ENDOPROTEOLYTIC PROCESSING AND STABILIZATION OF WILD-TYPE AND MUTANT PRESENILIN

Presenilin 1 (PS1), mutated in pedigrees of early-onset familial Alzhe imer's disease, is a polytopic integral membrane protein that is endop roteolytically cleaved into 27-kDa N-terminal and 17-kDa C-terminal fr agments, Although these fragments are the principal PSI species found in normal mammalian brain, the role of endoproteolysis in the maturati on of PS1 has been unclear. The present study, which uses stably trans fected mouse neuroblastoma N2a cells, demonstrates that full-length po lypeptides, derived from either wild-type or A246E FAD-mutant human (h u) PSII, are relatively short-lived (t(1/2), 1.5 h) proteins that give rise to the N- and C-terminal PS1 fragments, which are more stable (t (1/2) similar to 24 h), N-terminal fragments, generated artificially b y engineering a stop codon at amino acid 306 (PS1-306) of wild-type hu PS1, were short-lived, whereas an FAD-linked variant that lacked exon 9 (Delta E9) and was not endoproteolytically cleaved exhibited a long half-life. These observations suggest that endoproteolytic cleavage an d stability are not linked, leading us to propose a model ill which wi ld-type full-length huPS1 molecules are first stabilized then subseque ntly endoproteolytically cleaved to generate the N- and C-terminal fra gments, These fragments appear to represent the mature and functional Terms of wild-type huPS1.