Endoplasmic Reticulum and Protein Import - Vocabulary

Vocabulary flashcards covering key ER-related concepts, proteins, and mechanisms discussed in the lecture notes.

Endoplasmic Reticulum (ER)

A network of folded membranes that forms the start of the secretory pathway; consists of rough ER (with ribosomes) and smooth ER (lipid synthesis and calcium storage); connected to the nuclear envelope.

Rough Endoplasmic Reticulum (RER)

ER region studded with ribosomes; site where nascent polypeptides destined for secretion or membranes begin translation.

Smooth Endoplasmic Reticulum (SER)

ER region lacking ribosomes; involved in lipid synthesis and calcium storage; primary calcium reservoir in the ER.

Nuclear-ER connection

ER is continuous with the outer membrane of the nucleus, sharing membranes and linking nuclear processes to the secretory pathway.

Secretory Pathway

Cellular route for proteins to be secreted, delivered to membranes, or directed to organelles; begins with synthesis in the ER.

Terasaki ramps

Connecting membranous folds in the ER (parking-garage-like structure) that link different ER levels; facilitate intracellular transport.

Calcium storage in the ER

ER serves as a major calcium storage site essential for signaling and various cellular processes.

Sarcoplasmic Reticulum

Specialized ER in muscle cells that stores and releases Ca2+ to enable muscle contraction.

Ribosome (membrane-bound)

Ribosome attached to the ER membrane; translates proteins destined for secretion or membrane integration.

Ribosome (free)

Ribosome in the cytosol; synthesizes soluble cytosolic or organelle-targeted proteins.

Polyribosome

Multiple ribosomes translating a single mRNA to produce several polypeptide chains from one transcript.

Signal peptide

N-terminal hydrophobic sequence that targets a nascent protein to the ER and is typically cleaved after translocation begins.

Signal peptidase

ER-associated enzyme that cleaves off the signal peptide after the protein enters the ER.

Signal Recognition Particle (SRP)

RNA–protein complex that recognizes the signal peptide, pauses translation, and directs the ribosome–mRNA–nascent chain to the ER.

SRP receptor (SR)

ER membrane receptor that binds SRP to dock the ribosome at the Sec61 translocon.

Sec61 translocon

ER membrane pore complex through which the nascent polypeptide is threaded into the ER lumen or into the membrane.

Cotranlation import

Import of a protein into the ER while translation is ongoing; ribosome remains associated with the Sec61 translocon.

Posttranslational import

Import of a fully synthesized polypeptide into the ER after translation, often using chaperones and energy outside the ribosome.

BIP (ER HSP70)

ER lumenal HSP70 chaperone that helps pull the polypeptide through Sec61 and prevents premature folding; ATP-dependent.

HSP70 family

Family of ATP-dependent chaperones aiding protein folding and translocation in mitochondria and ER.

Transmembrane domain

Hydrophobic region of a polypeptide that spans the lipid bilayer and anchors the protein in the membrane.

Single-pass transmembrane protein

Protein that crosses the ER membrane once, containing a single transmembrane domain.

Multi-pass transmembrane protein

Protein that traverses the ER membrane multiple times, with several hydrophobic segments and alternating orientations.

Positive-inside rule

The side of a transmembrane domain enriched in positively charged residues tends to face the cytosol.

Maturation after entry

After translocation, the polypeptide folds in the ER lumen, the signal peptide is cleaved, and the protein traffics to its destination.