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

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