release calcium into the sarcoplasm.

Calcium ATPase Pumps

Returns calcium into the sarcoplasmic reticulum; The pumps are regulated by calcium-calmodulin complex.

Cytoplasmic streaming

Cell migration

Amoeboid motion

Cytokinesis in animal cells

Extension of the acrosome of sperm cells

What are examples of microfilament-based motility in non-muscle cells ?