T6 Cytoskeleton BIO 107

Cytoskeleton function in prokaryotes

Internal network of divers

• maintenance of cell shale in many species

• Cell separation during cell division

• Movement of some cell contents

Composition of cytoskeletal strands

• chains of globular protein

• Bundles of fibrillar protein

• different elements have different kinds of protein providing different functions

Green Fluorescent Protein (GFP)

Gree fluorescent protein from green fluorescent jellyfish

• allows to label things in cell (cytoskeleton)

• Can image LIVE CELLS

Tubulin superfamily (FtsZ)

Globular proteins, FtsZ is protein of tubulin superfamily

• Forms a band around midpoint of cell (Z ring)

• Creates strangulation in cell during cell division that leads to septatuon, eventually resulting in two daughter cells

  • segments cell

Inhibition of FtsZ

• results in filamentous morphology and lysis

• New avenue for strains of pathogenic bacteria with multiple resistance to current treatments

  • antiobiotics

Actin superfamily

Globular proteins

• MreB

• ParM

MreB

Globular protein, protein of actin superfamily

• gives rod-shaped bacteria their rod shape

• Probably used building scaffold in the deposition of new cell wall during growth

ParM

Protein of actin superfamily

• moves plasmids to opposite ends of cell prior to division

• Only for low count plasmids

  • 1 copy not high count (100 of copies)

Crescentin

A coiled-coil filamentous protein (CCRP), family bending bacilli into curved shape

• without crescentin is rod shaped, needs to bend bacilli into curved rod

Cell movement

Mobility: the cell can move but it is not propelling itself .

• Ex. Red blood cells (cells are mobile)

Motility: cell can move by propelling itself with its own means of locomotion

• ex. E.coli bacteria

• Power own movement

Prokaryotic flagellum

Unlike in eukaryotes, flagellum is not part of cytoskeleton in prokaryotes

• hollow tube filament made of self organizing flagellum proteins

  • Not related to cytoskeletal proteins

• Grows by addition at the tip

• Basal apparatus (motor) anchored in the cell wall, protruding into cytoplasm

• The flagella filament is attached to motor by the hook

  • own support mechanisms in membrane

  • Different from cytoskeleton proteins

Flagellar arrangements

Flagella not part of cytoskeleton, but shape of cell is controlled by the cytoskeleton and that influence movement

• e.coli is petrichous it means that flagella are all around perimeter

• Locomotion dependent on shape of cell which is controlled by cytoskeleton

Flagella form bundles

Because geometry is the sane when turning direction is the same, flagella bundle into propelling unit

• multiple flagella bundle together to form one large flagellum

Direction of flagella movement

Run: Counterclockwise motor rotation causes flagellar bundling and directional movement ( forward )

Tumble: clockwise rotation causes unbundling and spreading of flagellation and the cell rotates (in place)

• opposite direction if twist unbundled→ separate whip around independently

• Flagella move independently to rotate bacteria

Chemotaxis

Guidance system; movement directed by the concentration gradient of chemicals

• concentration gradient is external to cell

• chemicals must be present to direct movement

• cycles of runs and tumbles

• Higher concentration of chemicals → less bacteria tumbles

• Longer runs, less tumbles to go towards chemical attractant

Type IV pili

Twitchy motion,

Rectractile protein filaments that move the cell closer to anchor point

• like using grappling hook

Lengths of protein extend from cell and retract into cell

Archaellum

Archael flagellum

• shorter and simpler in structure

• Archael flagellum similar to type IV pili of bacteria

• Grows by addition at base of archaellum, not tip

• Rotational motion

• Powered by ArP, not proton gradient

• Lacking detailed knowledge about subject

Periplasmic flagellum

• known as axial filaments

• found in spirochaetes

External structures: internal to the outer membrane, but eternal to the plasma membrane

• outside cell between plasma membrane and outer membrane (periplasmic space)

• make entire cell rotate in corkscrew motion

Is flagella or axial filament more suitable for mucuous environment?

Flagella: have to push big bacteria through thick medium not efficient

Axial filaments: thin bacterium corkscrew entire body

How e.coli bacteria move?

Runs- swims in straight line, counterclockwise, flagella moves as singular bundle

Tumble - rotation and spreading of flagella, clockwise, flagellar bundle falls apart

Flagellum vs archaellum

Size

• archaellum is smaller and simpler

Filament growth

• Flagellum grows from tip

• Archaellum grows from base

Energy source

• flagellum is powered by proton gradient

• Archaellum is powered by ATP

Hook

• flagellum has hook, archaellum does not

Comparison of eukaryotic and prokaryotic cytoskeleton function

Maintaining shape: both

Move things during replication: both

Move things during regular physiological processes: eukaryotes

Propel the cell or its surroundings: eukaryotes

Cytoskeleton

Composed of different classes of fibers, it forms vast network throughout the cell

• dynamic scaffolding and locomotion of Eukaryotic cell

Cytoskeletal fibers:Microfilaments (cortex underneath membrane),micro tubules ( in every cell), intermediate filaments

DNA in nucleus

Broad functions of cytoskeleton

1. Maintain and change cell shape

2. Spatially organize cell contents

3. Connect cells and environment

3 classes of cytoskeletal fibres

1. Microtubules (tubulin family)

   • polar (directional) (not charged)

   • Globular monomers

2. Microfilaments (actin family)

   • polar (directional in different way)

   • Globular monomers

3. Intermediate filaments

• apolar

• Filamentous monomers

Microtubules

• hollow tubes made of tubulin heterodimers

• Stiff structural tubes

• Strong compression

• They can help maintain the cell shape under pressure

  • can push not pull

• Function as tracks to move cell contents and cell

  • microtubulin track for cell contents to move on

• tubulin assembles into hollow Microtubules made of dimers of alpha and beta tubulin

  • polar : 1 end alpha. 1 end beta

Microtubules as tracks for vesicle transport

Motor proteins attach to both Microtubules and cargo (usually vesicle) to transport it along Microtubules to specific destination

Motor proteins on Microtubules are polar

Kinesin moves toward plus end ( anterograde transport)

Dye in moves toward the minus end (retrograde transport)

Both walk on tube, hand over hand fashion

• Kinesins and dyenins are ATP powered and move in opposite directions

Eukaryotic cilia and flagella composition and structure

Eukaryotic cilia and flagella are both internal structures (within cell membrane) made of Microtubules

• both are ATP powered with same configuration

Basal body: 9 Microtubules triplets it’s no central Microtubules (9+0)

Motile portion: 9 Microtubules doublets with a central Microtubules pair (9+2)

Dyenins move flagellum

Dyenins attached to neighbouring doublet “walk” in surface pfmicrotubule outlet facing it

• dyenin moves toward minus end

1. Dynein arms attach:The dynein motor domain on the A-tubule binds to the B-tubule of the adjacent doublet.

2. ATP binds dynein → releases microtubule.

3. ATP causes dynein to detach from the B-tubule.

4. ATP hydrolysis → conformational change (“power stroke”).

5. Dynein undergoes a large structural shift that slides the A-tubule relative to the B-tubule.

6. ADP + Pi released → dynein reattaches at a new position.

7. The cycle repeats, producing continuous sliding motion between adjacent microtubules.

How flagella bend

Movement is restricted to cause bending, dyenin prevented from walking past each other

• Bending motion required to bend outlet, anchorage (basal body) , elastic component

• If microtubule doublets could slide freely, the flagellum would just extend and contract (like a telescope).

• But nexin links and radial spokes restrict that sliding.

• So, when dynein on one side of the flagellum is active, that side shortens, and the other side stays extended.

• This asymmetric activity causes the flagellum to bend.

Compare eukaryotic cilia and flagella structure

Eukaryotic cilia and flagella have same structure and same evolutionary origin

They differ in size and type of novement:

• flagellum: undulates symmetrically side to side

• Cilium: beats asymmetrically with a different stroke on each side

  • depending on which side bearing towards

  • Power stroke, extended recovery strokes

    • extend straight then push

Motility vs mobility: sperm and ova

Sperm is motile : soerm cells propel themselves with flagellum

Ova. (Eggs) are mobile: beating cilia on epithelial cells of Fallopian tubes propel themselves ovum

• do not propel themselves

• Propelled by cilia n fallopian tube

Microtubules radiate from MTOCs

Centrosome is an MTOC; Centrosome organize Microtubules

• Centrosome functions as Microtubules organizing center (MTOC)

• Other structures serve the sane purpose in a scenes of centrioles (such as I plants and fungi)

Microtubules role in mitosis

Microtubules guide chromosomes during mitosis

• Microtubules are part of spindle apparatus (cell division lectures)

Microfilaments (actin filaments/ thin filaments)

• G actin = protein monomer if Microfilaments (g for globular)

• G-actin is polymerized into F-actin to form filaments (f for filamentous)

• Microfilaments provide tensile strength to various parts of cell

  • good under tension, cannot resist compressive forces

• Not strong under compression

• Help shape cell

• Provide movement to the cell and cell membrane

• Formation of F-actin requires ATP

  • polymerization of G-actin

• Microfilaments form cell cortex (sub membrane shape cortex)

• Microfilaments give structure to some cells

  • microvilli (outfolds) on intestinal epithelial cells are internally supported by actin (Microfilaments)

Actin myosin system used for cell motion and cell contraction

How muscle cells are able to contract individually and collectively to produce muscle contraction; how muscle cells generate motion

• ATP powered

Actin myosin system for muscle cell contraction steps

The Cross-Bridge Cycle (Actin–Myosin Interaction)

1. ATP binds myosin head →
   Myosin detaches from actin.

2. ATP hydrolyzed (ATP → ADP + Pi) →
   Myosin head “cocks” into a high-energy position.

3. Myosin binds to actin (if Ca²⁺ present) →
   Cross-bridge forms between actin and myosin.

4. Power stroke:
   Pi released → myosin head pivots, pulling actin toward the M-line → sarcomere shortens.

   1. Aline is pulled closer together towards M line in center to make cell contact

5. ADP released, new ATP binds → cycle repeats.

Amoeboid movement

Most common types of crawling movement in Eukaryotes driven by actin filaments and myosin motor proteins pulling on them, forming pseudopodia

• shape membrane by changing shape of actin myofilaments

• Projections called pseudopodia

1. Extension of a pseudopodium

   • At the leading edge of the cell, actin filaments polymerize (assemble) beneath the plasma membrane.

   • This pushes the membrane forward, forming a bulge — the pseudopodium.

2. Attachment to the surface
   The pseudopodium adheres to the substrate using adhesion proteins (like integrins).

• This anchors the front end of the cell.

3. Cytoplasmic streaming
   The internal fluid (cytoplasm) flows forward into the pseudopodium — called endoplasmic streaming or sol–gel transformation.

• The actin cortex near the front remains more gel-like (dense), while the back becomes more fluid.

4. Contraction at the rear
   Myosin II interacts with actin filaments at the rear (posterior) of the cell, pulling the cell body forward.

• This contraction drags the rest of the cytoplasm toward the pseudopodium.

5. Detachment of the rear
   The old adhesion points at the rear detach, completing one “step.”

Then the process repeats: extend → attach → pull → detach.

Cellular streaming in plants

An actin myosin system

• actin myosin used to move organelles in plants

• Chloroplast move by actin and myosin heads

Bacteria that manipulate actin based cytoskeleton

Echlira chaffeensis (gram negative) is an obligate intracellularvpaparsite that hijacks cytoskeleton of neutrophils to find other neutrophils for infection

• parasitic bacteria hijack cytoskeleton of neutrophils → membrane reshaped to find other host cells

Intermediate filaments

• Large family of 50+ proteins

• Twist to form cable like structures of very high tensile strength

• Toughen parts of cell and morph cell into specific shape

• ANIMAL ONLY STRUCTURE

• diameter/ thickness is between Microfilaments and Microtubules

Intermediate filaments bundle formation

Ifs come in bundles of coiled coils

• monomers intertwine to form coiled coil dimers with strong disqualified bridges (and other interactions)

• Dimers pair up staggered to form tetramers

• Filamentous proteins , threads not globular , pair of intertwined threads stagger to form ropes of intermediate filaments

Desmosome fibers composition

Desmosome fibers are made of keratin (an IF protein)

• Desmosome needs high tensile strength to keep cells together

• Keratin IFs project from the Desmosome plate to inside both cells

Lamin

Lamin: protein that forms nuclear lamina, type of intermediate filaments

• net structure lines nuclear envelope where chromosome attached

Extracellular Keratin

Extracellular keratin is a type of extracellular intermediate filament

• extracellular keratin forms tough structures like claws, horns, beaks, feathers, hair, nails, etc.