Actin (Cytoskeleton)

Microtubule

a hollow tube formed from tubulin dimers

Microfilament

a double helix of actin monomers

Intermediate Filament

a strong fiber composed of protein subunits

ubiquitous

Actin is _________ in all cells

isoforms

Actin has multiple _______

G-Actin

Actin monomer

F-Actin

Actin polymer

two lobes; cleft

Each actin monomer is composed of _________ and a ______

ATP

Nucleotide that binds to actin monomer to activate the polymerization

positive

The barbed end represents the _________ end of actin

negative

The pointed end represents the ________ end of actin

ADP

Nucleotide within the actin monomer that inactivates its polymerization, weakening the polymer

after

Actin monomers hydrolyze ATP _____ polymerization

longitudinal; lateral

The actin filament is strengthened by ________ and _________ bonds

enzyme

Actin is an _______

irreversible

Actin’s ATP hydrolysis is _________

Nucleation

lag phase

Elongation

growth phase

Steady State

equilibrium phase

very little (if any) change in the net growth of the filament

Actin Nucleating Proteins

initiate the nucleation process

Formin (Nucleating Protein)

necessary for linear actin filament formation

builds a new filament from nothing

Arp2/3 Complex (Nucleating Protein)

necessary for branched actin formation

builds actin branch off of pre-existing filaments

deformity

Formin is named after a mouse gene ______

dimer; donut

Formin acts as a ______ forming a ______ shape around the actin

auto-inhibited

Formins exist in an _________ state

Rho GTPase; GBD

One Formin activates by the binding of ________ to its _______

dimer

After Formin activates, it forms a ________

GTPase Binding Domain (GBD)

Domain in a protein that GTPases bind to

actin

Arp 2 and Arp 3 are ______-related proteins

macromolecular; multimeric

Arp2/3 is __________ and _________

WASp (Wiskott-Aldrich Syndrome Protein)

Protein that regulates Arp2/3

side; webs

Arp2/3 complex binds to the ________ of the actin filament to form _____

itself

WASp is inhibited by ________

CDC42 GTPase; GBD

WASp activates by the binding of ________ to its ________

VCA; Arp2/3; actin

After WASp is activated, it’s ______ domain is exposed and it binds ______ and ________

Thymosin-beta 4

creates a pool of ATP-bound G-actin ready to polymerize

eukaryotes; motile

Thymosin-beta 4 is only in higher _________ and highly ________ cell

peptide

Thymosin is a small ______ that covers a large surface area of the monomer

inhibits

Thymosin-beta 4 _______ spontaneous polymerization

Profilin

ensures the addition of the actin monomer at the + end

ADP; ATP

Profilin exchanges _______ to ________ on an actin monomer to promote proliferation

Critical Concentration

concentration of actin monomers when rate of growth equals the rate of loss

grows

When G-actin concentration is above the CC, then filament _____

shrinks

When G-actin concentration is below the CC, then filament _____

Treadmilling

when the net loss is equal to the net gain of monomers

the actin filament remains constant

Positive side

Which side is the G-actin preferentially added onto

Cofilin

increases the helical twist of the filament

ADP

Cofilin has a preference for _____-bound actin