The secreted form of the Alzheimer's amyloid precursor protein with the Kunitz domain is protease nexin-II

Abstract

THE A4 protein (or β -protein) is a 42- or 43-amino-acid peptide¹ present in the extracellular neuritic plaques in Alzheimer's disease and is derived from a membrane-bound amyloid protein precursor (APP). Three forms of APP have been described^2–7 and are referred to as APP695, APP751 and APP770, reflecting the number of amino acids encoded for by their respective complementary DNAs. The two larger APPs contain a 57-amino-acid insert with striking homology to the Kunitz family of protease inhibitors^5–7. Here we report that the deduced amino-terminal sequence of APP is iden-tical to the sequence of a cell-secreted protease inhibitor, protease nexin-II (PN-II)⁸. To confirm this finding, APP751 and APP695 cDNAs were over-expressed in the human 293 cell line, and the secreted N-terminal extracellular domains of these APPs were purified to near homogeneity from the tissue-culture medium. The relative molecular mass and high-affinity binding to dextran sul-phate of secreted APP751 were consistent with that of PN-II. Functionally, secreted APP751 formed stable, non-covalent, inhibitory complexes with trypsin. Secreted APP695 did not form complexes with trypsin. We conclude that the secreted form of APP with the Kunitz protease inhibitor domain is PN-II.