Lecture 14: Actin Cell Migration

Actin

- determines cell shape
- cell contraction and motility (migration) in non-muscle cells
- muscle contraction (in muscle cells)
- participates in cell-cell and cell-matrix junctions
- participates in cytokinesis

Actin monomers

- Globular actin (G-actin) that bind to ATP/ADP, but can join a filament when bound to ATP.

Actin filaments

- F-actin - 2 parallel protofilaments of G-actin form a right-handed helix, G-actin bound to ATP assembles at the dynamic + end to form an actin microfilament
- Undergo dynamic remodeling:
-Plus "barbed" end - growth occurs at this end
- Minus "pointed" end - loss occurs at this end

Treadmilling of actin filaments

- Actin filaments can maintain a consistent length while actively being remolded.
- Occurs when G-actin- ATP is at the critical concentration

Kinetics of F-actin assembly in vitro

- 2-Actin monomers are added at the + end only.
- 3 - Action monomers are lost from the - end.
- 4-5- Equal numbers ofactin monomers areadded at the + end asare lost from - end(steady state =treadmilling)

Actin ability to assemble into different shapes

- Through interaction with actin binding proteins actin is able to assemble into different configurations in different cells and indifferent regions of the same cell

Actin Binding Proteins (ABPs)

- Regulate assembly of actin monomers.
- Nucleating proteins
- Monomer-sequestering proteins
- End-blocking (capping) proteins
- Monomer-polymerizing proteins.
- Actin filament depolymerizing proteins
- Cross-linking proteins
- Filament-serving proteins
- Membrane-binding proteins

Nucleating Proteins (ABP)

- Provide a template for adding actin monomers

Monomer-sequestering proteins

- Bind to actin-ATP monomers and prevent them from polymerizing

End-blocking (capping) proteins

- Regulate the length of actin filaments

Monomer-polymerizing proteins

- Promote the growth of actin filaments

Actin filament depolymerizing proteins

- Bind actin-ADP subunits at the minus end promoting rapid turnover of actin filaments

Cross-linking proteins

- Alter the 3D organization of actin filaments

Filament-serving proteins

- Shorten filaments and decrease cytoplasmic viscosity

Membrane-binding proteins

- Link contractile proteins to plasma membrane

Myosin II motor protein

- The head binds to the actin
- The head is an ATP dependent motor that interacts with the actin filaments
- The tail binds to other myosin molecules (AKA the cargo)
- Myosin II heavy chains assemble into a bipolar myosin filament
- Actin filaments slide along myosin filaments to mediate cellular contraction in non-muscle cells

Cell migration

- The movement of cells from site of origin to final location
- AKA cell location

Steps in cell migration

- Step 0: Polarization
- Step 1: Protrusion
- Step 2: Adhesion
- Step 3: Contraction
- Step 4: Disassemble

Step 0: Polarization

- A chemotactic (or other) initiating signal establishes direction of movement

Step 1: Protrusion

- Protrusion of a lamelipodium is actin-driven.
- Direction of cell movement

Step 2: Adhesion

- Cell forms new attachments with the substratum under the lamellipodia.
- Cells form adhesions with the substrate.

Step 3: Contraction

- The cell body is pulled forward by actin-myosin mediated contraction.

Step 4: Disassemble

- The force pulling on adhesions towards the rear of the cell causes them to disassemble.

Microtubules - Summary

- Subunits incorporated into polymer: GTP alpha-beta tubulin heterodimer.
- Preferred site of incorporation: + End (Beta tubulin)
- Polarity: Yes
- Enzymatic activity: GTPase
- Motor Proteins: Kinesins, Dynein
- Major group of associated proteins: MAPs
- Structure: Stiff, hollow tube
- Dimensions: 25 nm outer diam.
- Distribution: All Eukaryotes
- Primary Functions: Support, Intracellular Transport, Cell Organization

Intermediate Filaments - Summary

- Subunits incorporated into polymer: Various globular proteins
- Preferred site of incorporation: Internal
- Polarity: No
- Enzymatic activity: None
- Motor Proteins: None
- Major group of associated proteins: Plakins
- Structure: Tough, ropelike fibers
- Dimensions: 10 nm diam.
- Distribution: Animals
- Primary Functions: Structural Support

Actin Filaments - Summary

- Subunits incorporated into polymer: ATP - actin monomers
- Preferred site of incorporation: + End (Barbed)
- Polarity: Yes
- Enzymatic activity: ATPase
- Motor Proteins: Myosins
- Major group of associated proteins: Actin-binding proteins (ABPs)
- Structure:: Flexible, helical filament
- Dimensions: 8 nm diam.
- Distribution: All Eukaryotes
- Primary Functions: Motility, Contractility.