Transmembrane Substrate Determinants for γ-Secretase Processing of APP CTFβ

Marty A. Fernandez, Kelly M. Biette, Georgia Dolios, Divya Seth, Rong Wang, Michael S. Wolfe

Research output: Contribution to journalArticlepeer-review

31 Scopus citations

Abstract

The amyloid β-peptide (Aβ) of Alzheimer's disease (AD) is generated by proteolysis within the transmembrane domain (TMD) of a C-terminal fragment of the amyloid β protein-precursor (APP CTFβ) by the γ-secretase complex. This processing produces Aβ ranging from 38 to 49 residues in length. Evidence suggests that this spectrum of Aβ peptides is the result of successive γ-secretase cleavages, with endoproteolysis first occurring at the ϵ sites to generate Aβ48 or Aβ49, followed by C-terminal trimming mostly every three residues along two product lines to generate shorter, secreted forms of Aβ: the primary Aβ49-46-43-40 line and a minor Aβ48-45-42-38 line. The major secreted Aβ species are Aβ40 and Aβ42, and an increased proportion of the longer, aggregation-prone Aβ42 compared to Aβ40 is widely thought to be important in AD pathogenesis. We examined TMD substrate determinants of the specificity and efficiency of ϵ site endoproteolysis and carboxypeptidase trimming of CTFβ by γ-secretase. We determined that the C-terminal negative charge of the intermediate Aβ49 does not play a role in its trimming by γ-secretase. Peptidomimetic probes suggest that γ-secretase has S1′, S2′, and S3′ pockets, through which trimming by tripeptides may be determined. However, deletion of residues around the ϵ sites demonstrates that a depth of three residues within the TMD is not a determinant of the location of endoproteolytic ϵ cleavage of CTFβ. We also show that instability of the CTFβ TMD helix near the ϵ site significantly increases endoproteolysis, and that helical instability near the carboxypeptidase cleavage sites facilitates C-terminal trimming by γ-secretase. In addition, we found that CTFβ dimers are not endoproteolyzed by γ-secretase. These results support a model in which initial interaction of the array of residues along the undimerized single helical TMD of substrates dictates the site of initial ϵ cleavage and that helix unwinding is essential for both endoproteolysis and carboxypeptidase trimming.

Original languageEnglish
Pages (from-to)5675-5688
Number of pages14
JournalBiochemistry
Volume55
Issue number40
DOIs
StatePublished - 11 Oct 2016

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