Cytoskeleton

Structure and assembly steps for microfilaments

1. Actin is a globular protein and binds to ATP 2. Actin monomer molecules are joined together in a twisted double strand to form a microfilament. 3. Microfilaments have polarity, with actin monomers added to the plus end preferentially and disassembled at the minus end. 4. Microfilaments can be nucleated at different sites in the cells. 5. Microfilaments can self-assemble without using energy from ATP, but its disassembly requires the hydrolysis of ATP. 6. Microfilaments assembly is controlled by ABP and form higher order structures, such as bundles and networks.

Intermediate Filament Structure and Assembly

1. Alpha and Beta tubulin 2. The essential microtubule subunit is a heterodimer of alpha and beta tubulin 3. In cross-section, a microtubule consists of 13 dimers arranged around a hollow core in longitudinal section, the rows of dimers are called protofilaments. 4. Microtubules have polarity, with new subunits added preferentially to the plus end and disassembly occur at the minus end. 5. Assembly and disassembly occurs rapidly, in response to CA++ and temperature changes. Microtubules are capable of self-assembly without energy, but GTP is generally added to the ends in order to stabilize the microtubule. 6. Microtubules are nucleated at the microtubule organizing center near the nucleus. 7. Microtubules can change rapidly in response to environmental factors

Intermediate filament Structure

1. IFP join together as a coiled coil homodimer. 2. Two dimers combine to make tetramers, which align to form protofilaments. 3. Eight protofilaments combine to form one intermediate filament. 4. Intermediate filaments nearly always need to be polymerized. 5. Intermediate filaments are dynamic as well, but difficult to disassemble

Basal body

1 centriole

Microtubule Function in Nuclear Division

1. Astral MTs shrink (minus end directed motors) 2. Interpolar MTs pull apart (plus end directed motors) 3. Kineticore MTs shrink (unknown mechanism)

What contractile protein makes up microfilaments?

Actin, which is the most abundant cellular protein.

Dynamic Instability

Assembly at the plus end of a microtubule

Specialized mictrotubule structures include

Cilia and flagella that are motile surface projections of cells

Centriole

Microtubules and connecting fibers

Cancer

Paclitaxel of taxol is used to treat lung, ovarian, and breast cancer.

Pathogenic bacteria

Shigella and Listeria, hijack actin and ABPs in order to move across the cell membrane and promote infectivity

Microvilli

Smaller Actin microfilaments Less motile Absorption, cell binding

Stereocilia

Specialized microvilli involved in hearing and balance

What is the cytoskeleton?

The cytoskeleton is an essential component of the cytoplasm. It is a dynamic 3-dimensional structure that acts like skeleton and muscle of a cell and are important for cell movement and stability.

Microfilament cellular localization

1. Microfilaments are found throughout the cytoplasm with no central organizing structure. 2. Microfilaments are concentrated at the periphery of the cell, just below the plasma membrane. They are in microvilli, stress fibers, cleavage furrow during cell division protrusive structures such as filopodia, lamellipodia, and ruffles.

MF Organization and Localization

1. Muscle Cells - sacromeres; muscle contraction 2. Non-Muscle Cells - control shape Stress fibers allow cells to maintain and change shape. Filopodia and lammelipodia allow cell movement. Microvilli and stereocillia from specialized protrusive structure. Cleavage furrow allow cell division..

Microtubule doublets

9+2

Centrioles

A barrel structure with 9 triplets arranged in a circle. Each triplet consists of 3 microtubules joined together. When the cell is not dividing, they are found as pared structure in the MTOC, lying at right angles to each other. During cell division, centrioles duplicate.

Kartageners Syndrome

A form of primary ciliary dyskinesia that is caused by dyne mutation. Patients have recurrent respiratory infections and fertility problems. This disorder is associated with situs inversus, suggesting a role for cilia in early development.

What is each filament type associated with?

A specific filament-associated protein in order to form a filament system.

What happens with actin in muscle cells?

Actin assembled into filaments work with myosin filaments in order to generate contractile forces.

Microfilament cellular functions 4. Anchoring of cytoplasmic proteins

Actin connects with membrane proteins via ABPS and the membrane skeleton, allowing plasma membrane to be anchored to the cytoskeleton.

Intermediate Filament Cellular Functions 2. Space Fillers

As in axons of neurons. Intermediate filaments in the nervous system are called neurofilaments. These fill and enlarge the exxon because the size of the axon determines the conduction velocity.

Cilia and Flagella Structure

Axoneme which consists of MT core and extends from pericentriolar material to tip of cilia and flagella. Basal body is composed of MTOC and is below pericentriolar material. They are 9 microtubule doublets (9+2)

Stress Fibers

Bundles of microfilaments near the plasma membrane that are important for cell shape and cell adhesion. Microfilaments associate with actin binding proteins in order to anchor the plasma membrane.

Intermediate Filaments Primary Functions

Cell strength and space filling. Found in different cell types and regions of cells, such as nuclear lamina.

MTOC

Centrosome, serves as nucleating center for microtubules. Composed of two centrioles, pericentrioloar material.

Microtubules

Cylindrical tubules, that are 20-25 nm in diameter. Acts as scaffolds to detainee cell shape and provide tracks for cell organelles and vesicles to move along. They also form spindle fibers for separating chromosomes during mitosis. When arranged in geometric patters inside flagella and cilia, they are used for locomotion. Building block: tubulin dimer Resist compression, but flexible Hollow cylinder

Cell Motility

Forces generated in the front by actin polymerization. Forces generated at the back by myosin-dependent contraction.

Microfilament cellular functions 2. Contraction

In association with ABPs like myosin to form contractile structures such as sacromeres in muscle cells and stress fibers in non-muscle cells

Bundled microfilaments by ABPs in microvilli

Increase surface area. Functions: 1. Absorption/secretion - small intestine/kidney 2. Adhesion - lymphocytes, eggs 3. Sensory transduction - taste cell

Dynein

Is end directed and moves from positive to negative end.

Cilias

Larger Microtubules (9+2) Motile due to dynein motors Move surface particles

Intermediate filaments

Larger than microfilaments, but smaller than microtubules. 10 nm in diameter 50 different types, but all have similar structures A cell may have more than one type of intermediate filament proteins, but they can all form their own filament. Apart of the cytoskeleton, but not essential for survival.

What is the cytoskeleton made of?

Long fibers or polymers assembled from protein subunits. The primary fibers of the cytoskeleton are microfilaments, intermediate filaments, and microtubules.

Microtubule Function - Intracellular transport includes

MTOC Anterograde transport Retrograde transport **Transport dictates cell polarity.

Microfilament cellular functions 3. Cell Motility

Mediated by actin polymerization and hep cell movement, contribute to membrane protrusion in filopodia and and lamellipodia, membrane ruffles, often in migrating cells as well as phasecytosis.

What are microfilaments?

Microfilaments are fine, thread-like protein fibers, 7 nm in diameter. They are the smallest filaments of the cytoskeleton and also the most abundant.

What happens with actin in non-muscle cells?

Microfilaments contribute to a wide range of cellular functions including motility, contraction, membrane protrusion, and cytokinesis.

Microfilament cellular functions 1. Cytoplasmic Viscosity

Microfilaments interact with ABPs to create a criss-crossed meshwork within the cytoplasm

Microtubules are associate with

Microtubule Associated Proteins which tend to stabilize the microtubule structure or function as motors.

Epithelial Keratin

Most common, perinuclear. Abundant in epithelial cells. Connect with cell junction. Form skin, hair, and nail.

Flagella

Motility (sperm)

Dyneins

Motor protein that move toward the negative end from (+ to -) Energy dependent, ATP.

Kinestins

Motor proteins of along microtubules by using ATP. Chinese is positive directed and moves from negative to positive end.

Kinesins

Motor proteins that move towards the plus (- to +) Energy dependent, GTP These motors traffic vesicles to synapse along microtubules.

Cilia

Move fluid over the cell surface, such as the lungs and oviduct. Coordinated beating, move fluid over the tissue surface.

Microfilament Diseases - Duchenne Muscular Dystrophy

Mutation in dystrophin, an ABP which links actin to plasma membrane. Without normal dystrophin, the cell membrane becomes weak or permeable, so that extracellular components enter the cell and cause muscle cell to explode and die

MTOC anchors the

Negative disassembling end, thereby reducing disassembly. The positive end extends into the cells periphery.

Neurofilament

Neurofilament provides tensile strength and structural integrity to axons. Space filling: increases axon diameter to increase the speed of conduction.

Hutchinson Gilford Syndrome (Progeria)

Premature gaining and death. Very rare genetic disorder due to the fact that Lamin A protein is not properly processed and cannot assemble into nuclear lamina.

Intermediate Filament Cellular Functions 1. Structural Strength

Prickle cells in skin

Nuclear Lamins

Provides structure rigidily and regulates the nuclear envelopes assembly and disassembly.

Why are microtubules important?

They are important in mitotic spindle (mitosis) and locomotion (flagella)

Microfilament cellular functions 5. Structural rigidity

This happens when actin microfilaments are bundled by ABP, as in microvilli and sterocillia

Filopodia and Lammelipodia

Transient projects on the leading edge of motile cells. Actin polymerization.

Microtubule functions

Transport of organelles and vesicles within the cell. Cell polarity. During nuclear division form mitotic spindles and used for cell movement in motile cilia and flagella (locomotion).

Deafness

mutations in an ABP can cause degeneration of sterocillia.