PRESENILIN PROTEINS UNDERGO HETEROGENEOUS ENDOPROTEOLYSIS BETWEEN THR(291) AND ALA(299) AND OCCUR AS STABLE N-TERMINAL AND C-TERMINAL FRAGMENTS IN NORMAL AND ALZHEIMER BRAIN-TISSUE

Humans inheriting missense mutations in the presenilin (PS)1 and -2 ge nes undergo progressive cerebral deposition of the amyloid beta-protei n at an early age and develop a clinically and pathologically severe f orm of familial Alzheimer's disease (FAD). Because PS1 mutations cause the most aggressive known form of AD, it is important to elucidate th e structure and function of this multitransmembrane protein in the bra in. Using a panel of region-specific PS antibodies, we characterized t he presenilin polypeptides in mammalian tissues, including brains of n ormal, AD, and PS1-linked FAD subjects, and in transfected and nontran sfected cell lines. Very little full-length PS1 or -2 was detected in brain and untransfected cells; instead the protein occurred as a heter ogeneous array of stable N- and C-terminal proteolytic fragments that differed subtly among cell types and mammalian tissues. Sequencing of the major C-terminal fragment from PS1-transfected human 293 cells sho wed that the principal endoproteolytic cleavage occurs at and near Met (298) in the proximal portion of the large hydrophilic loop. Full-leng th PS1 in these cells is quickly turned over (T-1/2 approximate to 60 min), in part to the two major fragments. The sizes and amounts of the PS fragments were not significantly altered in four FAD brains with t he Cys410Tyr PS1 missense mutation. Our results indicate that presenil ins are rapidly processed to Nand C-terminal fragments in both neural and nonneural cells and that interference with this processing is not an obligatory feature of FAD-causing mutations. (C) 1997 Academic Pres s.