Rough endoplasmic reticulum

Site of synthesis of secretory (exported) proteins and of N-linked oligosaccharide addition to lysosomal and other proteins.

Nissl bodies (RER in neurons)—synthesize peptide neurotransmitters for secretion.

Free ribosomes—unattached to any membrane; site of synthesis of cytosolic, peroxisomal, and mitochondrial proteins.

N-linked glycosylation occurs in the endoplasmic reticulum.

Mucus-secreting goblet cells of small intestine and antibody-secreting plasma cells are rich in RER.

Proteins within organelles (eg, ER, Golgi bodies, lysosomes) are formed in RER.

Smooth endoplasmic reticulum

Site of steroid synthesis and detoxification of drugs and poisons. Lacks surface ribosomes.

Location of glucose-6-phosphatase (last step in both glycogenolysis and gluconeogenesis).

Hepatocytes and steroid hormone-producing cells of the adrenal cortex and gonads are rich in SER.

Cell trafficking

Golgi is distribution center for proteins and lipids from ER to vesicles and plasma membrane.

Posttranslational events in Golgi include modifying N-oligosaccharides on asparagine, adding O-oligosaccharides on serine and threonine, and adding mannose-6-phosphate to proteins for lysosomal and other proteins.

Endosomes are sorting centers for material from outside the cell or from the Golgi, sending it to lysosomes for destruction or back to the membrane/Golgi for further use.

I-cell disease

(inclusion cell disease/mucolipidosis type II)—inherited lysosomal storage disorder (autosomal recessive); defect in N-acetylglucosaminyl-1-phosphotransferase → failure of the Golgi to phosphorylate mannose residues (↓ mannose-6-phosphate) on glycoproteins → enzymes secreted extracellularly rather than delivered to lysosomes → lysosomes deficient in digestive enzymes → buildup of cellular debris in lysosomes (inclusion bodies). Results in coarse facial features, gingival hyperplasia, corneal clouding, restricted joint movements, claw hand deformities, kyphoscoliosis, and ↑ plasma levels of lysosomal enzymes. Symptoms similar to but more severe than Hurler syndrome. Often fatal in childhood.

Key:

• Clathrin
• COPI
• COPII
• Retrograde
• Anterograde

#boundingbox

This diagram illustrates the intricate process of vesicular trafficking within a cell, showing the Rough Endoplasmic Reticulum, Golgi apparatus, and various vesicles involved in transporting proteins and lipids.

Signal recognition particle (SRP)

Abundant, cytosolic ribonucleoprotein that traffics polypeptide-ribosome complex from the cytosol to the RER. Absent or dysfunctional SRP → accumulation of protein in cytosol.

Vesicular trafficking proteins

• COPI: Golgi → Golgi (retrograde); cis-Golgi → ER
• COPII: ER → cis-Golgi (anterograde). “Two (COPII) steps forward (anterograde); one (COPI) step back (retrograde).”
• Clathrin: trans-Golgi → lysosomes; plasma membrane → endosomes (receptor-mediated endocytosis [eg, LDL receptor activity]).

Topics Covered:

• Rough Endoplasmic Reticulum Functions
• Smooth Endoplasmic Reticulum Functions
• Golgi Apparatus and Endosome Functions
• I-Cell Disease (Mucolipidosis Type II)
• Vesicular Trafficking Diagram and Key
• Signal Recognition Particle (SRP)
• Specific Vesicular Trafficking Proteins