Identification of a β-secretase activity, which truncates amyloid β-peptide after its presenilin-dependent generation

The beta-amyloid precursor protein (betaAPP) is proteolytically processed by two secretase activities to produce the pathogenic amyloid beta-peptide (Abeta). N-terminal cleavage is mediated by beta-secretase (BACE) whereas C-terminal intramembraneous cleavage is exerted by the presenilin (PS) gamma-secretase complex. The Abeta-generating,gamma-secretase cleavage principally occurs after amino acid 40 or 42 and results in secretion of A/beta-(1-40) or Abeta-(1-42). Upon overexpression of BACE in cultured cells we unexpectedly noticed a reduction of secreted Abeta-(1-40/ 42). However, mass spectrometry revealed a truncated Abeta species, which terminates at amino acid 34 (Abeta-(1-34)) suggesting an alternative gamma-secretase cut. Indeed, expression of a loss-of-function variant of PS1 inhibited not only the production of Abeta-(1-40) and Abeta-(1-42) but also that of Abeta-(1-34). However, expression levels of BACE correlate with the amount of Abeta-(1-34), and Abeta(1-34) is produced at the expense of Abeta-(1-40) and Abeta(1-42). Since this suggested that BACE is involved in a C-teriminal truncation of Abeta, we incubated purified BACE with Abeta-(1-40) in vitro. Under these conditions Abeta-(1-34) was generated. Moreover, when conditioned media containing Abeta-(1-40) and Abeta-(1-42) were incubated with cells expressing a loss-of-function PS1 variant together with BACE, Abeta-(1-34) was efficiently produced in vivo. These data demonstrate that an apparently gamma-secretase-dependent Abeta derivative is produced after the generation of the non-truncated Abeta via an additional and unexpected activity of BACE.