Lecture 12: Cytoskeleton Part 2 (ACTIN)

Actin’s subunit

• polar monomer protein

• Asymmetric

Steps of Actin Polymerization

1. Nucleation

2. Elongation

3. Steady State

Nucleation Step

• formation of a stable protein nucleus with THREE actin monomers

• Slowest step 

How many actin do we need to even think about starting nucleation in vitro

3+

Critical Concentration

concentration of monomers required for polymerization to occur

Elongation Phase

the rapid addition of monomers to both ends (but obviously the plus end growing faster)

Steady State Phase

where the rate of loss & addition of monomers is equal 

TRUE OR FALSE: The binding properties at the plus and minus end of actin are unequal

True, because the plus side is strong binding (high ON rate) the minus side is weak binding (high OFF rate)

Why the plus and minus end of ACTIN have different binding properties 

Actin’s innate ATP binding & hydrolyzing abilities 

ATP-actin (where, how it’s diff)

•  at the plus end 

• bind nice and tight to the filaments creating the ATP CAP

• more stable 

• les

ADP-Actin

• at the minus end 

• binds very TERRIBLY to the suits 

• less stable 

TRUE OR FALSE: can ADP actin on its own still polymerize? 

Yes, but it’d, be so mid liek it’s not even funny,

also, the sides would both be ass 

(exactly what it is pls)Tread-milling

• ATP-Actin added and then is hydrolyzed to ADP actin and then dissociates

• rate of addition equals rate of loss 

• LENTGH REMAINS THE SAME 

Different actin structures in cells are determined by…

spending on the protein they interact with

Lamellipodia & folipodia

cell crawling

The FOUR classes of actin binding proteins

1. Nucleate Filaments 

2. Promote Disassembly 

3. Organize Filaments

4. Motor Proteins  

Actin Bidnding Protein: Nucleating Actin Filaments 

• ARP Complex 

• Formin 

Actin Binding Protein: Promote Disassembly

1. Severing Proteins (cut em up_

2. Capping (plus end blocking protein) 

3. Monomorer sequestering protein 

Monomer Sequestering Protein

•  “come her little actin”

• actin binding protein that binds monomers and prevents them from being added to filaments

 

Proteins that organize actin filaments (box like a fish)

• Crosslink Proteins

• Bundling Proteins

Crosslink Proteins

organize actin into networks by creating branches off filaments

Bundlign Protein

organize actin filaments into parallel bundles

True or false: Actin filaments Drive Cell Migration

True

Step by Step of Cell Migration

1. actin formwork forms at the leading edge of migrating cells where sheets (lamellipoidum) & protrusions (filopodia)

2. Front end is pushed out and the back pulls forward with it

Lamellipodium

branched actin network involved in cell migration

Filopodium 

straight actin filaments involved in cell migration (Phil is straight and Mitch is gay) 

Actin Cortex

dense region of actin underlying the PM

Focal Contancts

attachment points that occur between the cell & the surface crawling upon, think of ANCHORS

True or false: The actin is being polymerized at the leading edge and disassembling at the lagged edge of the cell

TRUE

ARP Complex

• an actin nucleator that builds bridges by binding to one side of a filament and nucleating a new one

• the arp complex stays bound and hold the negative end of the actin filament

• Importnat for lamellipodia and filopodia

Formin

• actin nucleator STABILIZES the initial trimer

• NOT BRANCHED

• Formin stays associated after & promotes the polymerization by helping the plus end.

• help nucleation and elongation 🙂