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Actin is a protein whose monomers assemble into filaments. An idealized actin filament has a bending modulus and persistence length; inferring its effective elastic modulus from those measurements additionally requires a cross-sectional model.

A actin filament assembled from actin subunits. actin filament mass and actin filament number are distinct quantities; subunit exchange can continue while the total mass is approximately steady.

An individual soluble actin subunit. Total actin monomer concentration and concentration available for assembly differ when actin monomers are bound by sequestering molecules.

A localized actin assembly in budding yeast, commonly associated with endocytosis. The lifetime of a visible patch and the lifetime of an individual constituent actin filament are different measurements.

Assembly of actin subunits into actin filaments. An unseeded bulk reaction can show a actin nucleation lag, a growth phase and an approximately constant final polymer mass; equal endpoints need not imply equal kinetics.

An assembly of actin filaments and myosin motors. Network architecture, motor activity and attachments determine how contractile activity produces cellular or tissue deformation.

actomyosin contractility generates force through interactions of myosin motors with actin networks. The resulting load depends on network organization, attachments and resistance from the surroundings.

An actomyosin ring is a contractile assembly containing actin filaments and myosin II. In typical animal cytokinesis, it forms in the cell cortex beneath the cell membrane. Its contraction draws the boundary inward to produce a cleavage furrow; an inward groove is not yet complete separation of the daughters.^[ref]

Coauthor of research on pulsed actin–myosin contraction during apical constriction. The study connected intermittent molecular forces with a sustained change in epithelial cell shape.^[ref]

Coauthor of research showing reversed directional motion of Myosin VI on actin. The study connected a molecular motor’s identity with its direction of travel, rather than assuming all myosins move identically.^[ref]

Armando del Rio coauthored research on vinculin.^[ref]

vinculin is an adhesion-associated protein that can bind exposed regions of talin and contribute to coupling adhesions to actin. Recruitment and load depend on the state of the surrounding linkage.

A transcript encoding a form of actin, with a localization element recognized by ZBP1.

Cat M. Haglund studied the bacterial protein Sca2 as a mediator of actin-based movement in Rickettsia. The work identified formin-like activity, connecting a bacterial surface protein with the assembly of host actin filaments.^[ref]

A focal adhesion is a multiprotein connection between integrins and the actin cytoskeleton. It can transmit traction and change its assembly in response to loading.

A histone H1 kinase assay measures protein kinase action on supplied histone H1, a protein substrate. When the transferred phosphate carries a radioactive isotope label, label incorporated into histone H1 reports catalytic action over the assay interval. It does not count CDK1 molecules, and a crude preparation can contain other protein kinases acting on histone H1. Attribution therefore needs controls.

A 2010 study prepared human HeLa S3 extracts after a double thymidine block, using swelling, a Dounce homogenizer, centrifugation, dialysis and concentration. Added recombinant cyclin B1 incubated for $20$ minutes.^[ref]

Aliquots received histone H1 and terminal-${}^{32}\mathrm{P}$-labeled ATP for a $15$-minute room-temperature reaction, stopped with Laemmli buffer. Dried SDS-PAGE gels exposed a phosphorimager screen; a BioRad Molecular Imager FX quantified labeling. No-added-cyclin B activity set the normalization baseline; RO-3306 reduced H1 activity approximately $85\%$. Companion Western blots monitored Tyr15 phosphorylation, with actin Western blot loading control.^[ref]

Mixed extracts lose compartment boundaries; inhibitor sensitivity supports a major CDK1 contribution, with residual signal.^[ref] Substrate choice and inhibitor specificity limit attribution. The cell-cycle hysteresis experiment supplies a separate history comparison with this kind of histone H1 readout; its dose thresholds should not be transferred to human biological cells.

Integrin signaling couples binding and mechanical loading of an integrin to changes inside a cell. An integrin spans the cell membrane: extracellular binding to the extracellular matrix can promote intracellular assembly of adhesion complexes connected to actin. Conversely, intracellular regulation can change integrin affinity and clustering. Signalling therefore runs in both directions across the boundary. ^[ref]

A change in matrix stiffness can change how an attached cell bears load and activates downstream signals. This makes adhesion a source of information as well as a tether. Measuring adhesion strength, phosphorylation of a signalling component and eventual cell proliferation addresses different stages; none alone specifies the whole response.

An intermediate filament is a rope-like protein assembly of the cytoskeleton, typically about 10 nanometres in diameter, between an actin filament and a microtubule in thickness. Different cells use different constituent proteins: keratins reinforce many epithelial cells, vimentin forms networks in many connective-tissue cells, and lamins support the cell nucleus. Their assembled subunits do not establish the uniform head-to-tail polarity characteristic of an actin filament.^[ref]

The mechanical role can be tested outside a biological cell. In 1991, P. A. Janmey and colleagues compared purified vimentin, actin and microtubule networks under deformation. The vimentin networks tolerated larger deformations before disruption, illustrating how an intermediate filament network can combine compliance with resistance to damage. This helps explain why load-bearing tissues need a network that can stretch rather than simply a rigid internal frame.^[ref]

An intrafusal muscle fibre is a specialized skeletal muscle fibre within a muscle spindle. Its mechanical state is monitored by endings of sensory neurons, contributing to proprioception, and motor input adjusts that state. The ordinary force-producing extrafusal muscle fibres lie outside the muscle spindle. Intrafusal contraction therefore helps regulate a sensory apparatus as well as changing local tension.

In a cat soleus preparation, researchers repetitively stimulated a single motor fibre supplying a muscle spindle, then imposed a ramp stretch while recording a primary sensory ending. Prior motor stimulation produced an extra burst of impulses at the start of the stretch. A subsequent stretch abolished this after-effect, whereas without that stretch it could persist for more than a minute.^[ref]

Two stretches with the same imposed movement can thus produce different sensory signals because the intrafusal muscle fibres have different recent activation histories. The authors proposed persistent interactions between actin and myosin as a mechanism; that proposal should be distinguished from the directly recorded impulses.^[ref]

An actin-rich cellular protrusion associated with localized extracellular matrix degradation during invasion. Formation, matrix digestion and whole-cell invasion are related but distinct experimental endpoints.

Kathryn R. Ayscough coauthored research on actin filament.^[ref]

A actin filament assembled from actin subunits. actin filament mass and actin filament number are distinct quantities; subunit exchange can continue while the total mass is approximately steady.