Endoplasmic Reticulum, Golgi, and Vesicular Transport

Vocabulary flashcards covering key terms from the lecture on lipid transport, ER functions, protein folding, glycosylation, quality control, Golgi processing, and vesicular trafficking.

Lipid Transfer

Movement of newly synthesized lipids from the ER to target membranes via vesicles or lipid-specific transfer proteins.

Vesicular Transport

General process where cargo-filled membrane buds off one compartment and fuses with another.

Lipid Transfer Proteins

ER enzymes that selectively pick specific lipids and insert them into mitochondria or chloroplast membranes.

Cytochrome P450

Smooth-ER heme proteins that hydroxylate hydrophobic molecules, aiding detoxification.

Hydroxylation

Addition of –OH groups to make molecules more water-soluble; catalyzed by P450 enzymes.

Smooth Endoplasmic Reticulum (SER)

ER domain lacking ribosomes; functions in lipid synthesis, detoxification, Ca²⁺ storage, and glycogen metabolism.

Detoxification

Metabolic conversion of hydrophobic toxins into hydrophilic products for urinary excretion.

Drug Tolerance

Need for higher drug doses after P450 induction increases drug clearance.

Barbiturates

Hydrophobic sedatives that induce P450 enzymes, leading to tolerance and alcohol synergy.

Aromatic Hydrocarbon Hydroxylase

P450 subclass that metabolizes polycyclic hydrocarbons; oxidation can create carcinogens.

Glucose-6-phosphatase

SER membrane enzyme that removes phosphate from glucose-6-P so glucose can exit hepatocytes.

Glycogen

Branched glucose polymer serving as cytosolic energy reserve in liver and muscle.

Glycogenolysis

Breakdown of glycogen to glucose-1-P by glycogen phosphorylase.

Glycogen Phosphorylase

Enzyme that releases glucose-1-phosphate from glycogen branches.

Phosphoglucomutase

Enzyme that converts glucose-1-P to glucose-6-P inside cells.

GLUT1

Plasma-membrane transporter that moves free glucose but not phosphorylated glucose.

Sarcoplasmic Reticulum

Specialized SER in muscle that stores Ca²⁺ for contraction.

Calcium ATPase

SR pump that uses ATP to sequester Ca²⁺ into the SR lumen.

Rough Endoplasmic Reticulum (RER)

ER domain studded with ribosomes; site of co-translational import and protein maturation.

Signal Peptide

N-terminal hydrophobic/basic sequence that targets nascent proteins to the RER.

Signal Recognition Particle (SRP)

Cytosolic ribonucleoprotein that recognizes the signal peptide and pauses translation.

SRP Receptor

RER membrane protein that binds SRP-ribosome complexes, positioning them on the translocon.

Translocon (Sec61)

Protein channel in RER membrane through which nascent chains enter the ER lumen.

Co-translational Translocation

Simultaneous protein synthesis and passage of the polypeptide into the ER lumen.

Signal Peptidase

ER lumen protease that cleaves the signal peptide once translocation begins.

Stop-Transfer Sequence

20–25 hydrophobic residues that halt translocation, anchoring membrane proteins.

Type I Membrane Protein

Single-pass protein with N-terminus in lumen; generated by signal peptide plus stop-transfer segment.

Type II/III Membrane Protein

Single-pass protein whose internal signal acts as both start and anchor, determining orientation.

Type IV Membrane Protein

Multipass protein inserted by alternating start- and stop-transfer sequences.

Chaperones

Proteins that assist folding of nascent chains and prevent aggregation.

BiP

ER-resident Hsp70 chaperone that binds emerging polypeptides and unfolded proteins.

Hsp70 Family

ATP-dependent chaperones that stabilize hydrophobic regions during folding.

Hsp60 (GroEL/GroES)

Barrel-shaped chaperonins providing an isolated folding chamber for polypeptides.

Protein Folding

Process by which polypeptides acquire native three-dimensional conformation.

N-linked Glycosylation

Attachment of a pre-assembled oligosaccharide to Asn within Asn-X-Ser/Thr in the ER.

O-linked Glycosylation

Addition of sugars to Ser or Thr hydroxyls, mainly in the Golgi.

Dolichol

ER membrane lipid carrier that builds and flips the oligosaccharide used for N-glycosylation.

Oligosaccharyl Transferase

ER enzyme that transfers the entire oligosaccharide from dolichol to the Asn residue.

Calnexin

ER membrane lectin chaperone that binds glycoproteins with one terminal glucose.

Calreticulin

Soluble ER lectin chaperone performing the same quality-control role as calnexin.

Glucosidase

ER enzyme that sequentially removes terminal glucoses from N-linked oligosaccharides.

ER-associated Degradation (ERAD)

Pathway that retro-translocates misfolded ER proteins to the cytosol for ubiquitin-mediated destruction.

GPI Anchor

Glycosylphosphatidylinositol lipid added to some proteins’ C-termini, tethering them to the membrane outer leaflet.

Disulfide Bond

Covalent S–S linkage between cysteines that stabilizes extracellular domains.

Protein Disulfide Isomerase (PDI)

ER enzyme that catalyzes formation and rearrangement of disulfide bonds.

Mannosidase

ER/Golgi enzyme that trims mannose; removal in ERAD marks terminally misfolded proteins.

Ubiquitin

76-aa protein covalently attached to lysines to signal various cellular fates, including degradation.

E1 Enzyme

Ubiquitin-activating enzyme that forms a high-energy thioester with ubiquitin.

E2 Enzyme

Ubiquitin-conjugating enzyme that receives ubiquitin from E1 and works with E3.

E3 Ubiquitin Ligase

Protein that recognizes the substrate and transfers ubiquitin from E2 to it.

Proteasome

ATP-dependent cylindrical complex that degrades polyubiquitinated proteins into peptides.

Unfolded Protein Response (UPR)

ER stress program that halts translation, up-regulates chaperones, and enhances degradation; prolonged activation triggers apoptosis.

Signal Sequence

Discrete amino-acid motif that directs a protein to a specific cellular compartment.

Protein Sorting

Overall process of sending proteins to their correct cellular destinations based on signal sequences.

Golgi Apparatus

Stack of cisternae that modifies, sorts, and packages proteins received from the ER.

Cis-Golgi

Golgi face nearest the ER; entry site for vesicles and initial processing reactions.

Trans-Golgi

Golgi face furthest from the ER; exit site where cargo is sorted into destination-specific vesicles.

ERGIC

ER-Golgi intermediate compartment where COPII vesicles fuse before entering the Golgi.

Anterograde Transport

Forward vesicle flow from ER through Golgi toward plasma membrane or lysosomes.

Retrograde Transport

Backward vesicle flow returning escaped or resident proteins to earlier compartments.

Clathrin

Coat protein forming triskelions that assemble into polyhedral cages on trans-Golgi and plasma-membrane buds.

COPI Coat

Coatomer complex that forms retrograde vesicles from Golgi to ER or between Golgi cisternae.

COPII Coat

Sar1-dependent coat that forms anterograde vesicles budding from the ER to the Golgi.

Triskelion

Three-legged clathrin subunit composed of three heavy and three light chains.

Adaptor Proteins

Bridge proteins (e.g., AP complexes, GGAs) that link cargo receptors to coat proteins.

ARF GTPase

Small G protein that initiates COPI and clathrin coat assembly at Golgi membranes.

Sar1 GTPase

ER-associated G protein that triggers COPII coat formation.

Rab GTPase

Large family of small G proteins that label vesicles and mediate tethering to target membranes.

Dynamin

GTPase that forms a collar around budding clathrin vesicles and constricts to effect fission.

SNARE Proteins

Complementary v- and t-membrane proteins whose coiling drives membrane fusion.

v-SNARE

SNARE located on the vesicle membrane, pairing specifically with its target counterpart.

t-SNARE

SNARE located on the target membrane that pairs with the matching v-SNARE.

Tethering Proteins

Long fibrous factors on target membranes that capture Rab-labeled vesicles from a distance.

Phosphatidylinositol

Phosphorylated membrane lipid whose varying head-group phosphorylation marks distinct trafficking domains.

Cargo Receptor

Transmembrane protein that binds soluble cargo in donor compartments and recruits coat machinery.