The molecular chaperone, α-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 a-crystallin (1-10 mug/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 a-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.