Vesicular Trafficking I – Key Vocabulary

Vocabulary flashcards covering major terms, molecules, and concepts from Lecture 10 on Vesicular Trafficking I.

Secretory Pathway

Series of membrane‐bound compartments through which proteins move from the ER to the Golgi, plasma membrane, lysosome, or extracellular space.

Early Secretory Pathway

Initial segment of the secretory route that includes ER export, transit through COPII vesicles, and processing in the cis‐Golgi.

Late Secretory Pathway

Portion of the secretory route that includes transport from the trans‐Golgi network to endosomes, lysosomes, secretory vesicles, or the plasma membrane.

Transport Vesicle

Small, membrane‐enclosed carrier that moves soluble and membrane proteins between organelles.

Vesicle Budding

Process by which a vesicle forms from a donor membrane while selecting specific cargo and coat proteins.

Vesicle Docking

Initial interaction of an uncoated vesicle with the target membrane, mediated by Rab GTPases and tethering factors.

Vesicle Fusion

Final merging of vesicle and target membranes driven by SNARE complex assembly, allowing cargo delivery.

Anterograde Transport

Forward movement of vesicles from the ER toward the Golgi and beyond.

Retrograde Transport

Return flow of vesicles, such as COPI vesicles, from the Golgi back to the ER to retrieve resident proteins.

Coat Proteins

Cytosolic proteins that polymerize on membranes to induce curvature and select cargo during vesicle budding.

COPII

Coat complex (Sec23/24 + Sec13/31) regulated by Sar1 that mediates ER → cis‐Golgi transport.

COPI

Coatomer complex regulated by ARF that mediates cis‐Golgi → ER and intra‐Golgi retrograde transport.

Clathrin

Triskelion coat protein that forms polyhedral cages for vesicles leaving the trans‐Golgi network or plasma membrane.

Adaptor Protein (AP) Complexes

Multisubunit adaptors (AP1, AP2, AP3) that link clathrin to cargo receptors and specify different trafficking routes.

Sar1

ER‐specific small GTPase that initiates COPII coat assembly when bound to GTP.

ARF

Small GTPase that recruits COPI or clathrin coats to Golgi membranes when GTP‐bound.

GTPase Switch

General regulatory mechanism in which a small GTP‐binding protein cycles between an active GTP‐bound state and an inactive GDP‐bound state.

GEF (Guanine Nucleotide Exchange Factor)

Protein that activates a GTPase by promoting GDP release and GTP binding.

GAP (GTPase‐Activating Protein)

Protein that stimulates GTP hydrolysis, turning a GTPase off.

SNARE Proteins

Family of membrane‐bound proteins whose v‐ and t‐forms pair to drive vesicle fusion.

v‐SNARE

SNARE located on the vesicle membrane (e.g., VAMP) that pairs with t‐SNAREs on the target membrane.

t‐SNARE

SNARE located on the target membrane (e.g., Syntaxin, SNAP-25) that pairs with v‐SNAREs to form the fusion complex.

Rab GTPase

Small GTPase that specifies vesicle identity and recruits tethering factors for docking.

Rab Effector

Tethering protein or complex that binds GTP‐Rab on incoming vesicles to facilitate docking.

NSF

ATPase that, with α-SNAP, disassembles cis‐SNARE complexes after membrane fusion.

KDEL Signal

Luminal tetrapeptide (Lys-Asp-Glu-Leu) that marks soluble ER‐resident proteins for retrieval from the Golgi via COPI vesicles.

KDEL Receptor

Cis‐Golgi membrane protein that binds KDEL sequences at low pH and packages them into COPI vesicles for return to the ER.

Mannose-6-Phosphate (M6P)

Carbohydrate tag added in the cis‐Golgi that targets soluble hydrolases to lysosomes via clathrin/AP1 vesicles.

Di-Acidic Motif

Cytosolic Asp-X-Glu sequence that directs membrane cargo into COPII vesicles through binding Sec24.

Coatomer

Seven‐subunit complex that constitutes the COPI coat.

Sec23/Sec24

Inner COPII coat subunits; Sec24 directly binds cargo sorting motifs, while Sec23 acts as a GAP for Sar1.

Sec13/Sec31

Outer COPII cage subunits that provide structural scaffolding for the budding vesicle.

Sec16

Large peripheral protein at ER exit sites that organizes COPII coat assembly.

Constitutive Secretion

Continuous, unregulated release of vesicles from the trans‐Golgi network to the plasma membrane.

Regulated Secretion

Vesicle release from the TGN that occurs only in response to a specific extracellular signal (e.g., hormone, neurotransmitter).

Endocytosis

Uptake of plasma‐membrane‐derived vesicles into the cell, often mediated by clathrin/AP2 coats.

Trans-Golgi Network (TGN)

Distal Golgi compartment that sorts proteins into vesicles destined for lysosomes, secretory granules, or the plasma membrane.

Temperature-Sensitive Mutant

Protein variant that folds and functions at lower temperatures but misfolds or blocks transport when cells are shifted to a higher temperature, useful for trafficking studies.

GFP Fusion Protein

Chimeric protein tagged with green fluorescent protein to visualize localization and movement in live cells.

Cell-Free Assay

In vitro experimental system using isolated organelles to reconstitute and measure vesicle transport events.

Endoglycosidase Sensitivity Shift

Biochemical method where glycoproteins become resistant or sensitive to specific enzymes as they move from the ER to Golgi, indicating transport.

Sorting Signal

Short amino-acid sequence or carbohydrate modification that directs a protein into the correct transport vesicle.