Actin Cytoskeleton: Non-Muscle Contraction, Adhesion, Intracellular Transport, and Cell Shape

Comprehensive vocabulary flashcards covering the molecular players and mechanisms of non-muscle contraction, cell adhesion, intracellular transport, and cell shape determination.

Non-muscle Myosin II Mini-filament

A dynamic bipolar filament of about $30$ ($28\text{--}30$) myosin molecules (~$300\text{ nm}$ long) that constantly polymerizes and depolymerizes to drive non-muscle contraction.

Auto-inhibited Myosin II Conformation

A compact, inactive state in which the coiled-coil tail bends into a Z-shaped folded bundle that binds its motor domains, preventing actin binding, ATP hydrolysis, and filament assembly.

Regulatory Light Chain (RLC)

A myosin subunit whose phosphorylation by MLCK or ROCK breaks auto-inhibition, causing the tail to straighten and assemble into bipolar contractile filaments.

Rho Kinase (ROCK)

A downstream kinase activated by GTP-bound Rho that phosphorylates the regulatory light chain of non-muscle myosin II to promote filament polymerization and contraction.

Formin

An actin nucleator and polymerase activated by GTP-bound Rho that binds the barbed end to generate linear, unbranched actin filaments required for contraction.

Front-to-Rear Polarity Signaling

A mutually inhibitory signaling system in migrating cells where local front Rac signaling stimulates protrusion (WAVE, Arp2/3) and inhibits Rho, while global rear Rho signaling stimulates contraction (ROCK, formin) and inhibits Rac.

Keratocyte Contractile Network

A disorganized network of randomly oriented bipolar myosin II mini-filaments pulling on converging actin networks; sufficient to retract the cell rear during fast migration without forming sarcomeres.

Stress Fibers

Quasi-sarcomeric bundles of actin filaments and non-muscle myosin II alternating with $\alpha$-actinin that retract the rear and exert contractile force on external extracellular matrix loads.

$\alpha$-Actinin

A cross-linking dimer with wide actin-binding domain spacing (~$40\text{ nm}$) that organizes actin filaments loosely enough to permit non-muscle myosin II entry between filaments.

Focal Adhesions

Multi-protein anchor complexes (comprising at least $60$ core proteins and ~$100$ cell-variable proteins) linking intracellular stress fibers to extracellular matrix proteins.

Integrin

A heterodimeric transmembrane adhesion receptor ($\alpha$ and $\beta$ subunits) that mediates attachment to extracellular matrix ligands such as fibronectin.

Talin

A mechanosensitive adapter protein that binds the cytoplasmic tail of $\beta$-integrin to activate it, links to actin, and unfolds under tension to uncover cryptic vinculin-binding sites.

Vinculin

An actin-binding protein recruited to exposed binding sites on tension-unfolded talin that reinforces focal adhesions by recruiting additional actin filaments.

Catch Bond

A specialized protein-protein interaction that strengthens rather than weakens when subjected to tensile pulling force.

Myosin V

A two-headed, processive motor with $6$ light chains per neck, a large step size of ~$36\text{ nm}$, and a high duty ratio (~$50\%$) that moves secretory cargo toward actin barbed ends.

Duty Ratio

The proportion of the ATPase cycle time that a motor protein remains strongly bound to its filament track (~$5\%$ for myosin II; ~$50\%$ for myosin V).

Rab27a

A small GTPase on the surface of melanosomes that recruits the adapter melanophilin to couple the organelle to Myosin Va for actin-based transport.

Melanophilin

An adapter protein (Rab27a effector) that bridges GTP-bound Rab27a on melanosomes to the cargo-binding domain of Myosin Va.

Griscelli Syndrome

A human genetic disorder characterized by hypopigmentation caused by mutations in Myosin Va, Rab27a, or melanophilin, which blocks melanosome transfer from microtubules to actin.

Myosin VI

A backward-directed motor possessing $2$ light chains and an extra neck $\alpha$-helix that moves cargo toward the pointed (minus) end of actin filaments, such as during endocytosis.

Fimbrin (Plastin)

A short monomeric cross-linking protein with closely spaced actin-binding domains that bundles actin filaments tightly together, excluding myosin II (e.g., in microvilli and filopodia).

Myosin 1a

A single-headed motor protein whose motor domain binds actin and whose short lipid-binding tail binds PIP2 to link the microvillus actin core to the plasma membrane.

Spectrin

A very long (~$200\text{ nm}$ extended) flexible tetrameric cross-linker consisting of two $\alpha$ and two $\beta$ subunits that forms an elastic network beneath the plasma membrane.

Spectrin Repeat

A triple-stranded $\alpha$-helical structural domain connected by flexible linkers that reversibly unfolds under force, allowing spectrin to extend like a spring.

Adducin

A capping protein that caps the barbed ends of short actin filaments at junctional complexes in the erythrocyte membrane skeleton.

Tropomodulin

A capping protein that caps pointed ends of the ~$35\text{ nm}$ actin filaments at junctional complexes in the erythrocyte membrane skeleton.

Ankyrin

The primary adapter protein in the erythrocyte membrane skeleton that tethers spectrin repeats directly to the cytoplasmic domain of the Band 3 anion channel.

Band 4.1

An ERM-family adapter protein that anchors the actin filament nodes of junctional complexes to transmembrane Band 3 and glycophorin.

Band 3

An abundant erythrocyte transmembrane anion transport channel that acts as a major anchor for ankyrin and Band 4.1.

Glycophorin

A single-pass, heavily glycosylated transmembrane protein in erythrocytes that interacts with Band 4.1 at the junctional complex and contributes to the glycocalyx.

Membrane Skeleton Fence

A submembrane actin-spectrin cortical network that corrals transmembrane proteins, confining their lateral diffusion to compartments until they hop across transient fence openings.