4 Protein Targeting

• protein completes translation in the cytoplasm by cytosolic ribosomes & will have 1 of 4 fates -

• 1. cytosol
• 2. mitochondria
• 3. nucleus
• 4. peroxisomes

• translation stops → moved to the ER → translation finished on the endoplasmic reticulum by RER ribosomes & will have 1 of 5 fates

• 1. plasma membrane protein
• 2. endoplasmic reticuclum
• 3. secretory vesicles (be secreted)
• 4. lysosomes
• 5. golgi

the first 2 dozen amino acids of a protein that specify which part of the cell the protein is going to

the signal is located on a protein's N-terminus, or 5' end, that's synthesized first

SKL (tri-peptide serene lysine leucine)

• located on the C-terminus

• a defect in SKL means the peroxisome won't have proteins to degrade toxins

KDEL (tetra peptide sequence)

• used to retrieve proteins that try to escape from ER

a short 5-7 basic amino acid sequence (small)

a LONG N-terminal AA sequence

• proteins that complex with nascent polypeptides & if they sense they're folded properly, direct them to their destination

• are widely conserved from bacteria to humans

• many are Heat-Shock Proteins (HSP)

• conditions that cause unfolding of newly synthesized proteins (eg. elevated temperature, toxins) induce chaperone proteins

• • they bind unfolded, mis-folded & aggregated proteins & CHAPERONE them to proteasomes for degradation
• - are a major mechanism of getting rid of defective proteins

• can be found in many cellular compartments (cytosol, Golgi, nucleus, mitochondria, ER, peroxisomes)

organelles that break down oxidative products, detoxify cells, & contain enzymes that destroy free radicals

1. Zellweger Syndrome

2. NALD (Neonatal Adrenoleukodystrophy)

the extracellular environment (the OUTSIDE of the cell)

packages proteins inside the cell before they are sent to their destination

• cis golgi is near the nucleus

• trans golgi is farther from the nucleus (closer to the plasma membrane)

in the ER, but not in the Golgi

in the ER & Golgi

cis-Golgi ONLY

ER: translation, sorting, initial sugar addition

cis Golgi network: Phosphorylation (+ trim, add, sort)

cis, medial, & trans cisterna: trim, add, sort

trans Golgi network: process & sort to either the lysosome, PM, or secretory vesicle

organelles with acid hydrolase enzymes that break down waste materials & cellular debris

• degradative enzymes work best at low pH achieved by H⁺ ATPase pump in lysosome membrane (pH = 5)

• • let’s say you start with 2 proteins in the RER
• • in the cis golgi network, mannose 6-phosphate kinase (a phosphotransferase) puts a phosphate group on the 6 position of a mannose residue on enzymes destined for the lysosome
• • as the proteins move into the medial + trans golgi they physically separate
• • the M6P receptor only recognizes mannose-6-phosphate groups
• • other protein will be concentrated in the trans golgi → vesicle containing this protein will bud off → travel to the PM → fuse → release contents to ECM
• • lysosomal protein + receptor has bud off in it’s own vesicle from the golgi & still exists at a neutral pH
• • hydronium pump pumps in protons, lowering pH
• • vesicle can now fuse with other highly acidic endosomes
• • as the pH drops, the M6P receptor can no longer hold onto the lysosomal enzyme protein [even the phosphate group comes off]
• • M6P receptors will physically separate & either be recycled to the trans golgi [to pick up more lysosomal enzymes] or will be shuttled to the plasma membrane [to pick up stray lysosomal membranes from neighboring cells]
• • pure vesicle of lysosomal proteins will fuse with a lysosome

mannose-6-phosphate

• mannose 6-phosphate kinase adds phosphate to 6th position on a mannose sugar in a protein destined to be a lysosomal enzyme

• caused by a defect in sphingolipid metabolism

• was the 1st lysosomal storage disease successfully treated with enzyme replacement therapy

• caused by a defect in sphingolipid metabolism

• successfully treated with enzyme replacement therapy

• caused by a defect in sphingolipid metabolism

• prevalent in Ashkenazi Jews (common on East Coast)

• caused by a defect in 1 of the several enzymes needed to break down Glycosaminoglycans

• a few can be treated with enzyme replacement or enzyme enhancement therapies

eg. Hunter, Hurler, Sanfillipo, Pompe, Morquio

• • proteins are not properly targeted for lysosome in the Golgi
• • M6P signal is missing
• • end up being secreted → build up of substances in lysosomes because there's no enzyme to degrade them
• • caused by a defect in the phosphotransferase that puts a phosphate group on the 6-position of specific mannose residues
• • can also be caused by a defective mannose-6-phosphate receptor protein (rare)
• • no lysosomal enzymes get to lysosomes → all substrates constipate the lysosome
• • rare disease that revealed mechanisms for LSD & lysosomal/organellar biogenesis

• 1. Fabry's
• 2. Goucher's
• 3. [Hunter’s, a MCPS]

another protein targeting disease: cytoplasmic proteins accumulate in nucleus because of mutation that creates NLS signal