The amyloid β-peptide (Aβ), strongly implicated in the pathogenesis of Alzheimer’s disease (AD), is produced from the amyloid β-protein precursor (APP) through consecutive proteolysis by β- and γ-secretases. The latter protease contains presenilin as the catalytic component of a membrane-embedded aspartyl protease complex. Missense mutations in presenilin are associated with early-onset familial AD, and these mutations generally both decrease Aβ production and increase the proportion of the aggregation-prone 42-residue form (Aβ42) over the 40-residue form (Aβ40). The connection between these two effects is not understood. Besides Aβ40 and Aβ42, γ-secretase produces a range of Aβ peptides, the result of initial cutting at the ε site to form Aβ48 or Aβ49 and subsequent trimming every 3–4 residues. Thus, γ-secretase displays both overall proteolytic activity (ε cutting) and processivity (trimming) toward its substrate APP. Here we tested whether a decrease in total activity correlates with decreased processivity using wild type and AD-mutant presenilin-containing protease complexes. Changes in pH, temperature and salt concentration that reduced overall activity of the wild type enzyme did not consistently result in increased proportions of longer Aβ peptides. Low salt concentrations and acidic pH were notable exceptions that subtly alter the proportion of individual Aβ peptides, suggesting that the charged state of certain residues may influence processivity. Five different AD-mutant complexes, representing a broad range of effects on overall activity, Aβ42-to-Aβ40 ratios, and ages of disease onset were also tested, revealing again that changes in total activity and processivity can be dissociated. Factors that control initial proteolysis of APP at the ε site apparently differ significantly from factors affecting subsequent trimming and the distribution of Aβ peptides.

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Background: Mutations in presenilin-1 (PS1) cause early-onset familial Alzheimer disease (FAD). Results: The PS1⅐␥-secretase complex trims the C terminus of long amyloid ␤-peptides (A␤), and FAD mutations significantly reduce the efficiency of trimming. Conclusion: This loss of carboxypeptidase function results in a gain of toxic A␤42 compared to A␤40. Significance: Understanding the effects of FAD mutations on ␥-secretase function is critical for developing effective treatments.