Organelles, Protein Trafficking & Peroxisomal Disorders

Protein targeting and the endoplasmic reticulum

Free vs bound ribosomes. Proteins for the cytosol, nucleus, mitochondria and peroxisomes (e.g., hexokinase, histones, catalase) are made on free ribosomes. Proteins that will be secreted, inserted into the plasma membrane or delivered to lysosomes carry a hydrophobic N-terminal signal sequence that routes their ribosomes to the rough ER, so translation and translocation happen together.

A protein whose signal sequence is deleted is made in the cytosol, is never secreted and is usually degraded.

Rough ER. Ribosome-studded RER makes secretory, membrane and lysosomal proteins and begins N-linked glycosylation, transferring a preformed oligosaccharide from dolichol phosphate en bloc to asparagine. It dominates cells that export protein: plasma cells (whose enlarged Golgi forms a pale perinuclear hof beside the clock-face nucleus), pancreatic acinar cells and goblet cells. In neurons it forms Nissl bodies in the cell body and dendrites but not the axon.

Smooth ER. SER lacks ribosomes. It synthesizes lipids and steroid hormones (adrenal cortex, Leydig cells, ovarian theca), carries hepatic cytochrome P450 detoxification (it expands with inducers such as phenobarbital), holds glucose-6-phosphatase for the last step of gluconeogenesis and glycogenolysis, and stores Ca2+ in muscle as the sarcoplasmic reticulum.

Folding and quality control

Chaperones. Heat-shock proteins (Hsp70, Hsp90 and chaperonins such as Hsp60) bind exposed hydrophobic stretches and help new chains fold; stress such as heat or hypoxia induces them. In the ER, BiP and the lectin chaperones calnexin and calreticulin hold glycoproteins until they fold correctly.

Ubiquitin–proteasome system. Proteins that stay misfolded are pulled back into the cytosol (ER-associated degradation), tagged with polyubiquitin by an E1–E2–E3 cascade and destroyed by the 26S proteasome. F508del CFTR and the PiZ variant of α1-antitrypsin, which also aggregates in hepatocytes, are classic examples. Defective E3 ligases cause disease: parkin in early-onset Parkinson disease, UBE3A in Angelman syndrome.

Unfolded protein response. When misfolded proteins accumulate, ER sensors slow general translation, raise chaperone production and expand degradation; unresolved stress triggers apoptosis. Antibody-secreting myeloma cells run near this limit, so the proteasome inhibitors bortezomib and carfilzomib push them into apoptosis.

Golgi apparatus and lysosomal sorting

Golgi functions. Cargo enters the cis face and leaves from the trans-Golgi network toward secretory granules, the plasma membrane or lysosomes. In transit the Golgi remodels N-linked oligosaccharides, adds O-linked sugars to serine and threonine, sulfates proteoglycans and selected tyrosines, and marks lysosomal enzymes with mannose-6-phosphate (M6P).

The M6P pathway. In the cis-Golgi, N-acetylglucosamine-1-phosphotransferase attaches GlcNAc-1-phosphate to mannose residues of lysosomal hydrolases, and an uncovering enzyme then exposes M6P. M6P receptors in the trans-Golgi gather the enzymes into clathrin-coated vesicles that fuse with late endosomes, where acidic pH releases the cargo and the receptors recycle.

I-cell disease (mucolipidosis II). Autosomal recessive GlcNAc-1-phosphotransferase deficiency leaves hydrolases untagged, so they are secreted instead of delivered. Plasma lysosomal enzyme activity is paradoxically high while fibroblasts fill with inclusions (the "I-cells"). Infants develop coarse facies, gingival hyperplasia, corneal clouding, restricted joints, skeletal dysplasia and developmental delay, and usually die in early childhood of cardiorespiratory complications. Unlike Hurler syndrome (α-L-iduronidase deficiency), urinary glycosaminoglycans are not markedly raised.

High lysosomal enzymes in plasma plus a Hurler-like child means the enzymes were made but mislabeled: think I-cell disease.

Vesicular transport

Peroxisomes and their disorders

Structure and import. Peroxisomes are single-membrane organelles whose proteins are made on free ribosomes and imported fully folded by peroxins (PEX proteins) that recognize a C-terminal Ser-Lys-Leu (SKL) signal. Their oxidases produce H2O2, which catalase, the marker enzyme, converts to water and oxygen.

• β-oxidation of very-long-chain fatty acids (22 or more carbons), which are shortened here and finished in mitochondria.
• α-oxidation of phytanic acid, a branched fatty acid from dairy, ruminant fat and some fish: phytanoyl-CoA hydroxylase starts removal of one carbon so the 3-methyl group no longer blocks β-oxidation, yielding pristanic acid.
• Plasmalogen synthesis: ether phospholipids that are major components of myelin.
• Bile acid synthesis: shortening the side chain of cholesterol-derived intermediates.

X-ALD in practice. Newborn screening now finds many affected boys before symptoms, and MRI surveillance lets transplantation happen at the first sign of cerebral involvement, when it works best. Because adrenal failure can precede neurologic signs, plasma VLCFA should be measured in any boy or man with unexplained primary adrenal insufficiency.

Adrenal crisis can be the first presentation of X-ALD: diagnosed boys need regular adrenal testing, glucocorticoid replacement and stress dosing.