Module 9- Cytoskeleton II

Protein complex that binds to both actin filaments and tropomyosin molecules. Binding of Ca2+ causes a conformational change that leads to this protein releasing its hold on the actin filament.

Troponin

Proteins that are located in the outer nuclear membrane that bind directly to actin filaments and indirectly to microtubules (via motor proteins).

KASH proteins

A protein that regulates actin filament elongation by sequestering actin subunits and making them unavailable for elongation.

Thymosin

This actin nucleating protein attaches to the side of actin filaments and nucleates the formation of branched actin networks. It remains bound to the (-) end, which allows rapid elongation at the (+) end.

ARP complex (ARP 2/3)

Sites of tight adhesion to the underlying ECM. They provide a structural link between the actin cytoskeleton and the ECM and are regions of signal transduction that relate to growth control. These sites are the traction for cell movement provided by transmembrane integrins.

Focal adhesions

An ion channel in the membrane of cells or cell structures (i.e. T-tubules) that opens/closes in response to a stimulus causing sufficient depolarization and thereby allows Ca2+ to cross the membrane.

Voltage-gated Ca2+ channels

When Ca2+ channels in the T-tubules open, this leads to the opening of a second type of channel in the sarcoplasmic reticulum.

Ca2+ release channels

A seven-pass cell surface receptor that when activated by its extracellular ligand, activates a G protein. This G protein will activate either an enzyme or an ion channel in the plasma membrane.

G protein coupled receptors (GPCRs)

An actin web-forming protein common in red blood cells. Creates a strong, yet flexible cell cortex providing mechanical support for the overlying cell membrane.

Spectrin

A 2-D protrusion at the leading edge of epithelial cells and fibroblasts, as well as by some neurons. They are formed by actin networks nucleated by ARP complexes. These sheetlike structures contain a cross-linked mesh of actin filaments, most of which lie in a plane parallel to the solid substratum.

Lamellipodia

In contractile cells, these are powered by active transport to rapidly transport Ca2+ from the cytosol back into the lumen of the sarcoplasmic reticulum (SR).

ATP-dependent Ca2+ pumps

Defines the length of the bipolar myosin thick filaments. The ends of this structure contain overlapping thin filaments. Under TEM, these are the dark regions.

A bands

Cell-surface receptors of a monomeric GTPase family that specifically promote the formation of lamellipodia.

Rac-GTP

Protein complex that promotes the formation of branched actin networks by activating ARP 2/3 complexes.

Cdc42-GTP

Channel-like structures used to efficiently convey the electrical signal of depolarization into the cell. Each triad contains one of these structures in the middle.

Transverse tubules (T-tubules)

A Ca2+ binding (sensing) protein that is used to regulate contraction of smooth muscle cells and contractile non-muscle cells. This protein undergoes a conformational change when it binds Ca2+ and can regulate many target proteins.

Calmodulin

A bundling protein that produces a parallel array of actin filaments in muscle cells. Its actin-binding sites are separated by a long spacer, so that it forms more loosely packed actin bundles.

α-actinin

A protein that acts as a molecular ruler to maintain the precise length of the thin actin filaments. Stretches from the Z disc toward the (-) end of each thin filament.

Nebulin

Intercellular anchoring cell-cell junctions that are usually formed between 2 epithelial cells. Contain dense plaques of protein into which intermediate filaments in the 2 adjoining cells insert.

Desmosomes

This dimeric actin nucleating protein nucleates the growth of straight (unbranched) actin filaments that can be cross-linked by other proteins to form parallel bundles.

Formins

A cylindrical structure that can be as long as an entire muscle cell. It is composed of a long, repeated chain of tiny contractile units called sarcomeres.

Myofibril

Cell-anchoring junctions that anchor the intermediate filaments in a cell to the extracellular matrix.

Hemidesmosomes

Regulates actin filament elongation by incorporating monomer actin subunits into growing filaments. Helps to concentrate actin monomers at sites of filament assembly.

Profilin

Binding of chemoattractant to GPCRs leads to this ligand-based signaling molecule that activates downstream Rac GTPase.

PI(3,4,5)P3

Process by which filaments remain the same length overall due to addition of subunits at the (+) end at the same rate as loss of subunits at the (-) end.

Treadmilling

Composed of part of the thin filaments of 2 adjacent sarcomeres, which do not overlap with the thick filaments. Under TEM, these are the light regions.

I bands

Proteins that bind actin filaments together to form organized arrays. There are 2 classes, bundling proteins and gel-forming proteins.

Cross linking proteins

Proteins that are expressed in epithelial cells where they provide mechanical strength and abrasion resistance.

Keratins

Rope-like fibers with high tensile strength found in most animal cells. They form a network surrounding the nucleus and extend out to the cell periphery.

Intermediate filaments

A bundling protein that produces a parallel array of actin filaments in muscle cells. It has 2 directly adjacent actin-binding sites, so that it holds its two actin filaments very close together aligned with the same polarity.

Fimbrin

Located in the middle of the A bands, this structure contains proteins that link adjacent thick filaments into a hexagonal lattice.

M line

The repetitive functional subunit of striated muscle cells that extends from Z disc to Z disc. Many of these subunits together form a myofibril.

Sarcomeres

A stable rod-like protein structure formed by 2 or more ⍺ helices coiled around each other.

Coiled-coil dimers

A 1-D protrusion at the leading edge of neurons (growth cones) and some types of fibroblasts. They contain a core of long, bundled actin filaments, similar to those in microvilli but longer and thinner, as well as more dynamic.

Filopodia

Large, bipolar thick filament protein that contains several hundred heads and found in striated muscle cells (skeletal and cardiac).

Myosin II

Proteins that link intermediate filaments to each other and to the rest of the cytoskeleton.

Plakins

Proteins that bind to the (+) or (-) end, respectively, and prevent gain or loss of subunits from the filament.

Capping proteins

An accessory actin-severing protein that interacts with the side of the actin filament and has subdomains that binds 2 different sites. After breaking the filament, this protein remains attached to actin and caps the (+) end.

Gelsolin

A gel forming protein that produces a 3-D actin network with gel-like consistency. Its actin-binding sites contain a V-shaped linkage between them, so that it cross-links actin filaments into a network with the filaments oriented almost at right angles to one another

Filamin

A ball or sphere-shaped monomeric subunit that contains an associated ADP or ATP.

G-actin

Proteins that form a 2-D meshwork underneath the inner membrane of the nuclear envelope. This gives the nucleus strength and also plays a role in nuclear-envelope disassembly during cell division.

Nuclear lamins

Enzyme that regulates actin filament assembly in smooth muscle cells and contractile non-muscle cells by phosphorylation.

Myosin light chain kinase (MLCK)

An accessory protein that caps the (-) ends of actin filaments, preventing growth or loss at the (-) ends.

Tropomodulin

Non-striated muscle cells that are regulated by intracellular Ca2+ levels.

Smooth muscle

A transmembrane adhesion protein that is involved in the attachment of cells to the ECM and to each other.

Integrins

A 3-D protrusion at the leading edge of rapidly migrating cells that looks like a "false-foot" such as in amoeboid cell migration.

Pseudopodia

A tight, right-handed helix that is approximately 8nm wide composed of subunits assembled head-to-tail.

F-actin

An accessory protein that caps the (+) ends of actin filaments, preventing growth or loss at the (+) ends.

CapZ

Cortical fibers that connect a cell to the ECM through focal adhesions or by forming a circumferential belt in an epithelial cell, or connecting it to adjacent cells via adherens junctions.

Stress fibers

Found in smooth muscle and contractile non-muscle cells and is phosphorylated by MLCK.

Regulatory myosin light chain

Movement of a cell towards or away from a diffusible chemical.

Chemotaxis

Intermediate fibers contain a central α-helical domain composed of a motif that occurs 40 times or more to form an extended coiled-coil structure with another monomer.

Heptad repeats

Cell-surface receptors of a monomeric GTPase family that specifically promote the formation of stress fibers.

Rho-GTP

Filaments found in nerve axons that provide tensile strength to the long axon processes.

Neurofilaments

An actin filament destabilizing protein that binds ADP-actin along the length of the filament forcing the filament to twist more tightly. Binding of this protein accelerates filament disassembly.

Cofilin

The diffusible chemical molecules that cells move towards or away from during the process of chemotaxis.

Chemoattractants

Proteins that produce parallel arrays of actin filaments, such as in microvilli.

Bundling proteins

A specialized layer of cytoplasm on the inner face of the plasma membrane. In animal cells, this layer is actin-rich and is responsible for movements of the cell surface.

Cell cortex

An elongated accessory protein that binds to the side of actin filaments. This binding stabilizes actin filaments as well as modulating binding of other accessory proteins.

Tropomyosin

Monomeric GTPase proteins that are cell-surface receptors. These proteins act as molecular switches that cycle between an active GTP-bound state and an inactive GDP-bound state.

Rho protein family

Located in the middle of each I band and binds the (+) ends of the actin thin filament.

Z discs

A spring-like protein found in muscle cells that helps position the thick filaments midway between Z discs.

Titan

A weblike sheet of modified endoplasmic reticulum that surrounds each myofibril like a net stocking. Serves as a reservoir to store Ca2+.

Sarcoplasmic reticulum (SR)

Proteins that are located in the inner nuclear membrane and bind to the nuclear lamina or chromosomes.

SUN proteins