Cell Biology: Protein Trafficking, Secretion, and Endocytosis

Vocabulary flashcards covering cellular protein import, membrane targeting, secretory pathways, and endocytosis based on lecture notes.

Mannose-6-phosphate

An oligosaccharide tag formed in the Golgi when mannose residues are phosphorylated at the sixth carbon, which ensures the protein will be delivered to lysosomes.

Multivesicular Endosomes (MVEs)

Intermediate structures or early lysosomes that sequester and degrade unused cellular components.

Stop-Transfer Sequence

A hydrophobic sequence of amino acids that halts the translocation of a polypeptide through the ER pore and moves laterally to anchor permanently into the membrane.

Internal Start-Transfer Sequence

A sequence located inside the polypeptide chain (rather than at the terminus) that initiates segmented translocation into the membrane to create multi-pass transmembrane proteins.

Post-Translational Import

The mechanism where proteins are first synthesized in the cytosol and subsequently imported into the ER lumen, rather than being translated directly into the ER co-translationally.

Hsp70

A heat shock protein chaperone that binds to newly synthesized proteins in the cytosol to maintain them in an unfolded state prior to entering the ER lumen.

Sec Complex

A protein complex composed of several Sec proteins that forms the pore or translocator through which polypeptides pass into the ER lumen.

BiP

A protein recruited to the ER lumen side of the translocator that couples ATP hydrolysis to pull the polypeptide completely into the lumen.

Zymogen

An inactive precursor form of an enzyme that requires chemical modification or cleavage to become active.

Constitutive Secretion

An unregulated, continuous secretory pathway where vesicles continuously bud and fuse with the plasma membrane without requiring an external signal.

Regulated Secretion

A secretory pathway where proteins condense, mature, and accumulate in vesicles that only fuse with the plasma membrane upon receiving a specific signal.

Protein Aggregates

Clusters of highly concentrated secretory proteins formed in the trans-Golgi network that help exclude nonsecretory proteins from secretory vesicles.

Polarized Secretion

The asymmetrical release of vesicle contents exclusively toward one specific surface or face of a cell, such as toward the intestinal lumen.

Colchicine

A chemical agent that inhibits the assembly of microtubules, used to demonstrate that microtubules are involved in the movement and fusion of secretory vesicles.

Exocytosis

The process by which intracellular vesicles fuse with the plasma membrane to discharge their contents extracellularly, adding phospholipids to the plasma membrane.

Endocytosis

The process by which the plasma membrane invaginates and pinches off inward to take up external substances into vesicles, removing lipids from the plasma membrane.

Phagocytosis

A form of endocytosis responsible for internalizing solid materials or particles into the cell.

Pinocytosis

A form of endocytosis that involves the uptake of extracellular liquids into the cell.