The molecular chaperone, alpha-crystallin, inhibits amyloid formation by apolipoprotein C-II.
Under lipid-free conditions, human apolipoprotein C-II (apoC-II) exists in an unfolded conformation that over several days forms amyloid ribbons. We examined the influence of the molecular chaperone, alpha-crystallin, on amyloid formation by apoC-II. Time-dependent changes in apoC-II turbidity (at 0.3 mg/ml) were suppressed potently by substoichiometric subunit concentrations of alpha-crystallin (1-10 microg/ml). alpha-Crystallin also inhibits time-dependent changes in the CD spectra, thioflavin T binding, and sedimentation coefficient of apoC-II. This contrasts with stoichiometric concentrations of alpha-crystallin required to suppress the amorphous aggregation of stressed proteins such as reduced alpha-lactalbumin. Two pieces of evidence suggest that alpha-crystallin directly interacts with amyloidogenic intermediates. First, sedimentation equilibrium and velocity experiments exclude high affinity interactions between alpha-crystallin and unstructured monomeric apoC-II. Second, the addition of alpha-crystallin does not lead to the accumulation of intermediate sized apoC-II species between monomer and large aggregates as indicated by gel filtration and sedimentation velocity experiments, suggesting that alpha-crystallin does not inhibit the relatively rapid fibril elongation upon nucleation. We propose that alpha-crystallin interacts stoichiometrically with partly structured amyloidogenic precursors, inhibiting amyloid formation at nucleation rather than the elongation phase. In doing so, alpha-crystallin forms transient complexes with apoC-II, in contrast to its chaperone behavior with stressed proteins.
Hatters
D M
DM
Department of Biochemistry and Molecular Biology, the University of Melbourne, Parkville, Victoria 3010, Australia.
Lindner
R A
RA
Carver
J A
JA
Howlett
G J
GJ
D000682:Amyloid / Q000737:chemistry*; D000818:Animals; D053304:Apolipoprotein C-II; D001056:Apolipoproteins C / Q000737:chemistry*; D052160:Benzothiazoles; D002417:Cattle; D002467:Cell Nucleus / Q000378:metabolism; D002850:Chromatography, Gel; D002942:Circular Dichroism; D003459:Crystallins / Q000494:pharmacology*; D005456:Fluorescent Dyes / Q000494:pharmacology; D007700:Kinetics; D007908:Lens, Crystalline / Q000737:chemistry; D008954:Models, Biological; D011485:Protein Binding; D011487:Protein Conformation; D013844:Thiazoles / Q000494:pharmacology; D013997:Time Factors; D014461:Ultracentrifugation
D000682:Amyloid; D053304:Apolipoprotein C-II; D001056:Apolipoproteins C; D052160:Benzothiazoles; D003459:Crystallins; D005456:Fluorescent Dyes; D013844:Thiazoles; C009462:thioflavin T