Download PROTEIN MISFOLDING

PROTEIN MISFOLDING
CLINICAL CORRELATIONS
• Folding means arriving at the right combinations of angles for every
amino acid residue in the sequence.
• Proteins fold on a defined pathway (or a small number of alternative
pathways); they don't randomly search all possible conformations until
they arrive at the most stable (lowest free energy) structure.
• Proteins that don't (re)fold on their own, need "molecular
chaperones" (which are also proteins) to keep them from slipping off
the folding pathway or to help them to get back on it.
–Some chaperones require ATP to carry out their function.
MANY DISEASES ARE THE RESULT OF DEFECTS IN PROTEIN FOLDING.
• Examples:
– CYSTIC FIBROSIS involves misfolding and resulting in lack of a protein
involved in Cl– transport across membranes.
– MANY NEURODEGENERATIVE DISORDERS INVOLVE ABNORMAL PROTEIN
AGGREGATION.
• PRION DISEASES (e.g., CJD, Creutzfeldt-Jakob disease) = spongiform
encephalopathies (also includes “mad cow” disease, scrapie in sheep, etc.)
• Alzheimer disease
• Parkinson Disease
• Huntington Disease
• PARTLY FOLDED OR MISFOLDED POLYPEPTIDES OR FRAGMENTS MAY
SOMETIMES ASSOCIATE WITH SIMILAR CHAINS TO FORM AGGREGATES.
• Partial unfolding of correctly folded proteins may also lead to aggregation.
• Aggregates vary in size from soluble dimers and trimers up to insoluble
fibrillar structures (amyloid).
CYSTIC FIBROSIS
• defective protein = CFTR (Cystic Fibrosis Transmembrane
conductance Regulator)
• LACK of normal protein, not the abnormal protein itself, causes
disease.
• Normal protein is a membrane protein (an ATP-regulated chloride
channel) in plasma membranes of epithelial cells, that pumps Cl–
ions OUT of cells
• Defective (mutant) protein doesn’t fold properly.
• Folding intermediates don't dissociate from chaperones, preventing
CFTR from insertion into membrane.
• High intracellular Cl– concentration makes cells take up H2O from
surrounding mucus by osmosis.
• Thick mucus accumulates in lungs and other tissues, and its
presence in lungs causes difficulty breathing and makes affected
individuals very subject to infections like pneumonia.
HUMAN NEURODEGENERATIVE DISORDERS
e.g., PRION DISEASES like
Creutzfeldt-Jakob Disease (CJD),
Alzheimer Disease,
Parkinson Disease,
Huntington Disease
• formation of protein aggregates, amyloid plaques/tangles,
lesions in the brain (Other amyloid-forming diseases affect other
organs, e.g., liver or heart.)
• Fibrils resulting from aggregation of different proteins (different
diseases) have common structural feature: central core of β sheets
known as a "cross-beta" structure.
SPONGIFORM ENCEPHALOPATHIES e.g.,
bovine spongiform encephalopathy (BSE, "mad cow disease"),
scrapie (sheep),
kuru (humans),
Creutzfeldt-Jakob disease (CJD, humans),
• Fatal, neurodegenerative diseases, with characteristic "holes" appearing in
brain ("sponge"-like appearance)
• Infectious, but causative agent is an abnormal protein, a "prion”
("proteinaceous infectious only" protein, PrP) (Stanley Prusiner, Nobel Prize in
Physiology/Medicine 1997).
1. Transmissible agent: various sized aggregates of a specific protein
2. Aggregates are resistant to treatment by most protein-degrading enzymes.
3. Protein is largely or completely derived from a cellular protein, PrP, (PrPC)
normally present in brain.
4. PrPC has a lot of α-helical conformation; abnormal conformation, PrPSC, has
much more β conformation, that tends to aggregate with other PRP molecules.
• Abnormal protein can be acquired by infection, or by inheritance (dominant),
or spontaneously ("sporadic" -- unknown cause).
(SPONGIFORM ENCEPHALOPATHIES, continued)
• A tiny bit of ABNORMALLY folded protein, PrPSC, can aggregate to form “crossbeta” structures, and small aggregates ("nuclei") can shift folding of normal
proteins to join the aggregation process and adopt the abnormal conformation
• In a sort of cascading "domino effect", a high concentration of abnormally
folded protein (PrPSC) forms amyloid plaques (large insoluble fibrous aggregates
with β conformation prevalent) in brain.
• These cross-beta structures fortunately don't form very fast, but if concentration
of the aggregation-prone intermediates increases, rate of cross-beta formation
can increase dramatically.
(SPONGIFORM ENCEPHALOPATHIES, continued)
• Hypothesis: misfolded proteins in all kinds of cells are always
forming cross-beta structures, but most cell types have adequate
degradative machinery to clean up the "garbage" so it doesn't
accumulate. Maybe neurons lack adequate molecular machinery to
dispose of them.
Alzheimer Disease
• Symptoms: memory loss, dementia, impairment in other forms of
cognition and behavior
• Not transmissible between individuals
• Intracellular aggregates (fibrillar tangles) of protein called tau
• Extracellular plaques contain aggregates of β-amyloid peptides
(Aβ):40-42-residue segments derived by proteolytic cleavage of a
much larger protein (amyloid precursor protein, APP) attached to
plasma membrane of neurons (function unknown)
• Peptide has flexible structure, "poised" to form fibrils (ordered
peptide aggregates).
OTHER DISEASES INVOLVING ABNORMAL FOLDING/AGGREGATION OF OTHER
PROTEINS
• PARKINSON DISEASE
– Protein that forms fibrils is α-synuclein.
– Lesions ("Lewy bodies") form in cytosol of dopaminergic neurons in substantia
negra of brain, accompanied by muscle rigidity and resting tremor.
• HUNTINGTON DISEASE (a "polyglutamine disease", trinucleotide expansion
disorder)
– Hereditary: autosomal dominant allele, caused by mutation that‘s an expansion
(abnormal lengthening) of repeated sequence CAGCAGCAG… in gene for the protein
huntingtin, encoding polyGln sequences in the protein.
– Huntington disease affects about 1 in 10,000 people in general population -- late
onset (middle age), but longer triplet expansions seem to cause earlier onset of
more rapidly progressing disease (due to the more highly aggregation-prone
mutant protein with longer polyGln sequences)
– Protein aggregates accumulate in nuclei and cytoplasm of neurons in striatum
region of the brain, which is required for coordinated movements, ultimately killing
the neurons.
– Disease leads to rigidity and dementia, always fatal.
– Other trinucleotide expansion diseases, e.g., spinocerebellar ataxias.