Cytoskeleton: Actin Filaments Flashcards by Rebecca Ruthberg

What is the protein subunit of actin?

Actin

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Actin is made up of actin ____

monomers

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actin monomers are also called what?

G-actin

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What are the main functions of actin?

maintain cell shape
changes cell shape
muscle contraction
cytoplasmic streaming
cell motility
cell division

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G-actin polymerizes into ____

F-actin

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What does F-actin stand for?

actin filaments

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Explain how actin is important in maintaining cell surface shape and cell motility

Cell surface shape &amp; Cell motility
Cell division &amp; cytokinesis
Lamellipodia, filipodia, phagocytic cups
Microvilli (intestinal epithelial cell surface) &amp; stereocilia
Cell junctions

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How does actin polymerize?

Both ends grow, the + end grows faster than the - end.

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Are actin polar? describe

yes, they have - and + side

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What shape is a F-Actin?

double helix

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In order for actin monomers to attach and grow a filament, they have to be ____ bound

ATP

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Why do monomers add more quickly to + end of actin filament?

They have higher affinity for + end

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ATP will eventually be _____ on actin filament

hydrolysed to ADP

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What prevents the actin filament from falling apart into its monomers?

The actin is “capped” by ATP bound actin that surround the ADP bound and prevent depolymerization

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The rate of elongation/shortening is determined by what?

level of ATP-G actin

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What happens if hydrolysis catches up with - end?

ADP bound monomers are exposed and actin is depolymerized - treadmilling

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What is treadmilling?

length of filament remains the same b/c monomers are being added to + end and removed from - end at same rate

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Nucleation occurs at ____ ___

plasma membrane

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Where is the highest density of actin filaments?

cell periphery

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What determines the changing of actin to make cell a different shape or stiffness?

signaling pathways, responding to their enviornemtn

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2 or 3 actin together is called what?

nucleus

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A G-actin prefers to join what?

It prefers to join a filament over nucleus, and that over just a few monomers that are joined

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What is actin nucleation?

binding actin monomers together

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What does Arp stand for?

actin-related proteins

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What is the function of Arp2/3?

binds to actin monomers and stimulates the creation of a new filament

What is the function of prolifin?

exchanges ADP for ATP, readies monomer for attachment

Activation of 2/3 will stimulate the formation of what?

branching actin networks

What is the function of formin?

nucleate growth of straight, unbranched actin filaments

How does formin function?

Bound to + end and move up, help insert monomers into + end

What is thymosin and what is its function?

actin binding protein, binds G-actin and prevents F-actin assembly

What is prolifin and what is its function?

actin binding protein | binds g-actin and promotes filament assembly at + end

What competes with thymosin?

prolifin

What is cofilin and what is its function?

actin bining protein - it promotes disassembly at - end. it is severing protein

What is gesolin and what is its function?

Actin binding capping protein. It caps + end and prevents further growth

Name 4 non-muscle actin functions

microvilli stress fibers cell surface protrusions & cell migration cytokinesis

What kind of motor protein is on microvilli?

Myosin I

What kind of motor protein is on muscle?

Myosin II

What kind of motor protein is on vesicle and organelle transport for actin?

Myosin V

What motor protein has contractile activity?

Myosin II

Actin organization in the cell is stabilized by what?

cross-linking proteins

What is filopodium?

tightly packed parallel bundles of actin

What is the function of filopodium?

Sensors, to deform the membrane. No contractile ability

Explain how stress fibers are packed

Very loosely, myosin II can enter. Contractile.

Name actin cross linking proteins that help form parallel cables/bundles

alpha-actinin, fimbrin, villin

Name actin cross linking proteins that help form webs/gels

spectrin, filamin

Describe the characteristics of alpha actinin

cross linking protein loosely packed bundles found in stress fibers and during cytokinesis contractile

Describe the characteristics of fimbrin

cross linking protein tight packed bundles found in microvilli non-contractile

What are stress fibers made out of?

contractile bundles of actin filaments

How is actin connected to the ECM?

focal adhesions - they are like footholds for actin to attach to the ECM

What is the function of focal adhesions and cell movement

they act like a foothold and are a place for actin to push off from to help move cell forward

What is the function of dystrophin?

anchors actin cytoskeleton to the sarcolemma (muscle cell membrane) in skeletal muscles

What is the function of vinculin & talin?

attach actin cytoskeleton to membranes in focal adhesions

What spans near the apical surface of every cell?

loosely packed parallel bundles of actin filaments.

What is a cadherin?

attachment of actin bundles

What type of crosslinking proteins are in microvilli?

fimbrin & villin

Do actin in microvilli contract?

Where are steroecilia located?

epididymis for reabsorption | hair cells of inner ear - mechanosensory

Describe stereocilia

non-contractile, non motile | made of f-actin

What kind of cross links does filamin make?

3D crosslinks - web like structure

What kind of crosslinks does spectrin make?

2D web

Describe spectrin and its function

protein - links to adaptor proteins that bind to transmembrane proteins to short filaments in actin allows for structural support needed in RBC

What is the name of the family of proteins that control the actin cytoskeleton?

Rho GTPases

Where are Rho GTPases located and what is their function?

located @ cell surface, they activate nucleating proteins

When will lamellipodia, filipodia, and stress fibers be activated?

when cell is triggered to migrate

rac, cdc42, rho are all examples of

Rho GTPases

What is lamellipodia?

Leading edge - the thing that points out and forms when cell needs to migrate

What do Rho GTPases activate in first step for cell migration?

Arp2/3 complex → actin polymerization @ leading edge

What is a protein besides Arp2/3 that needs to be activated for cell migration?

Cofilin → depolymerisation @ trailing edge

If the cell forms a lamellipodia, will it move forward?

No - it also needs to attach to ECM so it can be pushed

Describe steps in cell movement if a cell needs to migrate

1. Rho GTPases activate Arp2/3 2. Arp 2/3 causes polymerization and lamellipodia @ leading end 3. myosin motor protein causes contraction at rear of cell → pushes cell forward 4. New contacts to ECM are made at front, contacts at back are diassembled

Key words: WASP mutation, haematopoietic cells are what disease?

Wiskott-Aldrich Syndrome

What is the defect in Wiskott-Aldrich Syndrome ?

Mutation of WASP

What is WASP?

Wiskott-Aldrich syndrome protein

What is the function of WASP?

WASP activates Arp2/3 in WBC.

A loss of function of WASP results in what?

inability of WBC to move - severe immunedefeciency

Characteristics: Thrombocytopenia, eczema, recurrent infections; death before adulthood without bone marrow transplant are what disease?

Wiskott-Aldrich Syndrome

What is the mode of inheritance for Wiskott-Aldrich Syndrome?

x-linked

What is the mechanism for Wiskott-Aldrich Syndrome?

Mutation causes failure of actin nucleation (Arp 2/3) and disruption of signaling pathways ! prevents WBC from migrating to site of infection

Describe how actin is important in cytokinesis?

actin & myosin II form a contractile ring under plasma membrane and pinch the cell, dividing it into two

Cytochalasins, Phalloidin, & Latrunculin are all drugs that do what?

drugs that inhibit actin-based function. they are used in research, not for treatments.