Organelles and ER: Quality Control & Secretory Pathway – Lecture 5

Vocabulary flashcards covering key terms from Lecture 5 on organelles, protein targeting, ER translocation, folding, glycosylation, and quality control.

Organelles

Membrane-bound compartments within a cell, each with specialized functions.

Cytoplasm

All cellular contents enclosed by the plasma membrane but outside the nucleus; major site of protein synthesis and degradation.

Cytosol

The soluble portion of the cytoplasm located outside all membrane-enclosed organelles.

Endomembrane System (Secretory Pathway)

Interconnected organelles (ER, Golgi, endosomes, lysosomes, transport vesicles, plasma membrane) through which proteins and lipids are trafficked.

Gated Transport

Protein movement between the cytosol and nucleus through nuclear pore complexes.

Protein Translocation

The passage of a polypeptide across a membrane into an organelle or its insertion into a membrane.

Vesicular Transport

Movement of cargo between compartments via membrane-bound vesicles.

Signal Sequence

Short amino-acid motif that directs a protein to a specific cellular destination.

Peroxisome

Simple, single-membrane organelle that houses enzymes for hydrogen-peroxide metabolism and other oxidative reactions.

PTS1 Signal

C-terminal tripeptide Ser-Lys-Leu directing soluble proteins to peroxisomes via Pex5 receptor.

mPTS

Membrane Peroxisomal Transport Signal that targets peroxisomal membrane proteins budding from the ER.

Nucleus

Double-membrane organelle containing the genome; site of DNA and RNA synthesis.

Perinuclear Space

Region between the inner and outer nuclear membranes, continuous with the ER lumen.

Nuclear Pore Complex (NPC)

Large multiprotein channel (≈500–1000 subunits) regulating traffic between nucleus and cytosol.

Nuclear Localization Signal (NLS)

Basic amino-acid sequence that directs proteins into the nucleus.

Nuclear Import Receptor

Cytosolic transporter that binds NLS-containing cargo and interacts with NPC FG-repeats to enter the nucleus.

Ran-GAP

Cytosolic GTPase-activating protein that converts Ran-GTP to Ran-GDP, facilitating cargo release in nuclear transport.

Ran-GEF

Nuclear guanine nucleotide exchange factor that converts Ran-GDP to Ran-GTP, driving directional nuclear import.

Endoplasmic Reticulum (ER)

Extensive membrane system; entry point for secretory pathway proteins and site of lipid synthesis, Ca²⁺ storage, and detoxification.

Rough ER (RER)

ER sub-domain studded with ribosomes; site of synthesis, folding, and modification of secretory and membrane proteins.

Smooth ER (SER)

ER sub-domain lacking ribosomes; functions in steroid synthesis, Ca²⁺ storage, and detoxification.

Signal Recognition Particle (SRP)

Ribonucleoprotein that recognizes ER signal sequences and pauses translation to target ribosome–nascent chain complexes to the ER.

SRP Receptor

ER membrane protein that binds SRP and guides the ribosome to the translocon.

Translocon

Protein-conducting channel in the ER membrane through which nascent polypeptides enter or integrate into the membrane.

Chaperone (BiP, GRP94)

ER lumenal proteins that assist in folding and prevent aggregation of newly translocated polypeptides.

Protein Disulfide Isomerase (PDI)

ER enzyme that catalyzes formation and rearrangement of disulfide bonds to stabilize protein structure.

Co-translational Translocation

Process in which a polypeptide is threaded into the ER lumen while still being synthesized by the ribosome.

Post-translational Translocation

ER import of proteins after their complete synthesis in the cytosol, often assisted by chaperones.

Stop-Transfer Sequence

Hydrophobic stretch that halts translocation and anchors a protein segment within the ER membrane.

Get Proteins

Cytosolic factors that recognize C-terminal signal sequences of tail-anchored proteins for post-translational insertion into the ER.

GPI Anchor

Glycosylphosphatidylinositol lipid added in the ER to tether certain proteins to the outer leaflet of membranes.

Glycosylation

Enzymatic addition of sugar chains to proteins; prevalent in the ER and important for folding and stability.

Oligosaccharyl Transferase

ER enzyme that transfers preassembled sugar trees to asparagine residues (N-linked glycosylation).

N-Linked Glycosylation

Attachment of an oligosaccharide to the amide nitrogen of Asn in the motif Asn-X-Ser/Thr during ER translation.

Dolichol Phosphate

Lipid carrier on which N-linked oligosaccharides are built before transfer to proteins.

Calnexin

ER membrane chaperone that binds monoglucosylated glycoproteins to assist folding.

Calreticulin

Soluble ER chaperone analogous to calnexin, aiding folding of glycoproteins.

Calnexin Cycle

Quality-control pathway where glucosidases trim glucose residues, calnexin binds, and UGGT reglucosylates misfolded proteins for another folding attempt.

Glucosidase I/II

ER enzymes that sequentially remove glucose residues from N-linked oligosaccharides during folding surveillance.

UGGT (Glucosyltransferase)

ER enzyme that adds a glucose back onto misfolded glycoproteins, allowing them to rebind calnexin.

Mannosidase (ERAD Signal)

Slow ER enzyme removing a mannose to mark chronically misfolded proteins for degradation.

ER-Associated Degradation (ERAD)

Pathway that extracts terminally misfolded proteins to the cytosol for proteasomal destruction.

Proteasome

ATP-dependent cytosolic multiprotease complex that degrades ubiquitin-tagged proteins, including ERAD substrates.

Lectin (ERAD Component)

Sugar-binding protein that recognizes demannosylated glycoproteins earmarked for extraction and degradation.

Secretory Pathway

Route by which proteins travel from the ER through the Golgi to lysosomes, plasma membrane, or extracellular space.