Cytoskeleton part 2

Function of Actin

Actin filaments have a wide array of functions, including:

• Support for microvilli in intestinal epithelial cells.

• Enabling cell movement.

• Forming the contractile ring during cell division.

• Muscle contraction.

• Vesicle transport.

structure and assembly of actin filaments

Structure: Actin filaments are thin, flexible protein threads. They are composed of identical actin monomers that stack into a two-stranded helical molecule. They are thinner (7 nm) than microtubules and intermediate filaments.

Polarity: They have polarity, with a distinct plus (barbed) end and a minus (pointed) end.

Assembly: Actin polymerizes by a similar mechanism to tubulin, including a process called treadmilling. Monomers bind to the plus end, and subunits with bound ADP dissociate from the minus end.

Regulation of actin assembly and organization

Actin-binding proteins control its behavior.

• Monomer-sequestering proteins: These make monomers unavailable for polymerization. An example is thymosin.

• Monomer-polymerizing proteins: These facilitate actin-ATP binding and assembly. An example is profilin.

• Other functions include filament nucleating, end-blocking (capping), cross-linking, bundling, membrane-binding, depolymerizing, and filament-severing.

.Describe how actins are used to drive cell movement

protrusion of the leading edge

adehesion of the lemellipodium to the substratum

movement of the bulk of the cell forward

cell prepares for the next protrusion of the lamellipodium

Characterize the structure of myosin, outline the roles of different myosin domains, state its direction of movement along actin

• Structure: Myosin (most types) consists of two heavy chains and various associated light chains. 

• Myosin Domains and Roles: 

• Head (S1 fragment): Attaches to the actin fiber; its movement is enabled by ATP hydrolysis. 

• Tail: Used to attach to cargo or to form polymers (in myosin II). 

• Light chains: Affect the movement of the head. 

• Direction of Movement: All myosins (except type VI) move from the pointed (minus) end to the barbed (plus) end of the actin filament. 

Contrast the structure of unconventional and conventional myosins

• unconventional Myosin (Type I, V, VI): Found in all cells. They can contain one or two subunits and don't form filaments.

• Conventional Myosin (Type II): Found in muscle cells and forms filaments (specifically, bipolar filaments) .

State that vesicles may be associated with motor proteins that interact with different kinds of cytoskeleton

Vesicles may be associated with different motor proteins that interact with different kinds of cytoskeleton (microfilaments or microtubules). Vesicle trafficking mostly happens along actin filaments at the periphery of the cell.

Draw and label the structure of a skeletal muscle and sarcomere

Skeletal Muscle: Muscle bundles contain specialized muscle cells (muscle fibers) that have multiple nuclei. Each muscle fiber contains hundreds of thin myofibril strands.

Sarcomere: Each myofibril is a linear array of contracting units called sarcomeres. Sarcomeres are the contractile units of muscle and are separated by Z discs

characterize the arrangement of actin and myosin filaments in a sarcomere, and use the sliding filament model to describe the contraction of a sarcomere

Filament Arrangement: Thin actin filaments are attached to the Z discs and extend towards the middle. Thick myosin filaments are in the center and partly overlap the actin filaments. Actin is sandwiched between myosin fibers in a hexagonal pattern.

Sliding Filament Model: Contraction is described by the sliding filament model, where actin filaments slide against myosin filaments. This sliding action pulls the Z discs closer together, shortening the sarcomere

Characterize the structure of myosin II and the actin filament in a muscle cell.

Myosin II (Thick Filament): Myosin II proteins assemble into a thick multimer. They form bipolar filaments that have a double-headed arrangement , with a bare region in the center (myosin tails only) and myosin heads extending outward.

Actin Filament (Thin Filament): Actin is arranged in a normal helical fiber. It is surrounded by the long tubular protein tropomyosi

Describe how ATP hydrolysis drives the sliding of myosin against actin filaments.

ATP binding: Causes myosin head to release the actin filament.

ATP hydrolysis: Cocks the myosin head.

Power Stroke: The release of inorganic phosphate triggers the power stroke, causing a force-generating change in shape that slides the actin filament. The myosin head then reattaches to a new position on the actin filament.

Outline the role of sarcoplasmic reticulum and Ca2+ ions in regulating muscle contraction

Sarcoplasmic Reticulum (SR): The SR surrounds myofibrils and stores Ca2+ ions. These ions are released into the cytoplasm in response to an action potential.

Ca2+ Ions: Ca2+ release causes a sudden rise in cytoplasmic Ca2+ concentration (up to 250x from rest) , which triggers muscle contraction. Ca2+ is reabsorbed using active transport to relax the muscle.

Describe how Ca2+ affects troponin and tropomyosin shape and how that regulates the interaction between actin and myosin

At Rest (No Ca2+): Tropomyosin blocks the myosin binding site on actin. Troponin stabilizes tropomyosin.

During Contraction (With Ca2+): Ca2+ influx changes the troponin conformation. This shift in troponin then shifts tropomyosin, which exposes the myosin binding sites on actin , enabling the interaction with myosin and muscle contraction.