Conformational Dynamics of Transmembrane Domain 3 of Presenilin 1 Is Associated with the Trimming Activity of γ-Secretase.

γ-Secretase is an intramembrane-cleaving protease that generates the toxic species of the amyloid-ß peptide (Aß) that is responsible for the pathology of Alzheimer disease. The catalytic subunit of γ-secretase is presenilin 1 (PS1), which is a polytopic membrane protein with a hydrophilic catalytic pore. The length of the C terminus of Aß is proteolytically determined by its processive trimming by γ-secretase, although the precise mechanism still remains largely unknown. Here, we identified that transmembrane domain (TMD) 3 of human PS1 is involved in the formation of the intramembranous hydrophilic pore. Notably, the water accessibility of TMD3 was greatly altered by point mutations and compounds, which modify γ-secretase activity. The changes in the water accessibility of TMD3 was also correlated with Aß42 production. Moreover, crosslinking between TMD3 and TMD7 resulted in a loss of sensitivity to a γ-secretase modulator that reduces Aß42 production. Therefore, our findings indicate that the conformational dynamics of TMD3 is a prerequisite for regulation of the Aß trimming activity of γ-secretase.SIGNIFICANCE STATEMENT Modulation of γ-secretase activity to reduce the level of toxic amyloid-ß species is thought to be a therapeutic strategy for Alzheimer disease. However, the detailed mechanism of the regulation of amyloid-ß production, as well as the structure-and-activity relationship of γ-secretase remains unclear. Here we identified that the water accessibility around transmembrane domain 3 in presenilin 1 was increased along with a reduction in toxic amyloid-ß production. Our findings demonstrate how the structure of presenilin 1 dynamically changes during amyloid-ß production, and provides insights toward the development of treatments against Alzheimer disease.

Cooperative roles of hydrophilic loop 1 and the C-terminus of presenilin 1 in the substrate-gating mechanism of γ-secretase.

γ-Secretase is a multisubunit protease complex that is responsible for generating amyloid-ß peptides, which are associated with Alzheimer disease. The catalytic subunit of γ-secretase is presenilin 1 (PS1), which contains an initial substrate-binding site that is distinct from the catalytic site. Processive cleavage is suggested in the intramembrane-cleaving mechanism of γ-secretase. However, it largely remains unknown as to how γ-secretase recognizes its substrate during proteolysis. Here, we identified that the α-helical structural region of hydrophilic loop 1 (HL1) and the C-terminal region of human PS1 are distinct substrate-binding sites. Mutational analyses revealed that substrate binding to the HL1 region is critical for both ε- and γ-cleavage, whereas binding to the C-terminal region hampers γ-cleavage. Moreover, we propose that substrate binding triggers conformational changes in PS1, rendering it suitable for catalysis. Our data provide new insights into the complicated catalytic mechanism of PS1.