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 e xamined the influence of the molecular chaperone, alpha -crystallin, on amy loid formation by apoC-II. Time-dependent changes in apoC-II turbidity (at 0.3 mg/ml) were suppressed potently by substoichiometric subunit concentrat ions of a-crystallin (1-10 mug/ml). alpha -Crystallin also inhibits time-de pendent changes in the CD spectra, thioflavin T binding, and sedimentation coefficient of apoC-II. This contrasts with stoichiometric concentrations o f a-crystallin required to suppress the amorphous aggregation of stressed p roteins 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 aff inity interactions between alpha -crystallin and unstructured monomeric apo C-II. Second, the addition of alpha -crystallin does not lead to the accumu lation of intermediate sized apoC-II species between monomer and large aggr egates as indicated by gel filtration and sedimentation velocity experiment s, suggesting that alpha -crystallin does not inhibit the relatively rapid fibril elongation upon nucleation. We propose that alpha -crystallin intera cts stoichiometrically with partly structured amyloidogenic precursors, inh ibiting amyloid formation at nucleation rather than the elongation phase. I n doing so, alpha -crystallin forms transient complexes with apoC-II, in co ntrast to its chaperone behavior with stressed proteins.