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Cleavage of Notch by the major intramembrane aspartyl protease complex γ-secretase is a central event in cell regulation and is also important to Alzheimer’s disease, with more than 200 mutations in the catalytic subunit of γ-secretase (PS1) causing severe early-onset forms of the disease. Recently, cryogenic electron microscopy (cryo-EM) has revealed the electron density of the protein-Notch complex in frozen solution, indicating major changes upon substrate binding and a possible helix unwinding to expose peptide bonds. In order understand the all-atom dynamics that cause this process, and to test the Notch binding in a membrane protein rather than solution, we developed an all-atom model of mature wild-type γ-secretase bound to Notch in a complete membrane-water system and studied the system using three independent 500-nanosecond molecular dynamics simulations. Our ensembles are in essential agreement with known cryo-EM data. As in previous simulations we find unusual β-strand transitions in exposed parts of PS1. We also observe the atomic helix motions that cause loss of helicity in bound Notch by direct comparison to corresponding 500 ns simulations of free Notch, in particular five residues to the N-terminal site of the primary cleavage site. Most importantly, we identify three conformation states, with two of them differing in the Notch-bound catalytic site. These dynamics produce a ping-pong relationship of positioning the S3 cleavage sites of Notch relative to the aspartates. These conformation states are not visible in the cryo-EM data; probably the density is an average snapshot of the two states. Our identified conformation states rationalize how Notch cleavage can be imprecise and yield multiple products. Our identified conformation states may aid efforts to develop conformation-selective drugs that target C99 and Notch cleavage differently.Statement of SignificanceThe atomic dynamics underlying cleavage of Notch by γ-secretase in the membrane is of major biological importance. Electron microscopy has revealed the protein-Notch complex in frozen solution, showing major changes upon substrate binding and helix unwinding to expose peptide bonds, but does not explain why substrate cleavage is imprecise and produces several products. Our model of wild-type γ-secretase bound to Notch in a complete membrane-water system equilibrated by 3 × 500 nanoseconds of molecular dynamics strongly complements the electron microscopy data: We identify the specific loop and helix motions that cause the β-strand transitions in PS1 and the loss of helicity in specific residues of bound Notch. We identify different conformations of Notch, which importantly affect the S3 cleavage site; the open state may cause the imprecise cleavage with earlier release of products. Our identified states can aid development of conformation-selective drugs that target C99 and Notch cleavage differently. |
