Cytoskeleton Structure and Cell Motility (2A) - MCAT Biological and Biochemical Foundations of Living Systems

About $25\ \text{nm}$. Microtubules are the largest cytoskeletal filaments, formed from tubulin dimers, aiding in rigidity and transport.

About $10\ \text{nm}$. Intermediate filaments have an intermediate size, providing mechanical strength without polarity.

G-actin (globular actin). G-actin monomers polymerize to form filamentous F-actin, the structural basis of microfilaments.

$\alpha$-tubulin and $\beta$-tubulin heterodimers. Tubulin heterodimers assemble into protofilaments that form the hollow tubular structure of microtubules.

Plus-end addition with minus-end loss at steady state. Treadmilling maintains constant filament length through net assembly at one end and disassembly at the other.

The plus end. The plus end exhibits faster polymerization due to favorable kinetics of tubulin addition.

The plus (barbed) end. The plus end has a lower critical concentration for actin addition, promoting faster polymerization.

GTP (tubulin is a GTPase). GTP hydrolysis by tubulin drives dynamic instability, enabling rapid microtubule remodeling.

ATP (actin is an ATPase). ATP hydrolysis by actin subunits regulates filament dynamics, including treadmilling and critical concentration differences.

Polarity from head-to-tail tubulin dimer assembly. Head-to-tail arrangement of tubulin dimers creates inherent polarity, directing growth and motor movement.

Axonemal dynein. Axonemal dynein arms produce sliding forces between microtubule doublets, causing ciliary bending.

Lamellipodia (branched actin network). Lamellipodia use Arp2/3-mediated actin branching to push the plasma membrane forward during cell migration.

Polarity due to asymmetric actin subunit orientation. Asymmetric orientation of actin subunits imparts structural polarity, influencing assembly and motor protein directionality.

Myosin (most myosins are plus-end directed). Myosins interact with actin to generate force, with most types walking toward the barbed end.

Dynein. Dyneins facilitate retrograde transport, moving cargo toward the microtubule minus end at the cell center.

Kinesin. Kinesins are ATP-dependent motors that transport cargo anterogradely along microtubules toward the cell periphery.

$\gamma$-tubulin ring complex ($\gamma$-TuRC). γ-TuRC acts as a template for tubulin dimer addition, initiating microtubule polymerization from the minus end.

The centrosome. The centrosome serves as the primary site for microtubule nucleation and organization in animal cells.

Switching between growth and rapid shrinkage (catastrophe/rescue). Dynamic instability allows microtubules to explore cellular space and reorganize rapidly during processes like mitosis.

Nine outer doublets surrounding two central singlets. The 9+2 axoneme structure enables coordinated bending for motility in cilia and flagella.

Microfilaments (actin), microtubules, and intermediate filaments. These filaments provide structural support, enable motility, and facilitate intracellular transport in eukaryotic cells.

Actin microfilaments with myosin II. Actin-myosin interactions generate the contractile force to divide the cytoplasm in cytokinesis.

Microtubules. Microtubules form the spindle apparatus to separate chromosomes during cell division.

Tensile strength and mechanical stability. Intermediate filaments resist mechanical stress, maintaining cell integrity under tension.