Microtubules and Microtubule Motors I

Vocabulary practice flashcards covering microtubule structure, dynamic instability, associated proteins (MAPs, +TIPs, gamma-TuRC, CAMSAP), cell-type-specific arrays, and motor proteins (kinesin, cytoplasmic dynein, and dynactin).

Tubulin Dimer

The elementary building block of microtubules consisting of an $\alpha$-tubulin and a $\beta$-tubulin subunit; both bind GTP, but only the GTP bound to $\beta$-tubulin is exposed at the dimer surface, exchangeable, and hydrolyzable.

Protofilament

A linear, head-to-tail assembly of tubulin heterodimers; 13 parallel protofilaments associate laterally in a staggered arrangement with a 3-subunit vertical pitch to form the hollow cylindrical microtubule wall.

Plus End (Microtubule)

The fast-growing, dynamic structural end of a microtubule terminated by exposed $\beta$-tubulin subunits, exhibiting kinetic rate constants of $K_{on} = 9$ and $K_{off} = 45$ for GTP-tubulin.

Minus End (Microtubule)

The slower-growing structural end of a microtubule terminated by exposed $\alpha$-tubulin subunits, exhibiting kinetic rate constants of $K_{on} = 4.5$ and $K_{off} = 23$ for GTP-tubulin.

Axoneme

The core bundle of parallel, uniformly oriented microtubules derived from cilia or flagella, historically used as a stable seed to demonstrate asymmetric tubulin addition between the plus and minus ends.

Dynamic Instability

The intrinsic behavior of microtubules characterized by stochastic transitions between periods of steady elongation (growth) and rapid shortening (depolymerization) at their ends.

GTP Cap

A stabilizing terminal layer of GTP- or $GDP\cdot P_i$-bound tubulin subunits at the growing plus end that forces protofilaments into a straight conformation, preventing lateral splaying and catastrophic disassembly.

Catastrophe (Microtubules)

The abrupt transition of a microtubule end from persistent elongation to rapid shortening, triggered when GTP hydrolysis catches up to the tip and exposes unstable, curved GDP-tubulin protofilaments.

Rescue (Microtubules)

The stochastic switch of a rapidly depolymerizing microtubule end back to persistent growth, which occurs when a new GTP cap is successfully re-acquired.

Centrosome

The primary microtubule-organizing center (MTOC) of animal cells, composed of two orthogonal centrioles surrounded by an amorphous protein matrix known as the pericentriolar material (PCM).

Centriole

A barrel-shaped organelle within the centrosome consisting of a cylindrical pinwheel array of 9 microtubule triplets linked together by accessory structural proteins.

Pericentriolar Material (PCM)

The proteinaceous cloud surrounding the centrioles that contains $\gamma$-tubulin ring complexes ($\gamma$-TuRCs) and serves as the functional site of centrosomal microtubule nucleation and anchoring.

$\gamma$-Tubulin Ring Complex ($\gamma$-TuRC)

A ring-shaped protein complex that nucleates microtubules by mimicking the plus end with a 13-subunit helical pitch and remains stably attached to cap and prevent depolymerization of the minus end.

Microtubule Seam

The single longitudinal line of lateral discontinuity along a 13-protofilament microtubule wall where $\alpha$-tubulin subunits directly contact $\beta$-tubulin subunits due to the helical rise of the lattice.

Augmin

An accessory protein complex that binds along the side of a pre-existing mother microtubule and recruits $\gamma$-TuRC to initiate branched microtubule nucleation preserving the original filament polarity.

Stathmin

A microtubule-associated protein that binds and sequesters free $\alpha\beta$-tubulin heterodimers, lowering the pool of available subunits and inhibiting microtubule polymer assembly.

Katanin

A microtubule-severing enzyme (named after the Japanese word for sword) that breaks microtubules along their length, releasing them from nucleating centers such as the centrosome.

XMAP215

A plus-end tracking polymerase that stabilizes microtubule plus ends and accelerates growth rates by recruiting and delivering free tubulin dimers directly to the polymerizing tip.

Plus-Tip Interacting Proteins (+TIPs)

A specialized group of microtubule-associated proteins that accumulate at growing plus ends to stabilize the GTP cap, promote rescue, suppress catastrophe, and mediate attachment to cellular structures such as the cell cortex.

End-Binding (EB) Proteins

Core +TIP family members (e.g., EB1 and EB3) that specifically recognize and bind the GTP cap at growing microtubule tips, serving as master recruitment hubs for other +TIP factors.

CAMSAP

A protein that specifically recognizes, binds, and stabilizes non-centrosomal microtubule minus ends, protecting them from disassembly and anchoring them to subcellular sites such as the apical cortex of epithelial cells.

Kymograph

A space-time plot displaying position or length along a linear track on one axis and time on the other, commonly used to quantify microtubule growth, shortening velocity, and motor protein dynamics.

Radial Microtubule Array

The organization typical of fibroblasts and undifferentiated cells where minus ends converge at a central centrosome and plus ends project outward radially toward the cell periphery.

Neuronal Microtubule Polarity

The distinct structural layout in neurons wherein axons contain strictly unipolar microtubules with plus ends pointing toward the terminal (+ end out), whereas dendrites display mixed microtubule polarity (+ ends pointing both outward and inward).

Polarized Epithelial Microtubule Array

A non-radial, linear arrangement of microtubules in epithelial cells where minus ends are severed by katanin and anchored at the apical surface by CAMSAP, with plus ends extending downward toward the basal surface.

Kinesin-1 (Conventional Kinesin)

A plus-end-directed motor protein dimer comprising two heavy chains with N-terminal globular ATPase motor domains, flexible neck linkers, a coiled-coil stalk, and light chains at the C-terminal tail for cargo binding.

Neck Linker

A flexible peptide segment linking kinesin's motor domain to its coiled-coil dimerization stalk; docking of this linker onto the catalytic core upon ATP binding drives the power stroke that throws the partner head forward.

Kinesin-13

A non-motile kinesin family member with a central catalytic domain that acts as a microtubule depolymerase by binding ends and promoting curved protofilament conformations to stimulate catastrophe.

Cytoplasmic Dynein

The primary minus-end-directed microtubule motor in the cytoplasm, structured as a two-heavy-chain homodimer with ring-shaped hexameric AAA domains, projecting stalks that contact the microtubule, and tail domains that associate with accessory chains.

Stalk (Dynein)

The long coiled-coil antiparallel projection protruding from dynein's AAA ring that bears the microtubule-binding domain at its tip, physically separating the catalytic ATPase machinery from the microtubule lattice.

Dynein Power Stroke

An approximate $8\,\text{nm}$ step driven by ATP hydrolysis in dynein's major AAA domain that causes a conformational shift between the linker and ring, swinging the stalk to shift the motor toward the minus end.

Dynactin

An essential multi-subunit co-activator complex for cytoplasmic dynein comprising a short Arp1 filament capped by CapZ, an extended $p150^{\text{Glued}}$ arm, and linking components that mediate cargo binding and processive tracking.

Dynamitin

The $p50$ subunit of the dynactin complex that connects the Arp1 filament base to the $p150^{\text{Glued}}$ shoulder; its overexpression acts like 'dynamite' by dissociating dynactin into nonfunctional fragments.

$$p150^{\text{Glued}}$$

A projecting dimeric arm of the dynactin complex that possesses an independent microtubule-binding domain at its tip and a dynein-binding site, functioning as a 'walking stick' to prevent dynein detachment.

Organelle Positioning by Microtubules

The coordinated spatial arrangement of membrane-bound compartments wherein cytoplasmic dynein compacts the Golgi apparatus around the centrosome near the cell center, while kinesin-1 pulls the endoplasmic reticulum outward toward the cell edge.