PROTEOLYTIC PROCESSING AND SECRETION OF HUMAN BETA-AMYLOID PRECURSOR PROTEIN IN YEAST - EVIDENCE FOR A YEAST SECRETASE ACTIVITY

Human beta-amyloid precursor protein (APP), the transmembrane precurso r of the Alzheimer's disease beta-amyloid peptide, was expressed in th e yeast Saccharomyces cerevisiae by fusion to prepro-alpha-factor, Fro m analysis of protease deficient yeast strains, the fusion protein und erwent partial processing by Kex2 protease to generate full-length APP and a fraction of the molecules were degraded in the vacuole. A solub le APP ectodomain fragment bearing lumenal but not cytosolic epitopes was released into the media, indicating cleavage by a ''membrane prote in-solubilizing proteinase'' or ''secretase'' activity. Yeast cells co ntained a C-terminal APP fragment that co-migrated with authentic C-te rminal fragment derived from alpha-secretase cleavage of full-length A PP in human cells. The N-terminal sequence of immunoaffinity purified C-terminal APP fragment from yeast was identical to that reported in m ammalian and insect cells. These results demonstrate the existence of a secretase activity in yeast. Furthermore, this yeast secretase activ ity may be related to an APP processing activity present in metazoan c ells.