Endoproteolytic processing and stabilization of wild-type and mutant presenilin

Presenilin 1 (PS1), mutated in pedigrees of early-onset familial Alzheimer's disease, is a polytopic integral membrane protein that is endoproteolytically cleaved into 27-kDa N-terminal and 17-kDa C-terminal fragments. Although these fragments are the principal PS1 species found in normal mammalian brain, the role of endoproteolysis in the maturation of PS1 has been unclear. The present study, which uses stably transfected mouse neuroblastoma N2a cells, demonstrates that full-length polypeptides, derived from either wild-type or A246E FAD-mutant human (hu) PS1, are relatively short-lived (t1/2 1.5 h) proteins that give rise to the N- and C-terminal PS1 fragments, which are more stable (t1/2 approximately 24 h). N-terminal fragments, generated artificially by engineering a stop codon at amino acid 306 (PS1-306) of wild-type huPS1, were short-lived, whereas an FAD-linked variant that lacked exon 9 (DeltaE9) and was not endoproteolytically cleaved exhibited a long half-life. These observations suggest that endoproteolytic cleavage and stability are not linked, leading us to propose a model in which wild-type full-length huPS1 molecules are first stabilized then subsequently endoproteolytically cleaved to generate the N- and C-terminal fragments. These fragments appear to represent the mature and functional forms of wild-type huPS1.