CYTOSKLELETON

 Cytoskeleton :- 

Definition: 

The cytoplasm of eukaryotic cells is spatially organized by a network of protein filaments known as the cytoskeleton. 

Or

A cell’s shape, internal organization, and functional polarity are provided by a three-dimensional filamentous protein network called the cytoskeleton.  

Functions of cytoskeleton:

       I. Cytoskeletons are responsible for large-scale cellular polarity, enabling cells to tell the difference between top and bottom, or front and back.

    II. Cytoskeleton must also provide stable, large-scale structures for cellular organization.

A.    Origin and Functions

This network contains three principal types of filaments:

        i.            Actin filaments: They are helical polymers of the protein actin.

a.          Actin F filament is made up of monomer i.e. Globular actin or G-actin.

b.         The G actin binds with ATP to carry out the polymerization to form F actin (Actin filament.

c.          It has polarity where one end is minus end where de-polymerization occurs whereas plus end is the end where polymerization occur by hydrolyzing ATP.

Function:

a)      They determines the shape of the cell’s surface.  

b)      They are necessary for whole-cell locomotion.

c)      They also drive the pinching of one cell into two.

d)     It underlie the plasma membrane of animal cells, providing strength and shape to its thin lipid bilayer.

      ii.            Microtubules: Microtubules are long, hollow cylinders made of the protein tubulin. They are much more rigid than actin filaments.

a.       It use kinesins and dyneins motor proteins to transport cargo and perform a variety of other functions within the cell.

b.      Tubulin heterodimer is made up of α tubulin and β tubulin subunit

c.       Both heterodimer can bind to GTP however only β subunit can hydrolyze GTP

d.      The α and β subunit joined together to form polar structure where one is always α subunit/ minus end and another end there is β subunit/plus end which is end for the addition of more heterodimer to elongate the microtubule. Thus ultimately forming the protofilament.

e.       Such 13 filaments comes together to form hollow tube like structure.

Functions:

a)      They determine the positions of membrane-enclosed organelles.

b)      They direct intracellular transport.

c)      They form the mitotic spindle that segregates chromosomes during cell division.

    iii.            Intermediate filaments:

a.       They have diameter of ~10 nm

b.      They form strong rope like structure

c.       It has alpha helical monomer having N and C terminal

d.      Dimer is formed in parallel helical structure.

e.       Two dimer come together to form staggered tetramer by joining in anti-parallel fashion with no polarity.

f.       These staggered tetramer comes together to form twisted rope like intermediate filaments of 10 nm diameter.

Functions:

a)      They provide mechanical strength.

b)      They form tough appendages such as hair and fingernails.

c)      They line the inner face of the nuclear envelope (nuclear lamina), forming a protective cage for the cell’s DNA.

All of these cytoskeletal filaments interact with hundreds of accessory proteins that regulate and link the filaments to other cell components, as well as to each other.

The accessory proteins are essential for the controlled assembly of the cytoskeletal filaments in particular locations.

They also include the motor proteins, which convert the energy of ATP hydrolysis into mechanical force that can either move organelles along the filaments or move the filaments themselves.

In living cells, accessory proteins modulate the dynamics and organization of cytoskeletal filaments, resulting in complex events such as cell division or migration.

Actin Binding Proteins:

1.      Formins: Promotes the addition of G-actin to the end of unbranched filament

2.      Profilin: binds subunits and speeds up elongation

3.      Arp2/3 complex: Binds to the side of an existing actin filament to nucleate actin branching

4.      Tropomyosin: Stabilizes the actin filament

5.      Tropomodulin: prevent assembly and disassembly at minus end

Actin and Myosin:

Myosin:  

a.       It is an elongated protein formed from two heavy chains and two copies of each of two light chains.

b.      Each heavy chain has a globular head domain at its N-terminus that contains the force-generating machinery, followed by an extended coiled-coil structure that mediates heavy-chain dimerization.

c.       The two light chains bind close to the N-terminal head, while the long coiled-coil tail bundles itself with the tails of other myosin molecules.

d.      These tail–tail interactions form large, bipolar “thick filaments” that have several hundred myosin heads, oriented in opposite directions at the two ends of the thick filament

Skeletal muscle is made up of myofibrils containing thousands of sarcomeres assembled from highly ordered arrays of actin and myosin II filaments, together with many accessory proteins.

A myofibril consists of a long, repeated chain of tiny contractile units— called sarcomeres, each about 2.2 μm long—which give the vertebrate myofibril its striated appearance.

Sarcomere structure:

        i. Each sarcomere is formed from a miniature, precisely ordered array of parallel and partly overlapping thin and thick filaments.

      ii. The thin filaments are composed of actin and associated proteins, and they are attached at their plus ends to a Z disc at each end of the sarcomere.

    iii.The capped minus ends of the actin filaments extend in toward the middle of the sarcomere, where they overlap with thick filaments,

Using their neck domain as a lever arm, myosins convert ATP hydrolysis into mechanical work to move along actin filaments in a stepwise fashion.

     Muscle contraction is stimulated by calcium, which causes the actin-filament-associated protein tropomyosin to move, uncovering myosin binding sites and allowing the filaments to slide past one another.

Centrosomes and Centriole:

Centrosomes:

       I.  The centrosome is the major microtubule organizing complex (MTOC) of animal cells.

    II.     Located in the cytoplasm next to the nucleus,

 III.       it consists of an amorphous matrix of fibrous proteins to which the γ-tubulin ring complexes that nucleate microtubule growth are attached.

 IV.    The minus end of each microtubule is embedded in the centrosome, having grown from a γ-tubulin ring complex, whereas the plus end of each microtubule is free in the cytoplasm.

Centrioles

        i.  It is a pair of cylindrical structures, embedded in the centrosome.

      ii.  It is arranged at right angles to each other in an L-shaped configuration.

    iii.  A centriole consists of a cylindrical array of short, modified microtubules arranged into a barrel shape with striking nine fold symmetry.

    iv. Together with a large number of accessory proteins, the centrioles organize the peri-centriolar material, where microtubule nucleation takes place.

      v The centrosome duplicates and splits into two parts before mitosis, each containing a duplicated centriole pair.  

    v  The two centrosomes move to opposite sides of the nucleus when mitosis begins, and they form the two poles of the mitotic spindle.