Actin

Surface

Where is actin generally found close to in the cell?

Actin filament functions

1. Transport

2. Force

3. Motility

4. Shape

5. Division

Microvilli

Small, finger-like actin projections on the surface of eukaryotic cells that increase surface area for absorption and secretion.

Cell cortex

A dense network of actin filaments just beneath the plasma membrane, providing structural support and maintaining cell shape.

Stress fibers

Bundles of actin filaments and myosin that help in generating tension and maintain the shape of the cell, especially during movement.

Lamellipodia

Flat, sheet-like protrusions at the leading edge of a migrating cell, formed by branched actin filaments, facilitates cell migration

Filopodia

Thin, spike-like projections extending from the lamellipodia, formed by parallel actin filaments, and used for sensing the environment

Contractile Ring

A ring of actin filaments that forms during cytokinesis and contracts to pinch the cell into two daughter cells.

G actin

Globular actin monomers that polymerize to form actin filaments.

ATP

____ binds in the cleft between the two lobes of the actin monomer

F actin

the polymerized form of G actin that forms the structural framework of the cytoskeleton.

ATPase activity

When monomeric, actin has very weak _____, but when a polymer, has stronger _______

minus end

The ATP binding cleft of an actin monomer always faces the _______

Myosin S1 fragment

A subfragment of myosin that binds actin and is involved in muscle contraction and other motility processes

Structural polarity

By treating myosin with proteases to isolate the S1 fragments, _______ can be revealed by “decorating” actin filaments with these myosin S1 fragments

Plus end/Barbed End

The fast-growing end of an actin filament, where ATP-actin subunits are added during polymerization.

Minus end/Pointed End

The slow-growing end of an actin filament, typically where depolymerization occurs.

Actin polymerization steps

1. Nucleation

2. Elongation

3. Steady state

Nucleus

initial aggregation of 3 actin monomers that serve as a starting point for the formation of a new actin filament

Nucleation

process by which a small group of actin monomers (G-actin) come together to form a stable nucleus

dimer

There’s a “lag” phase when intially polymerizing actin because the ____ of g-actin is very unstable but the trimer is stable

Elongation

The process by which actin filaments grow by adding subunits to both ends

Steady state

A dynamic state where the rates of polymerization and depolymerization are balanced, so the length of the filament remains constant.

Critical concentration

The concentration of free subunits of G-actin above which polymerization can take place

different

the two ends of actin filament have ______ critical concentrations

plus end

The _____ of the actin filament grows faster due to having a lower critical concentration than the other end

equal

At steady state for the actin filament, the depolymerization rates ________ the polymerization rates, making sure the length stays constant

Treadmilling

A phenomenon where actin filaments grow at the plus end while simultaneously depolymerizing at the minus end, resulting in the filament’s net movement.

ATP hydrolysis

_______ decreases the stability of actin-actin interactions

ATP bound

The g-actin that are added at the plus end during steady state are _______

dissociate

After the ATP bound actin join the plus end of the filament, given some time the ATP is hydrolyzed and then the ADP bound actin is more likely to ________

slow, catches up

mins-end addition is _______, so hydrolysis ________

fast, lags behind

plus-end addition is _______, so hydrolysis ________

Not required

ATP hydrolysis is ______ for polymer formation

between

at the steady state, the G-actin concentration is _____ the critical concentration of the minus and plus ends

Phalloidin

A toxin that binds to and stabilizes actin filaments, preventing their depolymerization.

Cytochalasin

A chemical that disrupts actin polymerization by binding to actin filaments and preventing their growth, acts as capping protein

Latrunculin

A drug that sequesters G-actin monomers, inhibiting actin polymerization by binding to G-actin and inhibiting their addition to filament

Thymosin beta 4

A protein that binds to G-actin, preventing it from polymerizing into actin filaments. Ex of a monomer-sequestering protein

Profilin

Protein that promotes the addition of ATP-actin to the growing end of actin filaments, stimulating polymerization.

Actin nucleator

Proteins that promote the nucleation of new actin filaments, like the Arp2/3 complex or formins.

Arp2/3

A protein complex that nucleates branched actin filaments and regulates their polymerization.

Formin

Actin nucleator that directs where unbranched filaments form in a cell

Rho GTPase

protein that binds to RBD on a formin protein and activates the formin

Branched actin filaments

Actin filaments that branch out from a nucleating site, often regulated by the Arp2/3 complex.

Unbranched actin filaments

Linear actin filaments that grow straight from a nucleating site, typically regulated by formins.

Cofilin

A protein that binds to ADP-actin filaments, destabilizing them and promoting filament disassembly.

Gelsolin

A protein that severs actin filaments and caps the newly created ends to regulate filament dynamics, example of filament-severing protein

Vilin

A protein that cross-links and organizes actin filaments in microvilli, the domains are close together

Fimbrin

A protein that binds to actin filaments, forming tight bundles and the binding domains are close together

Cross-linking proteins

Actin-binding proteins that alter the 3D organization of actin filaments (vilin, fimbrin)

Filamin

A protein that cross-links actin filaments into orthogonal networks and gel-like structures, the binding domains are far apart

CapZ

A protein that caps the plus ends of actin filaments, regulating filament growth.

monomer-polymerizing proteins

Actin binding proteins that promote the growth of actin filaments

filament-severing proteins

Actin binding proteins that shorten filaments and decrease cytoplasmic viscosity (gelsolin)

Nucleotide exchange

Profilin binds to ADP-G-actin and promotes ___________ (ATP replaces ADP) which promotes polymerization

RBD, FH1, FH2

Domain composition of formin protein

Rho-binding domain

RBD, binding site for Rho-GTPase

Formin homology

FH1, full name of this domain of a formin

Profilin-ATP-actin

Once the Rho-GTPase binds to the RBD and activates the formin, the FH1 recruits the _______

nucleates, plus end, capping proteins

After the FH1 of the formin recruits profilin-ATP-actin, FH2 _______ and remains attached to the ______, and facilitates elongation, preventing _______ from binding

Nucleation promoting factor

For branched filaments to form, first a _______ binds to the actin subunit

Arp2/3

Once an NPF component has bound to the actin subunit, an ________ then binds to the NPF

activate

NPFs ______ the Arp2/3 complex

mimics

The Arp2/3 complex _____ the G-actin structure

side, mother

the Arp2/3 complex (which now consists of an Arp2/3 connected to an NPF with an actin subunit at the end) binds to the _____ of an existing _______ filament

dissociates

The NPF component _____ after the G-actin is delivered to the Arp2/3

70

How many degrees is the angle created from an Arp2/3 branched filament

Actin Bundles

Tight, parallel bundles of actin filaments organized by proteins like fimbrin and villin.

Actin Networks

Mesh-like arrangements of actin filaments, often stabilized by proteins like filamin.

Myosin I

A motor protein that is not a dimer and has a tiny tail, involved in membrane trafficking and cell motility

Myosin II

A motor protein that generates contractile forces, particularly in muscle cells. (Muscle contraction) (power stroke)

Myosin V

A motor protein involved in transporting cargo along actin filaments, using a "hand-over-hand" mechanism.

Bipolar thick filament

Higher order structure formed from myosin II's to support cellular contractility by sliding on actin fiiaments

Myosin head domain

Domain of the myosin that has an actin and ATP binding site (ATPase)

In vitro actin gliding assay

An experiment where myosin is fixed to a surface and used to move actin filaments, helping to study myosin function. ATP is added.

plus end, minus end

Myosin walks towards the ____, which means in the in vitro actin gliding assay, actin moved with the ______ leading

Adaptor proteins

In order for myosin V to interact with cargo, it must bind to _______ with its tail, and the ______ bind to the cargo (vesicle)

Myosin power stroke

The conformational change that occurs in the myosin II head during ATP hydrolysis, causing it to "walk" along actin filaments and generate force.

no affinity

When myosin is bound to ATP, it has ______ to actin

detachment

For the power stroke, the binding of ATP to a cleft in the myosin head causes the ________ of the head from the filament

hydrolysis of ATP, bind weakly

In the powerstroke, the _________ energizes the head and causes the head to __________ to the actin filament

release, tighter

In the power stroke, after ATP has been hydrolyzed and the head is weakly bound to the actin filament, the _______ of Pi causes ________ attachment of the myosin head to the filament. then the power stroke moves the filament

release

For the power stroke, the _______ of ADP in the final steps set the stage for another cycle

Muscle fiber

A singular muscle cell that contains myofibrils

myofibril

A contractile unit of muscle fibers, composed of repeating sarcomeres that contain actin and myosin filaments.

Sarcomere

The basic structural unit of a myofibril, composed of actin and myosin filaments arranged in a repeating pattern.

Thick filament

Filament in sarcomere made of myosin

Thin filament

Filament in sarcomere made of actin

Tropomodulin

protein that caps the pointed end of actin filaments, regulating their length and stability in muscle cells, binds to minus end which is facing inside sarcomere

Z-disc

The boundary of a sarcomere, anchoring the plus ends of actin filaments, made up of CapZ binding protein

titin

protein with elasticity found in the sarcomere that connects the myosin thick filament to the boundary of the sarcomere, has propensity to restore to original length

Overlap

When the sarcomere is contracted, there is more _____ between the actin and myosin

Neuromuscular junction

The synapse where a motor neuron communicates with a muscle cell to initiate contraction by release neurotransmitters

Transverse tubule

Invaginating membrane from surface of cell (continuous with plasma membrane) that transmits electrical signals deep into the muscle fiber

Sarcoplasmic reticulum

Specialized ER in muscle cells that stores calcium ions, which are released to initiate contraction, they surround myofibrils

Ca2+ ATPase

After contraction is over, ________ pumps via active transport Ca back into the SR, lowering cytosolic Ca and allowing muscle to relax

Voltage-gated Ca2+ channel

A channel in the sarcoplasmic reticulum that generates a mechanical force that activates RyR in response to the action potential

Ryanodine receptor

A channel in the sarcoplasmic reticulum that is both mechanically-gated and ligand-gated

• Ligand-gated because there’s Ca2+ induced Ca2+ release

Tropomyosin

A protein that blocks the myosin-binding sites in a relaxed muscle.T