Cell Biology: Cytoskeleton, Signal Transduction, Cell Signaling, and Intracellular Transport

Vocabulary practice flashcards covering cell structure and function, including the cytoskeleton (intermediate filaments, microtubules, actin), signal transduction mechanisms (GPCRs, RTKs, Wnt, Notch), neurobiology and synapses, and intracellular transport systems (nuclear import, ER targeting, and secretion).

Intermediate Filaments

Rope-like cytoskeletal fibers approximately $25\text{ nm}$ in diameter that are the most stable and durable cytoskeletal fibers, enabling cells to withstand mechanical stress.

Central Rod Domain of Intermediate Filament Monomer

A conserved $\alpha$-helical structural region of $310\text{--}350\text{ amino acids}$ flanked by globular amino ($\text{N}$) and carboxyl ($\text{C}$) terminal domains of variable size and structure.

Desmosome

A cell junction connecting the cytoskeletons of adjacent epithelial cells, formed by keratin intermediate filaments linked via an intracellular linker protein plaque to transmembrane cadherin proteins across the intercellular space.

Hemidesmosome

A specialized cell junction where keratin filaments are anchored to the extracellular matrix (such as the skin basal lamina) via transmembrane integrins.

Epidermolysis Bullosa Simplex (EBS)

A genetic disorder caused by mutations in keratin genes, where basal epidermal cells rupture between the nucleus and hemidesmosomes, causing skin blisters to develop after minor trauma or pressure.

Neurofilaments

Intermediate filaments found in nerve cells that form bundles cross-linked by protein cross-bridges, providing mechanical strength to the axon and greatly outnumbering microtubules.

Nuclear Lamina

A mesh-like network of lamin intermediate filament fibers lining the inner face of the inner nuclear membrane that provides structural support and attachment sites for chromatin.

Tubulin Dimer

The subunit of microtubules consisting of $\alpha$-tubulin and $\beta$-tubulin, which binds $\text{GTP}$ or $\text{GDP}$ and creates structural polarity with an $\alpha$ end ($-\text{ end}$) and a $\beta$ end ($+\text{ end}$).

Microtubule GTP Cap

A stabilized region at the growing plus end of a microtubule formed when $\text{GTP}$-tubulin dimers add faster than the rate of $\text{GTP}$ hydrolysis by the dimers.

Vinca Alkaloids

Chemotherapy drugs that bind to free tubulin dimers, preventing microtubule assembly and polymerization.

Taxanes

Chemotherapy drugs that bind to microtubules and enhance their stability, preventing depolymerization and blocking the separation of chromosomes during anaphase.

Kinesin and Cytoplasmic Dynein

Microtubule-dependent motor proteins that transport vesicles and organelles using $\text{ATP}$ hydrolysis; kinesin moves toward the plus ($+\text{ end}$) end and dynein moves toward the minus ($-\text{ end}$) end.

Axoneme Microtubule Bending

The process in cilia and flagella where dynein head groups walking along the B tubule toward the minus end cause doublet microtubules to bend because they are anchored by nexin linking proteins.

G-Actin

Globular actin, a polarized $\text{ATP}$/$\text{ADP}$-binding monomer of $375\text{ amino acids}$ with a plus ($+\text{ end}$) and a minus ($-\text{ end}$) that polymerizes into filamentous actin.

Actin Treadmilling

A dynamic steady-state process where $\text{G-actin}$ bound to $\text{ATP}$ associates at the plus end of an $\text{F-actin}$ filament while $\text{ADP}$-bound actin dissociates from the minus end, causing a shift in filament position.

Sarcomere

The repeating contractile unit of a myofibril, measuring approximately $2.5\,\mu\text{m}$ in length, bounded by Z discs that anchor the plus ends of actin filaments overlapping central myosin thick filaments.

Myosin Power Stroke

The force-generating conformational change during muscle contraction where inorganic phosphate ($\text{P}_i$) release triggers tight binding of myosin to actin, displacing the filament and releasing $\text{ADP}$.

Tropomyosin and Troponin Complex

Accessory proteins on actin filaments where tropomyosin blocks myosin-binding sites until $\text{Ca}^{2+}$ binds to the troponin complex, shifting tropomyosin and exposing the sites for contraction.

T-Tubule / Sarcoplasmic Reticulum Coupling

The junction where depolarization of transverse (T) tubules opens voltage-gated $\text{Ca}^{2+}$ channels, which are physically linked to and open $\text{Ca}^{2+}$-release channels in the sarcoplasmic reticulum membrane.

Signal Transduction

The process by which an extracellular signal is relayed to its final intracellular target(s) through a cascade of intracellular signaling molecules to alter cell behavior.

Cortisol Receptor

An intracellular nuclear receptor protein that remains inactive in the cytosol until bound by lipid-soluble cortisol, undergoing a conformational change that allows it to enter the nucleus and regulate target gene transcription.

Trimeric G Protein Activation

The dissociation of an inactive $\alpha\beta\gamma$ complex upon signal-induced $\text{GDP}$ release and $\text{GTP}$ binding to the $\alpha$ subunit, releasing an active $\alpha$ subunit and an active $\beta\gamma$ complex to activate downstream effectors.

Protein Kinase A (PKA)

A cyclic $\text{AMP}$-dependent kinase whose inactive tetrameric form (two regulatory and two catalytic subunits) dissociates into active catalytic subunits when cyclic $\text{AMP}$ binds the regulatory subunits.

Phospholipase C (PLC)

A membrane-bound enzyme activated by $\text{G}_\alpha$ that hydrolyzes phosphatidylinositol bisphosphate ($\text{PIP}_2$) into membrane-bound diacylglycerol ($\text{DAG}$) and soluble inositol 1,4,5-trisphosphate ($\text{IP}_3$).

Calmodulin

A small cytosolic $\text{Ca}^{2+}$-binding protein that modulates cellular functions by activating $\text{Ca}^{2+}$/calmodulin-dependent protein kinases (CaM-kinases) and phosphatases upon binding $\text{Ca}^{2+}$.

Wnt Signaling Pathway

A pathway where Wnt binding to Frizzled activates Disheveled to inactivate an $\text{APC}$-containing degradation complex, stabilizing $\beta$-catenin so it can enter the nucleus, activate the $\text{TCF}$ complex, and drive cell proliferation.

Receptor Tyrosine Kinases (RTKs)

Enzymic cell-surface receptors with a single transmembrane domain that dimerize upon ligand binding, trans-autophosphorylate cytoplasmic tyrosine residues, and recruit downstream signaling proteins.

SH2 Domain

Sarcoma-oncogene Homology domain 2; a conserved protein domain in intracellular signaling adaptors that recognizes and binds specific phosphorylated tyrosines on activated receptor tyrosine kinases.

Ras-MAPK Pathway

A signaling cascade initiated by $\text{RTK}$ activation that recruits an adaptor protein and $\text{Ras-GEF}$ (Sos) to activate the monomeric $\text{G}$ protein Ras, triggering a sequential phosphorylation cascade of $\text{MAPKKK}$, $\text{MAPKK}$, and $\text{MAPK}$.

Sevenless and Bride of Sevenless (Boss)

A ligand-receptor pair in Drosophila eye development where Boss on the R8 cell binds Sevenless ($\text{RTK}$) on the neighboring cell to induce its differentiation into the R7 photoreceptor.

PI 3-Kinase / Akt Pathway

A cell survival pathway wherein activated $\text{RTK}$ stimulates $\text{PI 3-kinase}$ to generate $\text{PIP}_3$, recruiting and activating Akt, which then phosphorylates Bad to release active, anti-apoptotic Bcl2.

Notch Pathway

A direct contact-dependent signaling mechanism wherein binding of the transmembrane Delta signal protein cleaves the cytoplasmic tail of the Notch receptor, allowing it to migrate to the nucleus and activate target genes.

Endocrine Signaling

A mode of intercellular communication in which signaling cells secrete hormones into the bloodstream for distribution throughout the body to reach distant target cells.

Paracrine Signaling

A mode of cell signaling where signaling molecules (such as cytokines) are secreted into the extracellular fluid and act locally by diffusing to neighboring target cells without entering the bloodstream.

Autocrine Signaling

A specialized form of paracrine signaling where a cell secretes a signal molecule that binds to receptors on its own surface, triggering a response in the same cell.

EPSP and IPSP Synaptic Potentials

Post-synaptic membrane potential changes where excitatory neurotransmitters (e.g., glutamate) open $\text{Na}^+$ channels causing depolarization ($\text{EPSP}$), while inhibitory neurotransmitters (e.g., $\text{GABA}$) open $\text{Cl}^-$ channels causing hyperpolarization ($\text{IPSP}$).

Acetylcholinesterase

An enzyme located on target cells that degrades acetylcholine in the synaptic cleft to terminate synaptic transmission; inhibited by nerve gas, leading to spastic paralysis.

Cocaine Mechanism of Action

The pharmacological inhibition of presynaptic dopamine transporter proteins, preventing dopamine re-uptake and causing prolonged stimulation of pleasure pathways in the limbic system.

Receptor Down-Regulation

An adaptive response wherein post-synaptic cells decrease the number of surface receptors in response to excessive stimulation (such as sustained dopamine in the presence of cocaine), leading to tolerance and dependence.

Substance P

An $\text{11-amino-acid}$ neuropeptide released at central nervous system synapses by sensory neurons in response to painful stimuli, causing an $\text{EPSP}$ that transmits pain signals to the brain.

Nuclear Pore Complex

A large multi-protein complex composed of $50\text{--}100\text{ polypeptides}$ spanning both nuclear membranes, possessing an overall diameter of $120\text{ nm}$ and an aqueous channel opening of $25\text{ nm}$.

Nuclear Localization Signal (NLS)

A short, typically positively charged stretch of $7\text{ amino acids}$ on cargo proteins that is exposed on the folded protein surface and directs transport into the nucleus.

Importin (Nuclear Import Receptor)

A cytosolic receptor protein that binds to the nuclear localization signal ($\text{NLS}$) on prospective nuclear proteins and guides them through the gel-like meshwork of the nuclear pore complex.

Ran GTPase Cycle in Nuclear Transport

The compartmentalized cycle where nuclear $\text{Ran-GTP}$ binds Importin to release cargo in the nucleus, and cytosolic $\text{RanGAP}$ stimulates $\text{GTP}$ hydrolysis to $\text{Ran-GDP}$, releasing Importin back into the cytoplasm.

RanGAP and RanGEF

Accessory regulators of the Ran cycle: $\text{RanGAP}$ is localized in the cytosol and activates $\text{GTP}$ hydrolysis, whereas $\text{RanGEF}$ is localized in the nucleus and promotes the exchange of $\text{GDP}$ for $\text{GTP}$.

Signal Recognition Particle (SRP)

A cytosolic ribonucleoprotein complex that recognizes and binds an $\text{N}$-terminal hydrophobic $\text{ER}$ signal sequence on a growing polypeptide, pausing translation and escorting the ribosome to an $\text{SRP}$ receptor on the rough $\text{ER}$.

Signal Peptidase

An $\text{ER}$-associated enzyme that cleaves the amino-terminal $\text{ER}$ signal sequence from translocated proteins during or shortly after entry into the $\text{ER}$ lumen.

Stop-Transfer Sequence

A stretch of non-polar, hydrophobic amino acids that stops translocation of a growing polypeptide chain across the $\text{ER}$ membrane, anchoring it as a transmembrane protein.

Golgi Apparatus Secretion Pathway

The vesicular transport route where proteins synthesized in the rough $\text{ER}$ enter the cis Golgi network, undergo carbohydrate modifications across the medial cisternae, and are sorted at the trans Golgi network for delivery.

Constitutive vs. Regulated Secretion

The two exocytic transport routes from the Golgi: constitutive secretion operates continuously in all cells as default bulk flow, whereas regulated secretion occurs only in specialized secretory cells upon stimulation by an external signal.