IMS1 TBL3 AKT Revision AI Flashcards

Cytoskeleton

A network of intracellular fibrillar proteins that provides structural organisation, mechanical support and enables movement in cells

Three main cytoskeleton filament types

Microfilaments, intermediate filaments, microtubules

Microfilaments

The thinnest cytoskeleton filaments composed primarily of actin

Diameter of microfilaments

Approximately 7 nanometres

Intermediate filaments

Cytoskeletal filaments providing tensile strength and mechanical stability

Diameter of intermediate filaments

Approximately 7 to 12 nanometres

Microtubules

Hollow cytoskeletal filaments composed of tubulin dimers

Diameter of microtubules

Approximately 25 nanometres

What does the cytoskeleton provide to the cell?

(1) Structural scaffold (2) Compartmentalisation (3) Movement and transport control (4) Mechanical linkage between cells

Structural scaffold role of cytoskeleton

Maintains cell shape and adapts shape depending on environment

Compartmentalisation role of cytoskeleton

Separates regions of the cell and positions organelles

Movement role of cytoskeleton

Enables cell migration, phagocytosis, contraction, intracellular transport and mitosis

Strength role of cytoskeleton

Connects to other cells and substrates to provide rigidity and elasticity to tissues

Microfilaments main protein

Actin

What is actin?

A globular protein that polymerises into filamentous structures forming microfilaments

Functions of microfilaments

Cell shape control, membrane support, motility, cytokinesis and intracellular trafficking

Intermediate filament primary function

Mechanical strength and resistance to stress

Proteins forming intermediate filaments

Keratins, lamins and specialised variants

Where are neurofilaments found?

Neurons

Microtubules main protein subunit

Tubulin

Functions of microtubules

Intracellular transport, mitotic spindle formation, maintenance of polarity and structural organisation

Definition of cytoplasmic streaming

Movement of cytoplasm facilitating transport inside cells

How does the cytoskeleton support cell movement?

Through polymerisation and depolymerisation of filaments generating pushing or pulling forces

What is the actin-based membrane skeleton?

A submembrane network of actin supporting the plasma membrane

Function of actin membrane skeleton

Restricts lateral mobility of membrane proteins and stabilises membrane surface

What controls degree of diffusion restriction at membrane?

Strength of interaction between proteins and actin skeleton

What is the picket and fence model?

Model describing how actin filaments form compartments restricting membrane protein movement

How do membrane proteins move between compartments?

Some remain confined while others move between compartments by hopping

Example of cytoskeleton-driven movement

Leukocyte migration during immune response

How does a leukocyte move?

Forms a lamellipodium which protrudes forward through rapid actin assembly

Lamellipodium

Broad sheet-like protrusion containing dynamic branching actin networks

Filopodia

Thin finger-like protrusions extending beyond lamellipodia containing parallel bundled actin filaments

Role of lamellipodia in cell migration

Generate forward pushing force

Role of filopodia in cell migration

Environmental sensing and directional steering

Growth cone

Motile structure at axon tip using actin-based protrusions for navigation

Role of microtubules in motility

Provide polarity orientation and transport materials toward leading edge

What enables lamellipodium extension?

Rapid polymerisation and depolymerisation of actin monomers

What is polymerisation in cytoskeleton?

Assembly of individual protein subunits to form filaments

Dynamic equilibrium

Balance between monomer pool and polymerised filaments in steady state

What do regulatory proteins do in filament dynamics?

Bind monomers or filament ends to modify growth or disassembly

Comparison between actin filaments and microtubules dynamics

Both use polymerisation mechanisms but differ in regulation and nucleotide usage

Dynamics of intermediate filaments

Less dynamic, more stable and not highly polarised

Summary functional differences between filaments

Actin shapes and moves cell, intermediate filaments stabilise cell, microtubules organise polarity and transport

Which filament type supports cytokinesis?

Microfilaments

Which filament type forms mitotic spindle?

Microtubules

Which filament type gives tensile strength?

Intermediate filaments

Actin monomer name

G-actin

Actin filament name

F-actin

Polymerisation

Process of assembling smaller subunits to form linear filaments

Depolymerisation

Disassembly of polymerised filaments into monomers

Intracellular trafficking

Movement of vesicles and organelles along cytoskeletal tracks

Cell polarity

Spatial differences between cell front and rear determining movement direction

Intermediate filaments

Structural protein filaments providing mechanical strength and stability within cells

Why they are called intermediate filaments

Their diameter is between microfilaments and microtubules

Diameter range of intermediate filaments

About 8 to 12 nanometres

Main functional role of intermediate filaments

To provide tensile strength and resist mechanical stress

Where intermediate filaments are found

Almost all living cells including long-lasting dead structures such as hair and nails

Most common intermediate filament protein

Alpha-keratin

What is keratin

Structural protein forming tough filament assemblies in epithelial and skin-derived structures

Number of amino acids per keratin monomer

Approximately 300 amino acids

What bonds hold keratin bundles together

Disulphide bonds

Which amino acid enables disulphide bonding in keratin

Cysteine

Why keratin structures are strong

High cysteine content allowing dense cross-linking between protein chains

How hair and nails are formed

Cells accumulate keratin bundles, die and leave hardened keratin-filled structures

Examples of tissues rich in keratin

Hair, nails, claws, horns, wool, feathers, scales and outer skin layer

Keratin function in epithelial cells

Maintains structural support and resistance to stress

Why epithelial tissues need mechanical strength

To withstand stretching, bending, friction and fluid forces

How many major classes of intermediate filament proteins exist

Five

Type I and II intermediate filaments

Acidic and basic keratins

Where Type I and II filaments are found

Epithelial cells such as epidermis and bladder lining

Primary function of Type I and II keratins

Provide tissue rigidity and prevent tearing or blistering

Main examples of Type III filaments

Vimentin, desmin, GFAP and peripherin

Where vimentin is found

Fibroblasts, endothelial cells and leukocytes

Where desmin is found

Cardiac muscle, skeletal muscle and within desmosome junctions

Function of desmin

Aligns and stabilises sarcomeres by linking neighbouring myofibrils

Where GFAP is found

Astrocytes and other glial cells

Function of GFAP

Provides mechanical support in glial networks and binds to associated proteins

Where peripherin is found

Peripheral neurons

What are lamins

Intermediate filament proteins forming the nuclear lamina

Where lamins are located

Just beneath the inner nuclear membrane

Function of lamins

Provide nuclear structural support and enable nuclear envelope reformation after mitosis

Overall architecture of intermediate filaments

Helical rope-like fibres lacking polarity

What allows intermediate filaments to have no polarity

Antiparallel arrangement during assembly

Stages of intermediate filament assembly

Monomer, dimer, tetramer, protofibril, complete filament

What is a dimer

Two monomers aligned in parallel forming a helical pair

What is a tetramer

Two dimers aligned in opposite directions forming a symmetrical unit

Why tetramers lead to no filament polarity

Each end contains both amino and carboxyl-terminal regions

What are protofibrils

Linear chains of tetramers joined end-to-end

How a complete intermediate filament forms

Four protofibrils assemble side-by-side into a stable rope-like filament

What is a desmosome

Specialised junction connecting intermediate filaments between neighbouring cells

Role of desmosomes

Provide mechanical resistance allowing tissues to withstand pulling forces

Primary linker protein in desmosomes

Desmoplakin

Role of desmoplakin

Anchors intermediate filaments to cell–cell junction complexes

Desmosomal cadherins

Proteins bridging cell membranes between adjacent cells

Examples of filament types linked through desmoplakin

Keratin in epithelia, desmin in muscle and vimentin in connective tissue

Function of desmin in muscle

Maintains sarcomere alignment by anchoring contractile units

Why desmin is clinically important

Loss of desmin integrity disrupts muscle contraction

How intermediate filaments are regulated

Through controlled phosphorylation and depolymerisation under specific conditions

Why phosphorylation matters

Weakens filament interactions and can dismantle structures during mitosis