Cell motility: Microtubules and actin microfilaments

Question 1

Cell motility needs

Answer 1

Energy
Guidance
Mechanical interaction with something outside the cells
Microtubules (swimming) vs Microfilaments (crawling)

Question 2

Microtubules: hollow tubes of alpha and beta tubulin

Answer 2

Rigid structure because of tube and bonding
Tubulin diameter - made up of one alpha tubulin (-) and one beta tubulin (+), is 24 nm
Always come as pair (alpha and beta) as are oppositely charged
13 profilaments make up a microtubule

Question 3

Microtubules-based motility

Answer 3

Structure:
- Cilia
- Flagella
Cargoes:
- Of cells
- Protozoa
- Sperm
- Of fluids
- Respiratory
- Reproductive tract
Cilia used to move fluid around e.g. in trachea

Question 4

Cilia and flagella structure

Answer 4

Same structure in cilia and flagella
Different length:
- Cilia: 2-10 x 0.25 μm
- Flagella: 100-200 x 0.25 μm
Major functional structure is the axoneme

Question 5

The axoneme

Answer 5

9+2 microtubule assembly
9 doublets and 2 in the middle (inner pair)
There are radial spokes pointing from front outer doublet towards the inner pair on each doublet

Question 6

Linking structure to function - the outer doublets

Answer 6

Dynein arms reach from the complete fibre of one doublet towards to incomplete fibres of the doublet in front of it
The outer arm is the one on the outside and the inner arm is the one of the inside
A complete (A) fibres is made of 13 protofilaments
An incomplete (B) fibres is made of 10 protofilaments
There is not always contact between the dynein arms, mainly on one side
Gives polarity between different sides of axinein

Question 7

Dynein and sliding

Answer 7

In isolated doublet microtubules dynein and ATP produces microtubule sliding

Question 8

Nexin crosslinkers

Answer 8

In normal flagellum dynein causes microtubule bending due to the addition of cross linking proteins

Question 9

Cilia and flagella

Answer 9

Different waveforms:
- Cilia: has power strokes and then recovery strokes
- Flagella: Just has even waves across structure
Dynein arm-dependent functions:
- Inner arm: Waveform
- Outer arm: Power stroke

Question 10

The axoneme in full

Answer 10

Transition zone - where microtubule ends and basal body begins
Basal bodies - link cytoskeleton to axoneme
(Look at diagram)

Question 11

Basal bodies - go back over this section

Answer 11

9 x 3 microtubule array
9 triplets and no central ones
0.2 μm x 0.4 μm (diameter x length)
Basal body «< centriole «< centrosome

Question 12

Centrosomes

Answer 12

In a cell that doesn’t have flagella just see microtubules throughout cell that doesn’t extent past outside cell and is embedded in the centrosome

Question 13

Actin-based motility - A perspective of scale

Answer 13

Muscle - moves organism
Non-muscle - moves cells/tissues
Motility form:
- Motors
- Turnover

Question 14

Actin filament structure

Answer 14

Fast growing plus end and a slow growing/disassembling minus end
ATP hydrolysed to ADP when the filament is polymerised
Look at diagram

Question 15

Microtubule structure

Answer 15

Made of two tubulin monomers (one alpha and one beta)
Has a positive end that is slow growing/falling apart and negative end that is growing quickly during polymeraization

Question 16

Assembly of polarised filaments - go back over

Answer 16

Pointed end (-ve)
Barbed end (+ve)

Question 17

Actin treadmilling

Answer 17

Need to go over

Question 18

Actin filaments vs microtubules

Answer 18

Look at table
Alpha subunits always buried so never comes into action of hydrolysed so that’s why it stays as GTP

Question 19

Actin-binding proteins

Answer 19

Monomers can become monomer-sequestering or monomer nucleating
Monomer nucleating can become monomer polymerizing
Monomer polymerizing can become membrane-binding, depolymerizing, filament-severing, bundling, cross-linking or end-blocking (capping)
look at diagram it makes it more clear

Question 20

Myosin

Answer 20

The microfilament motor protein of cytoskeletons and muscle
Dynein is motor protein of Cilia/flagella

Question 21

Myosin strucuture

Answer 21

Hinge domain where ATP used, allows head domain to move and attach/disattach to myosin binding site on actin filament
Myosin binding site is covered by a protein tropomyosin which is controlled by calcium and this means that the muscle is not always contracted
Look at diagram

Question 22

Actin based motility

Answer 22

Filopodium - parallel bundles
Lamellipodium - branched and crosslinked networks
Stress fibres - antiparallel contractile structures
Cortical actin
Cortex - crosslinked networks
Guidance is given by growth factors - Cell reaches out towards what the stimulus is