Cell Function

Mannose-6-phosphate

A carbohydrate tag added to lysosomal enzymes in the Golgi that targets them to the lysosome.

Dolichol phosphate

Lipid carrier in the ER membrane to which sugars are first attached during N-linked glycosylation.

Flippase

Enzyme that transfers the growing oligosaccharide on dolichol from the cytosolic to the luminal side of the ER.

COPII coat

Protein coat that mediates vesicle transport from the ER to the Golgi; composed of Sec13/31, Sec23/24, and Sar1.

COPI coat

Protein coat mediating retrograde transport from Golgi to ER; contains coatomer complex and Arf1.

Clathrin coat

Vesicle coat involved in endocytosis and transport from the trans-Golgi network; includes clathrin, adaptors (AP-1 or AP-2), and dynamin.

SNARE proteins

Membrane-bound proteins that mediate specific fusion of vesicles with target membranes.

Taxol (paclitaxel)

Drug that stabilizes microtubules, blocking mitosis; developed with FSU involvement and used to treat breast cancer.

Arp2/3 complex

Actin-related protein complex that nucleates branched actin filaments and drives actin 'rockets' in Listeria.

Basal body

Modified centriole that nucleates and anchors the axoneme of cilia and flagella.

Axoneme

Core structure of cilia and flagella with a '9 + 2' arrangement of microtubules.

Sarcomere

Fundamental contractile unit of striated muscle, bordered by Z lines and containing actin and myosin filaments.

Rough endoplasmic reticulum

ER domain with bound ribosomes; site of protein synthesis, folding, and initial glycosylation.

Smooth endoplasmic reticulum

ER without ribosomes; involved in lipid metabolism, detoxification, calcium storage, and steroid synthesis.

Signal recognition particle (SRP)

Ribonucleoprotein that binds ER signal sequences and directs ribosome-mRNA complexes to the ER membrane.

Translocon

Protein channel complex in the ER membrane that allows nascent polypeptides to enter the ER lumen.

Stop-transfer sequence

Hydrophobic region in a nascent protein that halts translocation and anchors the protein in the ER membrane.

Start-transfer sequence

Internal hydrophobic signal that initiates insertion of a membrane protein into the ER membrane.

KDEL sequence

ER retrieval signal (Lys-Asp-Glu-Leu) that returns escaped ER proteins from the Golgi via COPI vesicles.

RXR retention signal

Arg-X-Arg motif that prevents premature exit of incompletely assembled proteins from the ER.

Golgi cis face

Side of the Golgi apparatus oriented toward the ER; entry site for ER-derived vesicles.

Golgi trans face

Side of the Golgi apparatus oriented toward the plasma membrane; site of sorting and exit in vesicles.

Stationary cisternae model

Model where Golgi cisternae are static and proteins move via shuttle vesicles from cis → trans.

Cisternal maturation model

Model where Golgi cisternae mature from cis to trans while cargo remains within a single cisterna.

Dynamic instability

Random switching between growth and shrinkage of microtubules due to GTP hydrolysis on β-tubulin.

Treadmilling

Simultaneous polymerization at the plus end and depolymerization at the minus end of a filament.

CapZ

Actin-binding protein that caps microfilament plus ends, preventing further polymerization or depolymerization.

Gelsolin

Actin-binding protein that severs filaments and caps the new plus ends.

Profilin

Binds G-actin and promotes polymerization at the barbed end.

Thymosin β4

Sequesters G-actin monomers to prevent polymerization.

Filamin

Crosslinks actin filaments into a gel-like network.

Fascin

Bundles actin filaments tightly in filopodia.

α-Actinin

Bundles actin filaments in stress fibers and focal adhesions.

Myosin I

Actin-based motor linking actin filaments to membranes.

Myosin II

Bipolar actin-based motor that drives muscle contraction and cytokinesis.

Myosin V

Processive actin motor for organelle and vesicle transport.

Rho GTPases

Family of small G proteins (Rho, Rac, Cdc42) that regulate actin organization; activated by GEFs and inactivated by GAPs.

Rho activation

Induces formation of stress fibers.

Rac activation

Induces lamellipodia formation.

Cdc42 activation

Induces filopodia formation.

Microtubule-organizing center (MTOC)

Cellular site (e.g., centrosome) that nucleates and anchors microtubules, with minus ends anchored.

Centrosome

Main MTOC in animal cells, containing two centrioles surrounded by pericentriolar material.

γ-tubulin ring complex (γ-TuRC)

Nucleation site in centrosomes that templates microtubule minus ends.

Ninein

Anchoring protein for microtubule minus ends at centrosomes.

Augmin

Protein complex that recruits γ-TuRCs to form branched microtubules.

Tau

Microtubule-associated protein that stabilizes axonal microtubules; hyperphosphorylated in Alzheimer's disease.

EB1

+TIP protein that binds and stabilizes microtubule plus ends.

Stathmin

Sequesters tubulin dimers and promotes microtubule depolymerization.

Catastrophin

Microtubule-destabilizing protein promoting depolymerization at microtubule ends.

Katanin

ATP-dependent enzyme that severs microtubules.

Kinesin

Plus-end-directed microtubule motor composed of two heavy and two light chains; transports cargo away from centrosome.

Dynein

Minus-end-directed microtubule motor; cytoplasmic dyneins move vesicles, axonemal dyneins power cilia/flagella.

Dynactin

Complex that links cytoplasmic dynein to its cargo.

Axonemal dynein

Dynein in cilia/flagella that drives sliding of microtubule doublets, causing bending.

Nexin

Protein linking adjacent doublets in axonemes, restricting sliding and producing bending.

Radial spoke

Projection in axonemes connecting outer doublets to the central pair; transduces sliding into bending.

Intraflagellar transport (IFT)

Bidirectional transport of materials along axonemes using kinesin (plus-end) and dynein (minus-end).

Intermediate filaments

Rope-like cytoskeletal elements 8-12 nm in diameter that provide tensile strength and mechanical stability.

Keratin

Intermediate filament protein in epithelial cells, hair, nails, and horns.

Vimentin

Class III intermediate filament in connective-tissue cells.

Desmin

Class III intermediate filament in muscle cells.

GFAP

Class III intermediate filament in glial cells.

Neurofilament proteins

Class IV intermediate filaments found in neurons.

Nuclear lamins

Class V intermediate filaments forming a network under the inner nuclear membrane.

Nestin

Class VI intermediate filament found in embryonic neurons.

Plectin

Spectraplakin that crosslinks microtubules, actin filaments, and intermediate filaments.

Titin

Protein linking thick filaments to Z lines in sarcomeres, providing elasticity.

Nebulin

Protein aligning thin filaments and linking them to Z lines.

Myomesin

M-line protein that crosslinks thick filaments in sarcomeres.

Troponin complex

Calcium-binding regulatory complex (TnT, TnI, TnC) controlling tropomyosin movement on actin filaments.

Tropomyosin

Rod-shaped protein lying along actin filaments that blocks myosin-binding sites in relaxed muscle.

Power stroke

Step in muscle contraction when ADP release from myosin causes the head to pivot and pull actin.

Cross-bridge formation

Myosin head binds actin filament using energy from ATP hydrolysis.

Cross-bridge cycle

Sequence of cross-bridge formation → power stroke → detachment (ATP binding) → cocking (ATP hydrolysis).

Endomembrane system

Network of organelles including the ER, Golgi apparatus, endosomes, lysosomes, and plasma membrane that work together in protein and lipid synthesis, processing, and transport.

Rough endoplasmic reticulum (RER)

ER domain studded with ribosomes; site of synthesis and folding of membrane and secretory proteins.

Smooth endoplasmic reticulum (SER)

ER region without ribosomes; functions in lipid synthesis, detoxification, calcium storage, and steroid biosynthesis.

Transitional elements (ER exit sites)

Regions of rough ER that form smooth tubular structures where transport vesicles bud off toward the Golgi.

Glycosylation

Addition of carbohydrate chains to proteins; begins in the ER and is modified in the Golgi apparatus.

N-linked glycosylation

Attachment of oligosaccharides to asparagine residues of proteins in the ER lumen.

Core oligosaccharide

Preassembled sugar chain of 14 residues added en bloc to proteins during N-linked glycosylation.

Protein disulfide isomerase (PDI)

Enzyme in the ER that catalyzes disulfide bond formation and rearrangement during protein folding.

BiP (Binding immunoglobulin protein)

ER-resident Hsp70 chaperone that binds nascent polypeptides to prevent misfolding and aggregation.

Chaperone

Protein that assists in folding or assembly of other proteins without being part of the final structure.

Signal sequence

Short N-terminal peptide that directs a growing polypeptide to the ER for cotranslational import.

SRP receptor

Membrane receptor that recognizes SRP-ribosome complexes and transfers them to the translocon.

Signal peptidase

Enzyme that removes the N-terminal signal sequence from proteins as they enter the ER lumen.

Cotranslational import

Process where a nascent polypeptide enters the ER as it is being synthesized on a ribosome.

Integral membrane protein

Protein embedded in the lipid bilayer through one or more hydrophobic transmembrane segments.

Peripheral membrane protein

Protein loosely bound to membrane surfaces or to integral proteins.

Multipass transmembrane protein

Protein that spans the membrane multiple times via alternating start and stop transfer sequences.

Rough ER functions

Protein synthesis, folding, assembly, initial glycosylation, and quality control.

Smooth ER functions

Drug detoxification, glycogen metabolism, calcium storage, and steroid biosynthesis.

Cytochrome P-450

Smooth ER enzyme that catalyzes hydroxylation of hydrophobic drugs for detoxification.

Hydroxylation

Addition of hydroxyl groups to hydrophobic molecules to increase solubility for excretion.

Pharmacogenetics

Study of how genetic differences in enzymes like P-450 affect drug metabolism and response.

Glucose-6-phosphatase

Smooth ER enzyme in liver cells that catalyzes the final step of glycogen breakdown, producing free glucose.

Sarcoplasmic reticulum

Specialized smooth ER in muscle cells that stores and releases calcium ions for contraction.

Hydroxymethylglutaryl-CoA reductase (HMG-CoA reductase)

Smooth ER enzyme catalyzing the rate-limiting step of cholesterol synthesis; inhibited by statins.

Membrane biosynthesis

Process of making new membrane lipids in the ER and distributing them to other organelles.