Cytoskeleton, Cell Junctions, and Motility

Microfilaments, Intermediate Filaments, and Microtubules

The three cytoskeletal filament systems are distinguished by diameter, protein composition, and function. Microfilaments (actin filaments, ~7 nm diameter) are two-stranded helical polymers of G-actin monomers. They concentrate beneath the plasma membrane, where they form a cortical meshwork that resists tension, drives cell shape changes, and powers crawling motility via polymerization at the leading edge. Intermediate filaments (~10 nm diameter) are ropelike fibers of fibrous proteins (keratins in epithelia, vimentin in connective tissue, neurofilaments in neurons, lamins in the nuclear lamina). They provide purely mechanical strength — they resist tensile stress and maintain cell integrity but have no motor proteins or polarity. Microtubules (~25 nm diameter) are hollow tubes of α/β-tubulin heterodimers. They radiate from the centrosome, resist compression, and serve as tracks for long-range intracellular transport (kinesin and dynein) and form the mitotic spindle, cilia, and flagella. All three systems are implicated in disease: keratin mutations cause epidermolysis bullosa simplex (skin blistering), lamin mutations cause progeria (premature aging), and tubulin defects disrupt mitosis.

Motor Proteins

Motor proteins are ATP-driven molecular machines that walk directionally along cytoskeletal tracks. Myosin motors walk along actin filaments. Myosin II powers muscle contraction by sliding actin filaments toward the center of the sarcomere; it forms bipolar thick filaments whose heads pull antiparallel actin arrays. Unconventional myosins (e.g., myosin V) transport vesicles and organelles along actin tracks, always moving toward the barbed (plus) end. Kinesin motors walk along microtubules — most (e.g., conventional kinesin-1) move toward the plus end (toward the cell periphery), carrying vesicles, mitochondria, and ER tubules outward. Dynein motors move toward the minus end of microtubules (toward the centrosome), powering retrograde transport. Cytoplasmic dynein carries cargo inward; axonemal dynein powers ciliary and flagellar beating. All three motor families undergo a cycle of ATP binding, hydrolysis, and release that drives conformational changes — the motor 'head' alternately binds, pivots (power stroke), and releases the filament, translating chemical energy into mechanical work.

Cilia and Flagella

Cilia and flagella are microtubule-based motile or sensory appendages organized around the axoneme — a '9+2' arrangement of nine outer doublet microtubules surrounding a central pair. Each outer doublet has dynein arms that walk toward the minus end of the adjacent doublet; when constrained by cross-linking proteins (nexin), this sliding is converted into bending. Motile cilia (e.g., in respiratory epithelium, oviduct) beat in coordinated waves to move fluid and particles across cell surfaces. Primary cilia (9+0 arrangement, no central pair) are non-motile sensory antennae found on most vertebrate cells — they detect mechanical and chemical signals and are essential for Hedgehog and Wnt signaling pathways. Flagella in eukaryotes have the same 9+2 structure as motile cilia but are longer and exhibit whip-like beating (sperm locomotion). Defects in ciliary dynein cause primary ciliary dyskinesia (Kartagener's syndrome: situs inversus, chronic sinusitis, male infertility).

Extracellular Matrix

The extracellular matrix (ECM) is a three-dimensional network of secreted proteins and polysaccharides that fills the space between cells, providing structural support, regulating cell behavior, and serving as a reservoir for growth factors. The major fibrous component is collagen — triple-helical proteins that form fibrils conferring tensile strength. Elastin provides elasticity. Proteoglycans (core proteins with glycosaminoglycan [GAG] side chains such as hyaluronan, chondroitin sulfate) form a hydrated gel that resists compression. Adhesive glycoproteins, including fibronectin and laminin, link ECM fibers to cell-surface receptors (integrins). The ECM is not passive scaffolding; it signals through integrin-mediated pathways (outside-in signaling) that regulate proliferation, differentiation, and migration. Basement membrane — a specialized ECM sheet rich in type IV collagen, laminin, and perlecan — underlies epithelial and endothelial sheets.

Tight Junctions, Desmosomes, and Gap Junctions

Cell junctions create multicellular tissues by coupling adjacent cells mechanically, electrically, and chemically. Tight junctions (zonula occludens, apical-most) form a continuous belt of transmembrane proteins (claudins and occludins) that seal the intercellular space, establishing epithelial polarity by preventing lateral diffusion of membrane proteins between apical and basolateral domains. They gate paracellular transport — 'leaky' tight junctions in kidney proximal tubule permit solute passage; 'tight' tight junctions in the blood-brain barrier block nearly all paracellular movement. Desmosomes (macula adherens) are spot welds: cadherin family proteins (desmogleins, desmocollins) link to intermediate filaments via plaque proteins, distributing mechanical stress across epithelial sheets. Gap junctions are channels formed by connexin hexamers (connexons) that align between adjacent cells, permitting direct passage of ions, second messengers (cAMP, Ca²⁺), and small metabolites (<1 kDa). They electrically couple cells — crucial in cardiac muscle (intercalated discs) and smooth muscle for coordinated contraction. Adherens junctions (cadherin-catenin complexes linked to actin filaments) form continuous adhesion belts just below tight junctions.

Cell Adhesion and Migration

Cell adhesion is mediated primarily by integrins (transmembrane heterodimers that bind ECM ligands like fibronectin via RGD sequences) and cadherins (homophilic cell-cell adhesion proteins that require Ca²⁺). Both participate in dynamic adhesion — cells constantly form and release attachments as they migrate. Cell crawling proceeds through a cycle: (1) actin polymerization pushes the leading edge forward (lamellipodium and filopodium formation), (2) new integrin-based focal adhesions form at the front, (3) actomyosin contraction pulls the cell body forward, and (4) adhesions at the rear disassemble. This cycle is regulated by Rho-family GTPases: Rac drives lamellipodium formation, Cdc42 drives filopodium formation, and Rho drives stress fiber and focal adhesion assembly. Chemotaxis — directed migration toward a soluble attractant — involves asymmetric activation of these GTPases by receptor signaling. Metastatic cancer cells hijack these same pathways to invade tissues.

How it works

Polymerization dynamics: Actin filaments and microtubules are polar polymers with structurally distinct plus and minus ends. The plus end grows faster because monomers add more readily; nucleotide hydrolysis (ATP → ADP for actin, GTP → GDP for tubulin) creates a cap of unhydrolyzed monomers at the growing end. When the cap is lost, catastrophic depolymerization ensues — a phenomenon called dynamic instability in microtubules and treadmilling in actin. Cells regulate polymerization through accessory proteins: formins nucleate unbranched actin filaments, the Arp2/3 complex nucleates branched networks, cofilin severs and depolymerizes old ADP-actin filaments, and profilin promotes ATP-actin monomer addition. For microtubules, γ-tubulin ring complexes at the centrosome nucleate new microtubules, MAPs (microtubule-associated proteins) stabilize them, and catastrophins (e.g., kinesin-13 family) promote disassembly. The drugs cytochalasin (actin depolymerizer), phalloidin (actin stabilizer), colchicine, and taxol (microtubule drugs) are classic MCAT tools — they reveal which processes depend on each filament system. Motor protein mechanisms: Each motor operates through a conserved chemo-mechanical cycle. Myosin II: ATP binding releases the head from actin, ATP hydrolysis cocks the head into a high-energy conformation, phosphate release triggers the power stroke (head pivots, pulling the actin filament), and ADP release resets the cycle. In muscle, hundreds of myosin II heads work asynchronously to produce sustained force. Kinesin-1 moves processively (many steps before dissociating) by a hand-over-hand mechanism in which the two motor domains alternate binding, each 8 nm step consuming one ATP. Dynein has a distinct AAA+ ring structure; its linker domain swings to produce the power stroke, making it structurally unrelated to kinesin despite sharing the microtubule track. Junction functions: Tight junctions create a permeability barrier through claudin strand networks — claudin composition determines paracellular ion selectivity (different claudins in different tissues). Desmosomes link to intermediate filaments, so tension is carried by the keratin network rather than by individual cell membranes, distributing force across tissue. Gap junction permeability is regulated by pH (low pH closes) and Ca²⁺ (high Ca²⁺ closes) — this closes connexons during cell injury to isolate damaged cells (a protective mechanism). ECM signaling through integrins activates focal adhesion kinase (FAK) and downstream MAP kinase pathways; changes in ECM stiffness (sensed through integrins) can alter stem cell differentiation fate.

How it works

Polymerization dynamics: Actin filaments and microtubules are polar polymers with structurally distinct plus and minus ends. The plus end grows faster because monomers add more readily; nucleotide hydrolysis (ATP → ADP for actin, GTP → GDP for tubulin) creates a cap of unhydrolyzed monomers at the growing end. When the cap is lost, catastrophic depolymerization ensues — a phenomenon called dynamic instability in microtubules and treadmilling in actin. Cells regulate polymerization through accessory proteins: formins nucleate unbranched actin filaments, the Arp2/3 complex nucleates branched networks, cofilin severs and depolymerizes old ADP-actin filaments, and profilin promotes ATP-actin monomer addition. For microtubules, γ-tubulin ring complexes at the centrosome nucleate new microtubules, MAPs (microtubule-associated proteins) stabilize them, and catastrophins (e.g., kinesin-13 family) promote disassembly. The drugs cytochalasin (actin depolymerizer), phalloidin (actin stabilizer), colchicine, and taxol (microtubule drugs) are classic MCAT tools — they reveal which processes depend on each filament system. Motor protein mechanisms: Each motor operates through a conserved chemo-mechanical cycle. Myosin II: ATP binding releases the head from actin, ATP hydrolysis cocks the head into a high-energy conformation, phosphate release triggers the power stroke (head pivots, pulling the actin filament), and ADP release resets the cycle. In muscle, hundreds of myosin II heads work asynchronously to produce sustained force. Kinesin-1 moves processively (many steps before dissociating) by a hand-over-hand mechanism in which the two motor domains alternate binding, each 8 nm step consuming one ATP. Dynein has a distinct AAA+ ring structure; its linker domain swings to produce the power stroke, making it structurally unrelated to kinesin despite sharing the microtubule track. Junction functions: Tight junctions create a permeability barrier through claudin strand networks — claudin composition determines paracellular ion selectivity (different claudins in different tissues). Desmosomes link to intermediate filaments, so tension is carried by the keratin network rather than by individual cell membranes, distributing force across tissue. Gap junction permeability is regulated by pH (low pH closes) and Ca²⁺ (high Ca²⁺ closes) — this closes connexons during cell injury to isolate damaged cells (a protective mechanism). ECM signaling through integrins activates focal adhesion kinase (FAK) and downstream MAP kinase pathways; changes in ECM stiffness (sensed through integrins) can alter stem cell differentiation fate.

Comparisons

• Biochemistry: ATP/GTP nucleotide binding and hydrolysis are central to all three motor protein cycles and to actin/microtubule polymerization dynamics. The MCAT often asks about the energetic coupling — why GTP hydrolysis is not directly coupled to microtubule polymerization but instead enables dynamic instability.
• Muscle and Organ Systems: The sliding filament model of muscle contraction (actin-myosin interaction) is a direct application of cytoskeletal motor principles. Gap junctions in cardiac intercalated discs enable the heart's electrical syncytium. Tight junctions in the blood-brain barrier are clinically relevant.
• Genetics and Disease: Mutations in keratin (epidermolysis bullosa), lamin (progeria), dystrophin (Duchenne muscular dystrophy), collagen (osteogenesis imperfecta), and connexins (Charcot-Marie-Tooth disease, congenital deafness) illustrate loss-of-function phenotypes that the MCAT tests as passage-based problems.
• Pharmacology: Colchicine (gout treatment) depolymerizes microtubules and inhibits neutrophil migration. Taxol/paclitaxel stabilizes microtubules and arrests mitosis. Cytochalasin and phalloidin are biochemical probes for actin-dependent processes.

Common confusions

• Filament diameter confusion: Microfilaments (actin) = smallest (~7 nm), intermediate filaments = middle (~10 nm), microtubules = largest (~25 nm). The MCAT will give you a diameter and ask which filament you're dealing with. Also: actin and microtubules have motor proteins and polarity; intermediate filaments have NEITHER.
• Motor direction on microtubules: Most kinesins walk to the plus end (cell periphery). Dynein walks to the minus end (centrosome). The MCAT will show you a diagram of cargo moving one direction and ask which motor is responsible. Remember: kinesin = outward, dynein = inward.
• Desmosomes vs. adherens junctions vs. hemidesmosomes: Desmosomes link to INTERMEDIATE FILAMENTS (keratin). Adherens junctions link to ACTIN. Hemidesmosomes link to INTERMEDIATE FILAMENTS but attach to the BASEMENT MEMBRANE (ECM), not to another cell. Mixing these up is a classic trap.
• Tight junction vs. gap junction permeability: Tight junctions block paracellular passage between cells (the space between cells is sealed). Gap junctions allow passage THROUGH cells (connexon channels connect the cytoplasm of adjacent cells). A question about cell-to-cell diffusion of cAMP is testing gap junctions, not tight junctions.
• Cilia vs. flagella structure: Both eukaryotic cilia and flagella use the 9+2 axoneme and axonemal dynein. The difference is in length and beating pattern, not in the underlying molecular machinery. Bacterial flagella are entirely different — they rotate via a basal motor powered by the proton gradient and are made of flagellin, not tubulin.
• ECM receptors: Integrins bind ECM components (fibronectin, laminin, collagen). Cadherins bind cadherins on adjacent cells (homophilic cell-cell adhesion, Ca²⁺-dependent). Selectins bind carbohydrates. When the MCAT describes a receptor that binds collagen and activates intracellular kinases, it's an integrin — not a cadherin.

Quick review

• Actin (microfilaments): ~7 nm, two-stranded helix, resists tension, forms cortex. Myosin motors. Polarity (+/-). Cytochalasin depolymerizes, phalloidin stabilizes.
• Intermediate filaments: ~10 nm, rope-like keratins/vimentin/lamins. NO polarity, NO motors. Pure mechanical strength.
• Microtubules: ~25 nm, hollow α/β-tubulin tubes. Radiate from centrosome. Resist compression. Dynamic instability (GTP cap). Kinesin (+) and dynein (-) motors.
• Myosin: walks actin (+ end), power stroke driven by Pi release. Myosin II = muscle contraction (bipolar filaments, sliding filament model).
• Kinesin: + end on microtubules, anterograde (outward). Dynein: - end on microtubules, retrograde (inward). Axonemal dynein: cilia/flagella bending.
• Cilia/flagella: 9+2 axoneme, outer dynein arms slide doublets, bending via nexin constraint. Primary cilium: 9+0, sensory, non-motile.
• Tight junctions: claudins/occludins, seal paracellular space, maintain polarity. Desmosomes: cadherins → intermediate filaments, mechanical. Gap junctions: connexons, ions/cAMP <1 kDa pass, electrical coupling.
• Integrins: α/β heterodimers, bind ECM (fibronectin RGD), focal adhesions. Cadherins: Ca²⁺-dependent homophilic cell-cell adhesion.
• Cell migration: lamellipodium (Rac/Arp2/3), filopodium (Cdc42), stress fibers (Rho), focal adhesion turnover, actomyosin contraction.
• Rho GTPases: Rac → lamellipodia, Cdc42 → filopodia, Rho → stress fibers and focal adhesions.