Endoproteolytic processing and stabilization of wild-type and mutant presenilin

Presenilin 1 (PSI), 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 un- clear. 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 (t 1/4 1.5 h) proteins that give rise to the N- and C-terminal PS1 fragments, which are more stable (t 1/4 ~ 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 (ΔE9) 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.