116A WEEK 8 – Cytoskeleton and Motility
Learning outcomes:
1. Define the cytoskeleton and its functions
2. Describe the structural elements of the cytoskeleton: cytoskeletal filaments
and motors
3. Explain how the cytoskeleton is regulated
4. Describe the microtubule- and actin-based systems for cell motility
5. Describe cell migration and its types
Prelecture notes:
- The cytoskeleton consists of three main components:
o Actin filaments: Thinnest, involved in muscle contraction, cell
movement, and division. Forms networks that allow cells to change
shape and move
o Microtubules: Thickest, responsible for intracellular transport, structural
support, and cell division. Forms tracks for vesicle movement and aid
in chromosome separation during cell division. Microtubules also
support structures like flagella and cilia for cell movement
o Intermediate filaments: Provides mechanical strength and stability.
These rope-like structures anchor organelles and cells to their
surroundings, and their types vary across cell types, including keratins
in skin and hair, vimentin in muscle cells, and lamins around the
nucleus.
What is the cytoskeleton?
- A network of interconnected filaments
- Polymers (provide structure)
- Dynamic (direct rearrangement)
Function:
- Cell structure and mechanics
- Force generation – motility
- Intracellular transport
- Cell division
Why is it important?
- Keeps the cell shape, rearranges contents
- Cell division, migration, motility, intracellular transport
- Force generation and sensing
- Dysregulation leads to pathological conditions (invasive cancer etc)
Types of cytoskeletal filaments
Actin filaments Microtubules Intermediate filaments
Diameter/repeat 7nm/36nm 25nm/4nm ~10nm/various
length
Monomer Actin Tubulin ( & tubulin) Lamin/desmin/vimentin/keratin
etc
Polar? Ends are Yes Yes No
different
,Functions Muscle contraction Mitotic spindle Cell shape/stiffness
Cell motility Intracellular trafficking Nuclear lamina
(crawling) of organelles
Cytokinesis Microtubule-based
Cell shape structures (cilia,
centrosomes,
axonemes)
Types of cytoskeletal filaments and motors:
- Actin filaments (microfilaments) myosin
- Microtubules kinesin, dynein
- Intermediate filaments doesn’t have any known
motors
Molecular motors
- Consumes ATP
- Makes steps along the filaments
- Transports cargo, rearranges membranes and
cytoskeletal networks
Mechano-chemical cycle of kinesin stepping
- 1. ADP: strongly bound state
- 2. ATP: power stroke
- 3. ADP.Pi: Release
- One step = 8nm (one tubulin dimer)
Cytoplasmic and axonemal dynein
- Cytoplasmic dynein: movement of cargos along
microtubules, cell division
- Axonemal dynein: beating of cilia and flagella
Kinesin and dynein motors:
- Dynein:
o Always minus-end directed
o Can back-step under load
o One motor, many adaptors (adaptors define cargo specificity)
, - Kinesin:
o Can be moving toward minus-end, plus-end or non-motile
o Typically, does not back-step
o Multiple sub-families with multiple members
Actomyosin motor system:
- Muscles (myosin II)
- ‘Leading edge’ of migrating cells (myosin I & V)
- Contractile ring during cytokinesis (myosin II)
- Myosin V makes steps along the actin filaments
Polymer theory:
- Typical polymers:
o Concentration polymerisation
- Actin: grows from + end, shortens from -end, “treadmilling”
Actin filaments’ polymerisation
- Two intertwined protofilaments
- Growth from barbed (+)- end
- Shortening from pointed (-)-end
- Polymerisation consumes ATP
- Treadmilling
Regulation of actin polymerisation
- Profilin – binds ATP actin and promotes polymerisation
- Thymosin – binds ATP actin and blocks polymerisation
- Arp2/3 – promotes nucleation and branching
, - Formins – binds actin filaments and promotes elongation
- Capping proteins – binds the ends of a filament; prevents further
loss/addition of subunits e.g. CapZ; tropomodulins
- ADF/Cofilin – binds G-actin and F-actin (also severs filaments)
Regulation of actin polymerisation
- 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 e.g. CapZ; tropomodulins
- ADF/cofilin- binds G-actin and F-actin (also severs filaments)
Intermediate filaments
- Eight protofilaments joined end-to-end with
staggered overlap
- No known polarity (symmetric)
- Dynamic properties poorly studied
- They provide resistance against rupturing (below)
Learning outcomes:
1. Define the cytoskeleton and its functions
2. Describe the structural elements of the cytoskeleton: cytoskeletal filaments
and motors
3. Explain how the cytoskeleton is regulated
4. Describe the microtubule- and actin-based systems for cell motility
5. Describe cell migration and its types
Prelecture notes:
- The cytoskeleton consists of three main components:
o Actin filaments: Thinnest, involved in muscle contraction, cell
movement, and division. Forms networks that allow cells to change
shape and move
o Microtubules: Thickest, responsible for intracellular transport, structural
support, and cell division. Forms tracks for vesicle movement and aid
in chromosome separation during cell division. Microtubules also
support structures like flagella and cilia for cell movement
o Intermediate filaments: Provides mechanical strength and stability.
These rope-like structures anchor organelles and cells to their
surroundings, and their types vary across cell types, including keratins
in skin and hair, vimentin in muscle cells, and lamins around the
nucleus.
What is the cytoskeleton?
- A network of interconnected filaments
- Polymers (provide structure)
- Dynamic (direct rearrangement)
Function:
- Cell structure and mechanics
- Force generation – motility
- Intracellular transport
- Cell division
Why is it important?
- Keeps the cell shape, rearranges contents
- Cell division, migration, motility, intracellular transport
- Force generation and sensing
- Dysregulation leads to pathological conditions (invasive cancer etc)
Types of cytoskeletal filaments
Actin filaments Microtubules Intermediate filaments
Diameter/repeat 7nm/36nm 25nm/4nm ~10nm/various
length
Monomer Actin Tubulin ( & tubulin) Lamin/desmin/vimentin/keratin
etc
Polar? Ends are Yes Yes No
different
,Functions Muscle contraction Mitotic spindle Cell shape/stiffness
Cell motility Intracellular trafficking Nuclear lamina
(crawling) of organelles
Cytokinesis Microtubule-based
Cell shape structures (cilia,
centrosomes,
axonemes)
Types of cytoskeletal filaments and motors:
- Actin filaments (microfilaments) myosin
- Microtubules kinesin, dynein
- Intermediate filaments doesn’t have any known
motors
Molecular motors
- Consumes ATP
- Makes steps along the filaments
- Transports cargo, rearranges membranes and
cytoskeletal networks
Mechano-chemical cycle of kinesin stepping
- 1. ADP: strongly bound state
- 2. ATP: power stroke
- 3. ADP.Pi: Release
- One step = 8nm (one tubulin dimer)
Cytoplasmic and axonemal dynein
- Cytoplasmic dynein: movement of cargos along
microtubules, cell division
- Axonemal dynein: beating of cilia and flagella
Kinesin and dynein motors:
- Dynein:
o Always minus-end directed
o Can back-step under load
o One motor, many adaptors (adaptors define cargo specificity)
, - Kinesin:
o Can be moving toward minus-end, plus-end or non-motile
o Typically, does not back-step
o Multiple sub-families with multiple members
Actomyosin motor system:
- Muscles (myosin II)
- ‘Leading edge’ of migrating cells (myosin I & V)
- Contractile ring during cytokinesis (myosin II)
- Myosin V makes steps along the actin filaments
Polymer theory:
- Typical polymers:
o Concentration polymerisation
- Actin: grows from + end, shortens from -end, “treadmilling”
Actin filaments’ polymerisation
- Two intertwined protofilaments
- Growth from barbed (+)- end
- Shortening from pointed (-)-end
- Polymerisation consumes ATP
- Treadmilling
Regulation of actin polymerisation
- Profilin – binds ATP actin and promotes polymerisation
- Thymosin – binds ATP actin and blocks polymerisation
- Arp2/3 – promotes nucleation and branching
, - Formins – binds actin filaments and promotes elongation
- Capping proteins – binds the ends of a filament; prevents further
loss/addition of subunits e.g. CapZ; tropomodulins
- ADF/Cofilin – binds G-actin and F-actin (also severs filaments)
Regulation of actin polymerisation
- 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 e.g. CapZ; tropomodulins
- ADF/cofilin- binds G-actin and F-actin (also severs filaments)
Intermediate filaments
- Eight protofilaments joined end-to-end with
staggered overlap
- No known polarity (symmetric)
- Dynamic properties poorly studied
- They provide resistance against rupturing (below)
