Download Actin - WordPress.com

MCB Lecture 13 – Actin











What is G-actin?
o Globular Actin, which assembles to form F-Actin
What is F-actin?
o Fibrous Actin, made from G-actin combining
There are two types of filaments made from F-actin. What are they?
o Microfilaments
o Thin Filaments
What are the important functions of Actin?
o Cell Surface Shape
o Muscle Contraction
Which end of Actin grows to form the filament?
o Both.
o – End is slow growing
o + End is fast growing
What happens when a high concentration of ATP is bound to G-Actin?
o Monomers add to both ends, but add faster at the + end.
o The Hydrolysis of ATP is slow.
What happens when a low concentration of ATP is bound to G-Actin?
o Treadmilling can occur if it is not too low. This is where monomers
add to + end and come off of the – end at the same rate, causing the
length of the filament to remain the same as it moves.
o Too much can lead to dissociation.
Where is the highest density of actin filaments located?
o At the cell periphery
In Actin Nucleation, proteins are needed to elongate the strands. There are
three main proteins. What are the names of these proteins, and what do they
do?
o Arp2/3: Stimulates nucleation of G-Actin into dimers/trimmers
(initiates polymerization)
o Formins: Nucleate growth of straight, unbranched actin filaments
o Profilin: Exchanges ADP for ATP, so monomers can continue
attaching.
What complex attaches to create a branched actin network?
o ARP Complex
There are four proteins that modify filament elongation. Given each protein
name, state what is does:
o Thymosin?
 Binds G-Actin
 Prevents F-Actin Assembly
o Profilin?
 Competes with Thymosin
 Binds G-Actin
 Promotes filament assembly








o Cofilin?
 Promotes disassembly at the – end
o Gelsolin?
 Caps + end to prevent further growth
What is the difference between Lamellipodia and Filipodia?
o Lamellipodia push the cell forwards for movement
o Filipodia act as sensors that respond to chemotaxic factors
What type of Myosin is used for Contractile Muscles?
o Type II Myosin
Actin is organized differently in the cell depending on if it is a Contractile
Bundle (Stress Fiber), it is Lamellipodia, or it is Filipodia. How are the fibers
arranged in these three?
o Contractile Bundle: Straight, loosely packed, anti-parallel
o Lamellipodia: Random, loose
o Filipodia: Tightly packed and parallel
What three cross-linking proteins form parallel cables/ bundles?
o A-Actin
o Fimbrin
o Villin
What two cross-linking proteins form webs/gels?
o Spectrin
o Filamin
Which Actin Protein is contractile?
o A-Actin
Which Actin Protein is non-contractile?
o Fimbrin
Answer the following questions about Stress Fibers and Focal Adhesions:
o What are Focal Adhesions used for generally?
 Cell movement
o What are Stress Fibers?
 Contractile bundles of actin filaments inside of the cell
o What are Focal Adhesions?
 A clustering of integrin receptors at the membrane
o Where to the integrin receptors bind?
 The extracellular matrix
o What do integrin receptors bind to on the inside of the cell?
 Adapter Proteins (Vinculin & Talin)
 Stress Fibers
o What promotes the disassembly of Focal Adhesions at the back end as
the cell moves forward?
 Cofilin
o What are the stress fibers cross-linked by?
 A-Actin (contractile)
o What anchors the actin cytoskeleton to the Sarcolemma in Skeletal
Muscles?






 Dystrophin
What type of junction is formed by actin? What protein is involved?
o Adherens (tight) junctions
o Cadherin is involved.
Microvilli and Stereocilia are made of actin. What two linking proteins are
involved?
o Fimbrin
o Villin
Filamin and Spectrin are actin filaments that make webs for support of the
cell. Which causes 2D crosslinks and which causes 3D crosslinks?
o Filamin: 3D
o Spectrin: 2D
Which G-Protein is used in Actin Polymerization?
o Rho
What are the three different type of Rho?
o RAC
o CDC42
o RHO
Which Rho interacts with each cytoskeleton protein?
o RAC: Lamellipodia
o CDC42: Filipodia
o RHO: Stress Fibers