Vesicular Traffic, Secretion, and Endocytosis

Vocabulary practice flashcards covering molecular mechanisms of vesicle coat assembly, docking, fusion, sorting signals, and Golgi trafficking based on Chapter 14.

Transport vesicles

Membrane-bounded containers that collect cargo proteins by budding from a donor compartment and deliver them by fusing with a specific target membrane.

Secretory pathway

The pathway directing the distribution of soluble and membrane proteins synthesized at the rough ER to final cellular destinations, including the cell surface (secretion) or lysosomes.

Constitutive secretion

A continuous process in all cells where transport vesicles constantly move from the trans-Golgi network and fuse with the plasma membrane, releasing soluble proteins and integrating membrane proteins.

Regulated secretion

A pathway in specialized cell types in which cargo proteins accumulate in regulated secretory vesicles and fuse with the plasma membrane only upon receiving a neuronal or hormonal signal.

Endocytic pathway

The cellular pathway wherein vesicles budding from the plasma membrane internalize soluble extracellular proteins and deliver them to lysosomes via late endosomes.

COPII-coated vesicles

Transport vesicles that mediate anterograde movement from the rough ER to the $cis$-Golgi, coated with Sec23/Sec24, Sec13/Sec31 complexes, and Sec16, governed by the Sar1 GTPase.

COPI-coated vesicles

Transport vesicles that mediate retrograde movement from the $cis$-Golgi back to the ER and from later to earlier Golgi cisternae, coated by coatomers with seven COP subunits and regulated by ARF.

Clathrin-coated vesicles

Coated transport vesicles that move materials from the trans-Golgi network to endosomes/lysosomes and from the plasma membrane to endosomes, utilizing clathrin together with adapter complexes such as AP1, AP2, or GGA.

Sar1

A monomeric GTPase switch protein that controls the assembly and disassembly of the COPII coat on ER donor membranes.

Sec12

An ER integral membrane protein that acts as a guanine nucleotide exchange factor (GEF), stimulating Sar1 to exchange GDP for GTP and insert its hydrophobic N-terminus into the ER membrane.

Sec23/Sec24 complex

A COPII coat complex recruited by Sar1·GTP; Sec24 binds sorting signals on cytosolic domains of membrane cargo proteins, while Sec23 functions as a GTPase-activating protein (GAP) for Sar1.

Di-acidic sorting signal

A cytoplasmic targeting signal (e.g., Asp-X-Glu) on ER cargo membrane proteins recognized and bound by the Sec24 subunit of the COPII coat.

KDEL sequence

A C-terminal luminal retrieval signal (Lys-Asp-Glu-Leu) found on soluble ER-resident proteins that directs their return from the $cis$-Golgi network to the ER via COPI vesicles.

KDEL receptor

A membrane receptor located in the $cis$-Golgi network that binds missorted KDEL-bearing proteins in the acidic pH of the Golgi and releases them in the higher pH of the ER.

KKXX signal

A C-terminal cytosolic sorting signal (Lys-Lys-X-X) found on ER-resident membrane proteins that binds directly to the $\alpha$ and $\beta$ subunits of the COPI coat for retrograde transport.

Rab GTPases

Lipid-anchored GTPase switch proteins on transport vesicles that bind specific effector complexes on target membranes to govern vesicle docking.

SNARE complex

A four-helix coiled-coil bundle formed by one v-SNARE (VAMP) and cognate t-SNAREs (one syntaxin helix and two SNAP-25 helices) that pulls opposing membranes together to drive fusion.

NSF (N-ethylmaleimide-sensitive factor)

A hexameric ATPase that, aided by $\alpha$-SNAP, binds post-fusion SNARE complexes and hydrolyzes ATP to dissociate and recycle SNARE proteins.

cis-Golgi network (CGN)

An interconnected tubular entry network on the cis face of the Golgi complex nearest the ER that receives incoming COPII vesicles and sorts proteins.

trans-Golgi network (TGN)

A tubular network at the exit face of the Golgi complex pointing away from the ER that sorts modified proteins into distinct transport vesicles directed toward lysosomes, endosomes, or the cell surface.

Cisternal maturation model

The mechanism of Golgi transport wherein individual cisternae progressively mature and move in an anterograde direction from $cis$ to $trans$, while Golgi-resident enzymes are retrieved backward by retrograde COPI vesicles.

Golgi matrix

A group of fibrous scaffold proteins that mechanically support the Golgi complex and play a key role in its disassembly and reassembly during cell division.

O-linked glycosylation

A post-translational modification taking place in the Golgi complex involving the addition of oligosaccharides to the hydroxyl oxygen of serine or threonine residues.