Protein Sorting I (L14)

Question 1

how do pathways for targeting soluble and membrane proteins to different compartments/organelles start?

Answer 1

with synthesis of polypeptides in cytoplasm

Question 2

how can post-translational uptake of precursor proteins into mitochondria be assayed w/o cells?

Answer 2

1. yeast mitochondrial proteins made by cytoplasmic ribosomes in a cell-free system
2. add energized yeast mitochondria
3. protein taken up into mitochondria -> uptake-targeting sequence removed and degraded
4. add protease -> proteins sequestered within mitochondria are resistant to protease

(if add trypsin w/o mitochondria, uptake-targeting sequence and mitochondrial protein are degraded)

Question 3

how do molecular chaperones assist in protein folding?

Answer 3

by binding and stabilizing unfolded or partly-folded proteins and preventing their aggregation or degradation

Question 4

what experiment proved that transport into mitochondrial matrix needs to be w/ unfolded polypeptide?

Answer 4

using DHFR - when uptake experiments were done in presence of methotrexate, which keeps protein folded, protein got trapped inside translocon

Question 5

describe peroxisomes

Answer 5

single membrane bound organelle w/ diverse morphology/enzymes - lack DNA and ribosomes

Question 6

function of peroxisomes

Answer 6

oxidation of organic substrates and fatty acids - metabolism of fatty acids producing acetyl-coA

Question 7

model of peroxisomal biogenesis and division

Answer 7

precursor membrane + peroxisomal membrane proteins -> peroxisomal ghost -> add PTS1 bearing matrix protein and PTS2 bearing matrix protein -> mature peroxisome

Question 8

mitochondria and peroxisomes… can they arise de novo?

Answer 8

mito - no

peroxisomes - yes, from precursor membranes derived from ER, as well as by division of pre-existing organelles

Question 9

Zellweger Syndrome

Answer 9

group I peroxisomal disorder - nearly complete loss of peroxisomal enzymes

Question 10

Zellweger-like syndrome

Answer 10

group II peroxisomal disorder - loss of only some enzymes

Question 11

clinical manifestations of Zellweger syndrome

Answer 11

• lack of peroxisomes in cells of liver, kidney, brain
• enlarged liver
• elevated levels of copper/iron in blood
• vision disturbances
• mental retardation, seizures, failure to grow, GI bleeding, dysphagia

Question 12

adrenoleukodystrophy (ALD)

Answer 12

group III peroxisomal disorder - defective oxidation of LCFA - missing only membrane transporter specific for uptake of LCFA-coA synthase

Question 13

clinical manifestations of ALD

Answer 13

classic form:

• progressive dementia
• learning disabilities, seizures, visual loss
• death 1-10 years