Distinct properties of wild-type and the amyloidogenic human cystatin C variant of hereditary cerebral hemorrhage with amyloidosis, Icelandic type.

[hereditary cerebral hemorrhage with amyloidosis]

Variant human cystatin C (L 68 Q ) is an amyloidogenic protein . It deposits in the cerebral vasculature of Icelandic patients with cerebral amyloid angiopathy , leading to stroke . Wild-type and variant cystatin C are cysteine proteinase inhibitors which form concentration dependent inactive dimers ; however , variant cystatin C dimerizes at lower concentrations and has an increased susceptibility to a serine protease . We studied the effect of the L 68 Q amino acid substitution on cystatin C properties , utilizing full length cystatin C purified in mild conditions from media of cells stably transfected with either the wild-type or variant cystatin C genes . The variant cystatin C forms fibrils in vitro detectable by electron microscopy in conditions in which the wild-type protein forms amorphous aggregates . We also show by circular dichroism , steady-state fluorescence and Fourier-transformed infrared spectroscopy that the amino acid substitution modifies cystatin C structure by destabilizing alpha-helical structures and exposing the tryptophan residue to a more polar environment , yielding a more unfolded molecule . These spectral changes demonstrate that variant cystatin C has a three -dimensional structure different from that of the wild-type protein . The structural differences between variant and wild-type cystatin C account for the susceptibility of the variant protein to unfolding , proteolysis and fibrillogenesis .