Cytoskeleton Structure, Motors, and Functions

Vocabulary flashcards covering the components, molecular motors, structural properties, and physiological roles of the eukaryotic cytoskeleton based on lecture materials.

Cytoskeleton

A dynamic intracellular network of filamentous structures—comprising microtubules, microfilaments, and intermediate filaments—that provides structural support, maintains cell shape, organizes organelles, and directs motility.

Microtubules

Hollow, cylindrical cytoskeletal tubes with an outer diameter of $25\,\text{nm}$ and a $15\text{-nm}$ lumen, composed of 13 columns of tubulin dimers ($\alpha$-tubulin and $\beta$-tubulin) that act as compression-resisting girders and pathways for intracellular transport.

Microfilaments (Actin Filaments)

Two intertwined strands of polymerized actin monomers (F-actin) with a diameter of $7\,\text{nm}$ that serve as tension-bearing elements involved in cell motility, cytokinesis, and muscle contraction.

Intermediate Filaments (IFs)

Tough, rope-like cytoskeletal fibers with a diameter of $8\text{--}12\,\text{nm}$ formed from fibrous polypeptides supercoiled into thicker cables that resist tension and maintain cell integrity.

Axonal Transport

The microtubule-mediated movement of vesicles, organelles, and neurotransmitters through a nerve cell, occurring either away from the cell body (anterograde) or toward the cell body (retrograde).

Molecular Motors

Specialized mechanochemical enzymes (including kinesins, dyneins, and myosins) that convert energy released by ATP hydrolysis into mechanical energy to travel unidirectionally and stepwise along cytoskeletal tracks.

Kinesin

A tetrameric motor protein composed of two identical heavy chains and two identical light chains, possessing a rod-like stalk of $80\,\text{nm}$ and a pair of globular catalytic heads that travel unidirectionally toward the plus end of microtubules.

Cytoplasmic Dynein

A massive, multi-subunit motor protein with globular, force-generating heads that moves cargo processively toward the minus end of microtubules.

Dynactin

A multisubunit protein adaptor complex that facilitates the attachment of cytoplasmic dynein to membrane-bounded organelle and vesicle cargo.

Microtubule-Organizing Centers (MTOCs)

Specialized cellular structures that control the nucleation, orientation, and assembly of microtubules within eukaryotic cells.

Centrosome

The primary MTOC responsible for initiating and organizing microtubule nucleation in animal cells, consisting of a pair of centrioles surrounded by pericentriolar material.

Pericentriolar Material (PCM)

The electron-dense amorphous proteinaceous cloud surrounding the paired centrioles of a centrosome that nucleates microtubule assembly.

Primary Cilia

Solitary, nonmotile sensory organelles protruding from many eukaryotic cells that monitor mechanical and chemical signals in the extracellular fluid; mutations in them can cause polycystic kidney disease.

Situs Inversus

A congenital disorder characterized by the complete mirror-image reversal of normal left-right internal organ asymmetry, caused by genetic mutations affecting ciliary motility during development.

Lamins

Sequence type V intermediate filament proteins that polymerize on the inner nuclear membrane to form the nuclear lamina in all cell types.

Keratins

A diverse family of intermediate filament proteins—categorized into type I (acidic, 28 polypeptides) and type II (basic, 26 polypeptides)—that form the protective structural network in epithelial cells.

Neurofilaments

Type IV intermediate filaments composed of three subunit polypeptides (NF-L, NF-M, and NF-H) that form the core structural framework of central and peripheral nerve axons.

Plectin

A plakin-type cross-bridging protein that links intermediate filaments to microtubules, actin filaments, and adhesive cell junctions.

Conventional (Type II) Myosin

A filament-forming motor protein composed of two heavy chains, two regulatory light chains, and two essential light chains, possessing two catalytic head domains and an elongated $150\text{-nm}$ coiled-coil tail.

S1 Fragment

The isolated catalytic head-and-neck domain of myosin II produced by proteolytic cleavage that contains the actin-binding site and the ATPase motor machinery.

Unconventional Myosins

A diverse group of non-filament-forming actin motors (such as Myosin V) that function primarily in organelle transport and intracellular trafficking.

Melanophilin

An adaptor protein that forms a molecular bridge connecting the globular tail of Myosin V to Rab 27a on organellar vesicle membranes.

Rab 27a

A small membrane-associated GTPase that serves as the membrane receptor on pigment granules and vesicles to recruit melanophilin and Myosin V.