Cytoskeleton and cell motility

What is the cytoskeleton? 2 points

• network of proteins within the cell

• gives shape and anchors organelles

What does the dynamic network of the cytoskeleton allow cells to do? (4 points)

• move

• contract

• divide

• pull/push

State the 3 proteins that the cytoskeleton is comprised of

• microfilaments (actin filaments)

• microtubules

• intermediate filaments

What are actin filaments made up of?

2 strands of actin wrapped around eachother

Where are action filaments located?

below the cell membrane and throughout the cytoplasm.

List four functions of microfilaments

• Muscle contraction

• Cell motility (crawling)

• Cytokinesis

• Cell shape

Are actin filaments polar?

yes

What motor does actin filaments use?

myosin

How do actin filaments facilitate changes in cell shape?

slide close to eachother and further apart

How do actin filaments facilitate muscle contraction?

they interact with myosin to generate force and shorten the muscle fiber.

How do actin filaments facilitate cell movement?

psuedopodia

What are pseudopodia?

structures used by neutrophils to crawl in and out of blo9od vessels during diapedesis

What is diapedesis?

the process by which white blood cells move out of the circulatory system and into tissues to reach sites of infection or inflammation.

How are pseudopodia formed?

rapid polymerisation of actin in one direction, creating a hook

Diapedesis: step 1

hook wedges between endothelial cells of blood vessels

Diapedesis: step 2

neutrophil squeezes through to the other side of the endothelial layer.

How do actin filaments facilitate cytokinesis movement?

ring of actin filaments form a contractile ring between the two nuclei that pinches the cell membrane, leading to cell division.

What is the largest protein structure in the cytoplasm?

microtubules

Describe the assembly of a microtubule (2 points)

• two α and β tubulin line up to form a protofilament

• 13 protofilaments come together to form a microtubule

List 4 functions of microtubules

• Mitotic spindle

• Intracellular trafficking of organelles

• Microtubule-based structures (cilia ,centrosomes, axonemes)

• Resist compressive forces and so maintain the cells structure

Are microtubules polar?

yes

What motors do microtubules use?

kinesin and dynein

How do microtubules facilitate intracellular trafficking of organelles?

they span across the cell, allowing them to be used as railroads for intracellular transport

Describe the assembly of a centrosome (2 points)

• 9 microtubule triplets make up a centriole

• 2 centrioles at a right angle make up a centrosome

How do microtubules facilitate the formation of the mitotic spindle?

microtubules from the centrosome polymerise in the direction of kietochores (mitotic spindle) embedded in the centromeres of chromatids

How do microtubules facilitate cell movement?

Formation of cilia and flagella

What are motile systems?

system of motor proteins and filaments that self-organise to produce movement and/or transport

List 2 types of microtubule based motility

• Cytoplasmic microtubules

• Axonemal microtubules

Cytoplasmic microtubules in cell motility

spindle fibre formation

Axonemal microtubules

cilia and flagella

What do centrioles form to nucleate cilia and flagella?

basal bodies

How long are cilia?

2 – 10μm

How do cilia move?

“oar-like” beating pattern

How long are flagellum?

10 – 200μm

How do eukaryotic flagella move?

propagated bending motion

How do prokaryotic flagella move?

rotating in a corkscrew-like motion

Describe the structure of cilia and flagella (2 points)

• 9-fold symmetry (9 + 2)

• Microtubule sliding thanks to stepping of dynein between microtubules

State 4 proteins intermediate filaments can be

• lamin

• desmin

• vimentin

• keratin

List 3 functions of intermediate filaments

• fastens organelles in place

• anchor cells to neighboring ones in cell to cell junctions

• anchor cells to environment

Are intermediate filaments polar?

no

Describe the structure of intermediate filaments

eight protofilaments joined end-to-end with staggered overlap

What do molecular motors do?

use ATP to transport cargo, rearrange membranes and cytoskeletal networks by making steps along the filament

List the mechano-chemical cycle of kinesin stepping (3 points)

• ADP: strongly bound state

• ATP: power stroke

• ADP∙Pi: release

What does cytoplasmic dynein handle?

movement of cargos along microtubules and cell division

What does axonemal dynein handle

beating of cilia and flagella

Dynein: direction

always minus-end directed

Dynein: can it backstep?

can back-step under load

Dynein: how many motors?

one motor, many adaptors (adaptors define cargo specificity)

Kinesin: direction

can be moving towards minus-end, plus- end, or non-motile

Kinesin: can it backstep?

typically does not back-step

Kinesin: how many motors?

multiple sub-families with multiple members

Myosin II (2 points)

• muscles

• contractile ring during cytokinesis

Myosin I & IV

“leading edge” of migrating cells

How do mictrotubules polymerise? (2 points)

• polymerisation and depolymerisation coexist in steady state conditions

• grow from both ends and switch stochastically

How do actin filaments polymerise?

• two intertwined protofilaments

• growth from barbed (+) end, shortening from pointed (-) end (treadmilling)

State 6 proteins that regulate actin polymerisation

• profilin

• thymosin

• arp2/3

• formins

• capping proteins

• ADF/cofilin

Profilin

binds ATP actin and promotes polymerization

Thymosin

binds ATP actin and blocks polymerization

Arp2/3

promotes nucleation and branching

Formins

bind actin filaments and promote elongation

Capping proteins

bind the ends of a filament; prevent further loss/addition of subunits

ADF/cofilin

binds G-actin and F-actin (also severs filaments)

What acts at microtubule ends to regulate microtubule dynamics? (2 points)

• Catastrophe-promoting factors (EB, MCAK)

• Rescue factors (CLASP, kinetochore proteins)

What acts at microtubule walls to regulate microtubule dynamics? (2 points

• Severing enzymes (katanin): extract

  a tubulin from microtubule lattice

• Lattice stabilising proteins (tau)

Thick filaments in muscles

bundles of myosin II motors arranged via staggered coiled coils

Thin filaments in muscles (2 points)

• actin that interdigitates with the thick filaments

• tropomyosin and troponin stabilise the filament

Actin-based motility: endocytosis (2 points)

• Clathin-coated pits

• Actin polymerisation and myosin V motility helps to form and detach a vesicle

Cell migration: crawling (3 points)

• crawling: extend protrusions

• filopodia: thin, finger-like

• lamellopodia: large, flattened

Cell migration: amoeboid (3 points)

• amoebas and white blood cells

• pseudopodia (false foot)

• 3D migration

Cell crawling and adhesion (4 points)

1. actin polymerisation drives protrusions of lamellopodia

2. new attachment sites form at the front of the cell

3. acto-myosin contraction pulls the rest of the cell forward

4. focal adhesions at the rear detach

Integrin receptors

link cells to the extracellular matrix (e.g. collagen)

Cadherin receptors

link cells to other cells