Endomembrane System and Protein Trafficking in Cells

Endomembrane system

Dynamic system of cytoplasmic membranes in cells.

Protein trafficking

Movement of proteins and lipids between organelles.

Endoplasmic reticulum (ER)

50-90% of cell membrane; includes rough and smooth ER.

Rough ER

Synthesizes membrane, secretory proteins, and glycoproteins.

Smooth ER

Synthesizes steroids, membrane lipids, and detoxifies drugs.

Glycosylation

Modification of proteins by adding sugar chains.

BiP (binding protein)

Chaperone preventing aggregation of nonpolar regions.

Protein disulfide isomerase (PDI)

Chaperone reforming disulfide bonds for correct folding.

Calcium ATPase

Moves calcium against its gradient in smooth ER.

Cis-Golgi network (CGN)

Interface between ER and Golgi apparatus.

Trans-Golgi network (TGN)

Interface between Golgi stack and plasma membrane.

Anterograde movement

Transport toward the plasma membrane.

Retrograde movement

Transport back toward the ER.

Signal sequence

Short peptide indicating protein's cellular destination.

Posttranslational import

Protein enters organelles after translation completion.

Cotranslational import

Protein enters organelles during translation process.

Nuclear localization signal (NLS)

Sequence directing proteins to the nucleus.

Mitochondrial transit sequence

Sequence guiding proteins into mitochondria.

Integral membrane proteins

Proteins embedded in cellular membranes.

Type 1 transmembrane protein

C-terminus located in cytosol.

Type 2 transmembrane protein

C-terminus located in ER lumen.

Retrieval tags

Signals for returning proteins to ER or Golgi.

Mannose-6-phosphate

Tag for lysosomal enzyme targeting.

I-cell disease

Genetic disorder causing lysosomal enzyme deficiency.

Chaperone proteins

Assist in proper protein folding.

Secretory proteins

Proteins destined for secretion outside the cell.

Lysosomal enzymes

Hydrolases required for breaking down cellular waste.

Polypeptide folding

Process of protein achieving functional three-dimensional structure.

Calcium storage

Smooth ER stores high concentrations of Ca2+.

Drug tolerance

Increased enzyme production in response to drugs.

Quality control

Mechanism ensuring proper protein folding and function.

Vesicular transport

Movement of materials in membrane-bound vesicles.

Cisternae maturation model

Model explaining Golgi protein maturation during transport.

O-linked glycosylation

Glycosylation involving attachment to serine or threonine.

N-linked glycosylation

Glycosylation involving attachment to asparagine.

Free ribosomes

Ribosomes synthesizing proteins in cytosol.

Ribosomes anchored to rough ER

Ribosomes synthesizing secretory and membrane proteins.

Transport vesicle

Vesicle carrying proteins to their destination.

Nuclear Pore Complex (NPC)

Structure facilitating transport between nucleus and cytoplasm.

Signal recognition particle (SRP)

Binds ER signal sequence during translation.

Start/stop transfer sequence

Control regions of polypeptide in membrane.

Phosphotransferase enzyme

Enzyme phosphorylating mannose on lysosomal enzymes.

Enzyme replacement therapy

Treatment involving direct introduction of functional enzymes.

Substrate reduction therapy

Reducing substrate production to prevent toxic buildup.

HeLa cells

Human cell line used for biological research.

Electrical impulse in muscle cells

Triggers calcium release from the ER.

Polypeptide

Chain of amino acids forming proteins.

Lysosome

Organelle containing enzymes for digestion.

Extracellular secretion

Release of substances outside the cell.