Microtubules II: Intracellular Transport, Cell Migration, and Ciliary Motility

Vocabulary practice flashcards covering intracellular axonal transport, melanosome trafficking, microtubule regulation of directional cell migration, MAP family stabilization, and ciliary/flagellar axoneme structure.

Anterograde axonal transport

The kinesin-mediated, plus-end-directed transport of organelles and materials—such as synaptic vesicles and healthy mitochondria—from the neuronal cell body along the axon to synaptic terminals.

Retrograde axonal transport

The dynein-mediated, minus-end-directed transport of damaged materials—including aged mitochondria, autophagosomes, and multivesicular bodies—from synaptic terminals back to the cell body for degradation in lysosomes.

Bidirectional organelle motility

The movement pattern of vesicles and organelles carrying both kinesin and dynein motors simultaneously, allowing them to reverse direction like a "rear gear" when encountering obstacles along a microtubule.

Melanophores

Specialized fish skin pigment cells containing a radial microtubule array that rapidly disperse or aggregate melanosomes (melanin granules) to alter coloration based on background signaling.

$\text{cAMP}$ regulation of melanosome transport

The secondary messenger pathway in fish melanophores wherein increased $\text{cAMP}$ activates kinesin and inhibits dynein to disperse pigment granules, whereas decreased $\text{cAMP}$ activates dynein and inhibits kinesin to aggregate granules at the cell center.

Latrunculin

An actin-depolymerizing drug that sequesters actin monomers; in fish melanophores, its addition forces dispersed pigment granules all the way to the cell periphery instead of remaining evenly distributed.

Myosin V (in pigment transport)

An actin-dependent motor protein bound to melanosomes that walks along randomly oriented cortical actin filaments, taking granules off microtubule tracks to maintain uniform dispersion throughout the cytoplasm.

Contact inhibition of locomotion

The property of normal cells to stop migrating and proliferating when they make physical contact with surrounding cells in a monolayer; this regulatory feature is characteristically lost in cancer cells.

Asymmetric microtubule array (in migrating cells)

A polarized cytoskeletal arrangement where the leading edge contains numerous, more stable microtubules oriented toward the front, while the trailing rear contains fewer, highly dynamic microtubules subject to breakage.

Stathmin

A regulatory protein that sequesters two $\alpha\beta$-tubulin dimers to prevent assembly and promote microtubule catastrophes; its activity is inhibited at the leading edge by downstream effectors of Rac GTPase to stabilize frontal microtubules.

GEF-H1

A guanine nucleotide exchange factor for RhoA that is inhibited when bound to intact microtubules; its release following microtubule depolymerization triggers GDP-to-GTP exchange on RhoA, inducing actomyosin contraction.

RhoA

A small monomeric GTPase activated at the rear of migrating cells that stimulates Rho-kinase (activating myosin II) and formin (polymerizing actin filaments) to generate contractile force for trailing-edge retraction.

Microtubule-mediated focal adhesion disassembly

The process whereby dynamic microtubules repeatedly target and contact focal adhesions at the cell rear, weakening adhesion complexes to enable smooth rear retraction without triggering catch-bond strengthening.

MAP2

A microtubule-associated protein enriched specifically in neuronal dendrites that stabilizes microtubules and possesses long projection domains that cross-link microtubules with wide inter-filament spacing.

Tau

A microtubule-associated protein localized specifically to neuronal axons that stabilizes microtubules and bundles them closely together via shorter projection domains.

MAP4

A microtubule-associated protein of the MAP2/tau family that binds, stabilizes, and cross-links microtubules in non-neuronal cells.

Ciliary motion

A coordinated, paddle-like motility cycle consisting of a rigid forward effective stroke (power stroke) to propel fluid and a flexible, low-resistance recovery stroke curled close to the cell surface.

Flagellar motion

A symmetrical, wave-like or undulating motion propagating along the length of a flagellum that produces a propeller-like forward thrust to drive cellular locomotion.

Basal body

A modified centriole (the mother centriole) anchored at the cell cortex composed of 9 microtubule triplets ($9 \times 3 + 0$ arrangement) arranged in a cartwheel structure, which nucleates the ciliary or flagellar axoneme.

Axoneme

The structural cytoskeletal core of motile cilia and flagella, consisting of 9 outer microtubule doublets encircling a central pair of singlet microtubules in a classic $9 + 2$ arrangement.

Microtubule doublet

The outer structural unit of the axoneme composed of a complete A tubule (13 protofilaments) fused to an incomplete B tubule (10–11 protofilaments) that shares part of the A tubule wall.

Axonemal dynein

Multi-headed minus-end-directed motor complexes anchored along the A tubule of outer doublets that generate sliding forces against the adjacent B tubule upon ATP hydrolysis.

Nexin

An elastic linking protein connecting adjacent outer microtubule doublets within the axoneme; it prevents unlimited sliding between doublets and converts dynein-driven shearing into regular axonemal bending.

$\gamma\text{-TuRC}$ ($\gamma$-tubulin ring complex)

A multi-protein ring complex that nucleates microtubule assembly at centrosomes or membranes and remains associated with the minus end to prevent spontaneous depolymerization.

Augmin

An eight-subunit protein complex that binds along the wall of an existing microtubule to nucleate branched microtubule growth.

Kinesin-13

A specialized non-motile kinesin motor family that binds to microtubule plus and minus ends to actively destabilize protofilaments and induce catastrophic disassembly.

+TIPs

Plus-end tracking proteins that specifically recognize and associate with growing microtubule plus ends to regulate filament dynamics and couple them to other targets, such as the cell cortex or membranes.

XMAP215

A microtubule polymerase protein that binds free tubulin dimers and stabilizes growing plus ends to dramatically accelerate microtubule elongation.

Katanin

An ATP-dependent microtubule-severing enzyme that breaks microtubules along their length, releasing them from the centrosome to establish non-centrosomal arrays in differentiated cells.