Chapter 15: Intracellular Compartments and Protein Transport

Vocabulary practice flashcards covering organelle functions, intracellular transport pathways, nuclear import/export, organelle translocation, vesicular coat mechanics, and degradation from Chapter 15.

Signal Sequence

A continuous stretch of 15–60 amino acids within a protein that acts as a sorting signal, directing the protein to its designated cellular compartment.

Nuclear Pore Complex

A large protein assembly composed of approximately 30 different proteins in multiple copies that perforates the double membrane of the nuclear envelope, creating a gated channel for selective macromolecular transport.

Nuclear Localization Signal (NLS)

A sorting sequence rich in positively charged lysine residues located near the beginning of an amino acid chain that directs cytosolic proteins into the nucleus.

Nuclear Import Receptor

A soluble cytosolic protein that recognizes and binds the nuclear localization signal on prospective nuclear proteins, guiding them through the nuclear pore complex.

Ran

A monomeric GTPase that provides energy and spatial directionality to nuclear transport, existing predominantly bound to GTP inside the nucleus and bound to GDP in the cytosol.

Ran-GAP (GTPase-Activating Protein)

An accessory protein located exclusively on the cytosolic side of the nuclear envelope that induces Ran to hydrolyze its bound GTP into GDP and inorganic phosphate.

Ran-GEF (Guanine Nucleotide Exchange Factor)

An accessory protein located exclusively in the nucleus that causes Ran-GDP to release GDP and bind GTP, maintaining high levels of Ran-GTP within the nucleoplasm.

Protein Translocator

A membrane-bound protein complex that forms a translocational channel, transferring unfolded polypeptide chains across the membranes of mitochondria, chloroplasts, and the endoplasmic reticulum.

Signal Peptidase

An enzyme associated with translocators that cleaves the signal sequence from translocating proteins during import into organelles such as the ER or mitochondria.

Signal Recognition Particle (SRP)

A cytosolic ribonucleoprotein complex that recognizes and binds both the ER signal sequence of a newly forming polypeptide and the ribosome, halting translation until docked at the ER membrane.

SRP Receptor

An integral membrane protein embedded in the cytosolic face of the rough ER membrane that binds the SRP-ribosome complex, facilitating docking to a protein translocator.

Stop-Transfer Sequence

A stretch of hydrophobic amino acids that halts translocation through the ER protein translocator, releasing the polypeptide laterally into the lipid bilayer to anchor it as a transmembrane segment.

Start-Transfer Sequence

An internal hydrophobic amino acid sequence that initiates protein translocation into the ER lumen without being cleaved, remaining within the lipid bilayer as a membrane-spanning alpha helix.

Dolichol

A specialized membrane-anchored lipid that holds a preformed 14-sugar branched oligosaccharide before its transfer onto an asparagine residue during N-linked protein glycosylation in the ER.

Oligosaccharyl Transferase

A membrane-bound enzyme with its active site situated in the ER lumen that catalyzes the en bloc transfer of an oligosaccharide from dolichol to an asparagine ($\text{Asn}$) residue of a translocating polypeptide.

Clathrin

A coat protein that polymerizes into basketlike, polyhedral cages on the cytosolic surface of membranes to induce curvature and shape budding transport vesicles.

Adaptin

A coat component that selects and secures specific cargo receptors while simultaneously anchoring clathrin to the cytosolic surface of budding vesicles.

Dynamin

A small cytosolic GTPase that polymerizes into a ring around the invaginated neck of a budding vesicle, constricting upon GTP hydrolysis to pinch off the vesicle from the donor membrane.

Rab Protein

A family of small GTPases present on vesicle surfaces that are recognized by matching tethering proteins on target membranes to ensure accurate docking specificity.

v-SNARE

A transmembrane protein incorporated into transport vesicle membranes that winds tightly with complementary t-SNAREs on target membranes, bringing bilayers within $1.5\,\text{nm}$ to drive membrane fusion.

t-SNARE

A transmembrane protein localized on the target membrane that forms an interlocking helical bundle with a matching v-SNARE on an incoming transport vesicle to catalyze bilayer fusion.

Constitutive Secretory Pathway

A continuous, unregulated exocytosis pathway operating in all eukaryotic cells that supplies freshly synthesized proteins and lipids to the plasma membrane and secretes soluble components into the extracellular fluid.

Regulated Secretory Pathway

A specialized exocytosis pathway where proteins are concentrated and stored in secretory vesicles, releasing their cargo only when prompted by a specific extracellular trigger, such as a hormone or neurotransmitter.

Pinocytosis

A form of endocytosis ('cell drinking') characterized by the continuous ingestion of extracellular fluid and small solutes in vesicles having a diameter of $< 150\,\text{nm}$.

Phagocytosis

A specialized form of endocytosis ('cell eating') in which dedicated phagocytic cells extend pseudopods to ingest large particles, such as microorganisms or dead cells, into vesicles measuring $> 250\,\text{nm}$ in diameter.

Transcytosis

An endosomal sorting pathway in polarized epithelial cells where endocytosed receptors and bound cargo are transferred across the cell from one plasma membrane domain to an opposing domain.

Acid Hydrolases

A diverse group of approximately 40 hydrolytic enzymes located in lysosomes—including nucleases, proteases, glycosidases, and lipases—that exhibit optimal catalytic activity at an acidic pH of approximately $5.0$.

Autophagy

A cellular self-degradation pathway in which worn-out organelles or cytosolic components are enclosed within a double-membrane autophagosome that subsequently fuses with a lysosome.

Peroxisome

A membrane-enclosed organelle that carries out oxidative breakdown of toxic molecules, synthesizes myelin, and imports folded proteins from both the cytosol and the endoplasmic reticulum.

Peroxins

A family of proteins encoded by at least 12 distinct genes that mediate peroxisome biogenesis by establishing the organelle membrane and facilitating the import of cytosolic enzymes.

Zellweger Spectrum Disorder

A severe congenital condition caused by mutations in peroxin genes that disrupts peroxisome biogenesis, causing myelin breakdown, hypotonia, feeding difficulties, sensorineural loss, and infant mortality typically within the first year.