Impas 1 possesses endoproteolytic activity against multipass membrane protein substrate cleaving the presenilin 1 holoprotein

Presenilins (PS1 and PS2) are supposed to be unusual aspartic proteases and components of the gamma-secretase complex regulating cleavage of type I proteins. Multiple mutations in PS1 are a major cause of familial early-onset Alzheimer's disease (AD). We and Others recently identified PS-related families of proteins (IMPAS/PSH/signal peptide peptidases (SPP)). The functions of these proteins are yet to be determined. We found that intramembrane protease-associated or intramembrane protease aspartic protein Impas 1 (IMP1)/SPP induces intramembranous cleavage of PS1 holoprotein in cultured cells coexpressing these proteins. Mutations in evolutionary invariant sites in hIMP1 or specific gamma-secretase inhibitors abolish the hIMP1-mediated endoproteolysis of PS1. In contrast, neither AD-like mutations in hIMP1 nor in PS1 substrate abridge the PS1 cleavage. The data suggest that IMP1 is a bi-aspartic polytopic protease capable of cleaving transmembrane precursor proteins. These data, to our knowledge, are a first observation that a multipass transmembrane protein or the integral protease per se may be a primary substrate for an intramembranous proteolysis.