Cytoplasmic Organelles

BiP (Binding Immunoglobulin Protein) uses ATP to mask exposed hydrophobic regions on nascent proteins, preventing aggregation and helping them fold properly.

The Sec61 complex forms the physical protein-conducting channel (translocon) across the RER membrane. Blocking it halts co-translational translocation.

The rough endoplasmic reticulum (RER) derives its descriptive name directly from the attachment of membrane-bound ribosomes on its outer cytosolic side.

Glucose-6-phosphatase is a resident enzyme of the SER membrane, playing a key role in regulating blood glucose levels.

The membrane network of the endoplasmic reticulum shares physical continuity with the outer nuclear membrane, allowing structural integration.

Because COPII vesicles carry cargo forward (anterograde), blocking them causes proteins to pile up within the ER.

The smooth endoplasmic reticulum (SER) lacks ribosomes and contains the specialized enzymatic machinery necessary for lipid and steroid metabolism.

Ribophorins I and II are structural glycoproteins found uniquely on the RER membrane that secure ribosomes in place during synthesis.

Cisternae are the flattened fluid-filled sacs making up the ER and Golgi. Cristae belong to mitochondria, and thylakoids belong to chloroplasts.

The active sites of lipid synthesis enzymes face the cytosol, adding new lipid molecules directly to the cytosolic leaflet of the SER.

The SER contains the Cytochrome P450 enzyme family, which hydroxylates hydrophobic compounds to increase solubility and ease excretion.

The sarcoplasmic reticulum is a specialized SER that hoards calcium ions, releasing them into the sarcoplasm to trigger muscle contraction.

The Signal Recognition Particle (SRP) binds the emerging N-terminal signal sequence of a peptide, stalling translation until it docks with the RER membrane.

The Lys-Asp-Glu-Leu (KDEL) sequence at the C-terminus of ER-resident proteins ensures they are captured and recycled back if they escape to the Golgi.

The Unfolded Protein Response (UPR) activates signaling loops to expand ER volume, increase chaperone expression, and halt translation to manage stress.

A core carbohydrate tree is built on dolichol phosphate, a membrane lipid anchor, before being transferred to an asparagine residue of the growing peptide.

Transmembrane proteins are threaded into the RER membrane during translation, moving via vesicles through the secretory pathway to reach the plasma membrane.

Emerin is a vital inner nuclear membrane protein that anchors the nuclear lamina. Its loss disrupts nuclear structure, causing Emery-Dreifuss muscular dystrophy.

The inner nuclear membrane contains integral proteins (like LBR) that bind to the nuclear lamina to stabilize chromatin positioning.