Folding and stability of the extracellular domain of the human amyloid precursor protein

The beta-amyloid peptide (Abeta), the major component of the senile plaques found in the brains of Alzheimer's disease patients, is derived from proteolytic processing of a transmembrane glycoprotein known as the amyloid precursor protein (APP). Human APP exists in various isoforms, of which the major ones contain 695, 751, and 770 amino acids. Proteolytic cleavage of APP by alpha- or beta-secretases releases the extracellular soluble fragments sAPPalpha or sAPPbeta, respectively. Despite the fact that sAPPalpha plays important roles in both physiological and pathological processes in the brain, very little is known about its structure and stability. We have recently presented a structural model of sAPPalpha(695) obtained from small-angle x-ray scattering measurements (Gralle, M., Botelho, M. M., Oliveira, C. L. P., Torriani, I., and Ferreira, S. T. ( 2002) Biophys. J. 83, 3513 - 3524). We now report studies on the folding and stabilities of sAPPalpha(695) and sAPPalpha(770). The combined use of intrinsic fluorescence, 4-4'-Dianilino-1,1' binaphthyl-5,5'-disulfonic acid ( bis-ANS) fluorescence, circular dichroism, differential ultraviolet absorption, and small-angle x-ray scattering measurements of the equilibrium unfolding of sAPPalpha(695) and sAPPalpha(770) by GdnHCl and urea revealed multistep folding pathways for both sAPPalpha isoforms. Such stepwise folding processes may be related to the identification of distinct structural domains in the three-dimensional model of sAPPalpha. Furthermore, the relatively low stability of the native state of sAPPalpha suggests that conformational plasticity may play a role in allowing APP to interact with a number of distinct physiological ligands.