Cell Biology: Cell Adhesions, ECM, Cytoskeleton, and Motor Proteins

Comprehensive practice flashcards in vocabulary style covering cell adhesions, the extracellular matrix, actin dynamics, myosin motors, microtubules, intermediate filaments, kinesin, dynein, and ciliary transport.

Collagens

The most abundant family of extracellular matrix proteins in the human body, characterized by repeating $(Gly\text{-}X\text{-}Y)_n$ sequence motifs that wind together into a triple-helical superhelix to provide tissue rigidity and tensile strength.

Basal Lamina

A thin, acellular sheet-like meshwork of extracellular matrix underlying epithelial cell layers, consisting of four primary components: collagen IV, laminin, entactin (nidogen), and perlecan.

Collagen IV

The primary basement membrane collagen that forms two-dimensional flexible sheets rather than fibers, containing interruptions in its Gly-X-Y repeats that introduce flexible bends in the triple helix.

Laminin

The principal multi-adhesive heterotrimeric glycoprotein of the basal lamina that anchors epithelial cells via integrins and crosslinks collagen IV and heparan sulfate proteoglycans.

Fibronectin

The primary multi-adhesive matrix protein in connective tissues, composed of modular repeating sequences (types I, II, and III) that contain binding domains for integrins, collagens, and fibrin.

Proteoglycans

Extracellular macromolecules consisting predominantly of carbohydrate, formed by sulfated glycosaminoglycan (GAG) chains covalently linked to a core protein to provide hydrated cushioning.

Glycosaminoglycans (GAGs)

Long, unbranched polysaccharide chains of repeating acidic, sulfated disaccharide units (including hyaluronan, chondroitin sulfate, heparan sulfate, and keratan sulfate) that carry strong negative charges to attract water.

Aggrecan

The major proteoglycan in cartilage, containing numerous chondroitin sulfate and keratan sulfate chains assembled along a giant hyaluronan backbone to generate a $1\,\mu\text{m}$ sphere of hydration for joint lubrication.

Scurvy

A nutritional deficiency of vitamin C (ascorbic acid) that impairs the post-translational hydroxylation of proline and lysine residues, preventing stable collagen superhelix formation and lysyl cross-linking.

G-Actin

Globular, monomeric actin that possesses a nucleotide-binding cleft coordinating a divalent $\text{Mg}^{2+}$ cation and either $\text{ATP}$ or $\text{ADP}$.

F-Actin

Filamentous actin polymer consisting of two parallel helical strands with structural polarity, exhibiting a barbed $(+)$ end and a pointed $(-)$ end.

Critical Concentration ($C_c$)

The steady-state monomer concentration at which the rate of subunit addition precisely balances the rate of subunit dissociation; for actin, $C_c^+ = 0.12\,\mu\text{M}$ at the $(+)$ end and $C_c^- = 0.6\,\mu\text{M}$ at the $(-)$ end.

Actin Treadmilling

The steady-state kinetic behavior where ATP-bound G-actin preferentially adds to the filament $(+)$ end while ADP-actin dissociates from the $(-)$ end at an equivalent rate, moving subunits through the filament without changing overall length.

Profilin

An actin-binding protein acting as an adenine nucleotide exchange factor (AEF) that converts ADP-G-actin to ATP-G-actin and funnels monomers onto the filament $(+)$ end.

Cofilin

An actin-depolymerizing factor that preferentially binds ADP-F-actin near the $(-)$ end, increasing filament twist to promote mechanical severing and subunit dissociation.

Thymosin-$\beta 4$

A peptide that sequesters ATP-G-actin to prevent its spontaneous assembly into filaments, maintaining a high reservoir pool of unpolymerized monomeric actin in the cytoplasm.

Formin

A dimeric actin nucleator containing FH1 and FH2 domains, activated by membrane-bound Rho-GTP, that processively directs the assembly of straight, unbranched actin filaments at their $(+)$ ends.

Arp2/3 Complex

A seven-subunit protein complex activated downstream of Rac by nucleation-promoting factors (NPFs) that binds to existing microfilaments to nucleate branched actin filament networks at a $70^\circ$ angle.

Capping Proteins (Actin)

Proteins that stabilize microfilament length by blocking subunit exchange at specific ends, including CapZ at the $(+)$ end and Tropomodulin at the $(-)$ end.

Gelsolin

A calcium-activated actin-binding protein that binds along microfilaments, severs the filament backbone, and remains attached as a cap on the newly exposed $(+)$ end.

Myosin II

A two-headed, conventional motor protein with a coiled-coil tail that self-assembles into bipolar thick filaments to drive muscle contraction and cytokinesis contractile ring constriction.

Myosin V

A processive, two-headed motor protein with an extended neck region that moves vesicular and organellar cargo toward the $(+)$ end of actin filaments via a $74\,\text{nm}$ hand-over-hand mechanism.

Myosin I

A single-headed, non-filament-forming class of myosin with a short step size of $10\text{--}14\,\text{nm}$ that links microfilaments to the plasma membrane and participates in endocytosis.

Myosin Power Stroke

The mechanical step in the myosin work cycle where the release of inorganic phosphate ($\text{P}_i$) triggers lever-arm rotation of the tightly actin-bound head, pulling the actin filament.

Microtubules

Hollow, cylindrical polymers with an outer diameter of $25\,\text{nm}$ assembled from $13$ parallel protofilaments of alternating $\alpha$- and $\beta$-tubulin heterodimers.

$\alpha$-Tubulin

The tubulin subunit permanently bound to non-exchangeable $\text{GTP}$, exposed at the structural $(-)$ end of the microtubule protofilament.

$\beta$-Tubulin

The GTPase-active subunit of the tubulin heterodimer exposed at the $(+)$ end of a microtubule, which exchanges GDP for GTP and hydrolyzes bound GTP after incorporation into the polymer.

Dynamic Instability

The rapid switching between phases of steady assembly (rescue) and rapid disassembly (catastrophe) at the microtubule $(+)$ end, regulated by the presence or loss of a stabilizing GTP cap.

Catastrophe

The abrupt transition of a microtubule from elongation to rapid shrinkage, triggered when GTP hydrolysis outpaces subunit addition, exposing curved GDP-tubulin protofilaments that peel and fray.

Rescue

The sudden transition of a depolymerizing microtubule back to the growth phase, initiated when fresh GTP-tubulin dimers re-bind to the shortening end to restore the GTP cap.

Centrosome

The primary microtubule-organizing center (MTOC) of animal cells during interphase, comprising two orthogonally arranged centrioles surrounded by pericentriolar material (PCM).

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

A multiprotein ring complex located in the pericentriolar material that acts as a helical template to nucleate straight microtubules and anchor their $(-)$ ends.

Tau

A microtubule-associated protein (MAP) family member that stabilizes microtubules and bundles them closely together ($25\,\text{nm}$ spacing) within neuronal axons.

MAP2

A neuronal microtubule-associated protein located in dendrites that binds along the microtubule wall and projects a long arm to maintain wider spacing between adjacent microtubules.

Intermediate Filaments (IFs)

Tough, flexible cytoskeletal filaments with a diameter of $10\,\text{nm}$ assembled from antiparallel tetramers; they lack enzymatic activity, nucleotide binding, motor protein transport, and structural polarity.

Keratins

The most diverse group of intermediate filaments (Classes I and II), forming obligate heterodimers of acidic and basic subunits to provide mechanical resilience to epithelial cells, hair, and nails.

Desmin

A Class III intermediate filament protein in muscle cells that encircles Z-disks in skeletal muscle fibers and links dense bodies to the cell cortex in smooth muscle.

Neurofilaments

Class IV intermediate filaments (NF-L, NF-M, and NF-H) located in neuronal axons, possessing extending C-terminal sidearm domains that maintain inter-filament spacing and determine axon caliber.

Nuclear Lamins

Class V intermediate filament proteins that polymerize into the two-dimensional nuclear lamina underlying the inner nuclear membrane to provide structural support and chromatin attachment sites.

Kinesin-1

A processive, two-headed motor protein that hydrolyzes $\text{ATP}$ to transport vesicles and organelles along microtubules toward the $(+)$ end (anterograde transport).

Cytoplasmic Dynein

A massive two-headed motor protein built around two heavy chains with an AAA ATPase head and a stalk microtubule-binding domain, driving retrograde transport toward the $(-)$ end of microtubules.

Dynactin

A multi-protein adapter complex containing Arp1, CapZ, and actin that binds to the dynein stem domain to link cytoplasmic dynein to its membrane cargo.

Axonemal Dynein

The motor protein permanently anchored to the A tubules of outer doublet microtubules that generates ciliary and flagellar beating by walking toward the $(-)$ end of adjacent B tubules.

Nexin

An interdoublet elastic crosslinking protein in ciliary and flagellar axonemes that converts dynein-mediated sliding forces into bending movement.

Intraflagellar Transport (IFT)

The bidirectional transport system moving cargo along axonemal microtubules to build and maintain cilia and flagella, driven by kinesin-2 to the tip and cytoplasmic dynein to the base.

Primary Cilium

A solitary, immotile sensory organelle found on most mammalian cells whose basal body is the older centriole of the centrosome, serving as a concentrated signaling platform for receptors like odorants and opsins.

Microtubule Seam

The single longitudinal line along the 13-protofilament microtubule lattice where adjacent protofilaments are out of register, resulting in heterotypic lateral contacts between $\alpha$- and $\beta$-tubulin subunits.

Augmin

An eight-protein complex that binds along the lattice of existing microtubules to nucleate branched microtubules during mitotic spindle assembly, functioning analogously to Arp2/3 in actin networks.