Nicastrin functions to sterically hinder γ-secretase–substrate interactions driven by substrate transmembrane domain
- Ann Romney Center for Neurologic Diseases, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA 02115
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Edited by Benjamin F. Cravatt, The Scripps Research Institute, La Jolla, CA, and approved November 24, 2015 (received for review July 1, 2015)
Significance
γ-Secretase is a conserved and ubiquitous intramembrane-cleaving protease (I-CLiP). Its normal function is required for proper notch signaling, and its processing of amyloid precursor protein is implicated in Alzheimer’s disease. Although γ-secretase has over 90 reported substrates, little is known about the mechanism by which γ-secretase recruits its substrates while at the same time distinguishing substrate from nonsubstrate within the protein-crowded environment of cellular membranes. In contrast to previous studies, our data demonstrate that substrate transmembrane domain drives its interaction with γ-secretase. We find that the γ-secretase component nicastrin acts to sterically block substrates with large ectodomains from interacting with γ-secretase, providing the mechanism by which γ-secretase selectively recruits ectodomain-shed substrates while also preventing cleavage of nonsubstrates.
Abstract
γ-Secretase is an intramembrane-cleaving protease that processes many type-I integral membrane proteins within the lipid bilayer, an event preceded by shedding of most of the substrate’s ectodomain by α- or β-secretases. The mechanism by which γ-secretase selectively recognizes and recruits ectodomain-shed substrates for catalysis remains unclear. In contrast to previous reports that substrate is actively recruited for catalysis when its remaining short ectodomain interacts with the nicastrin component of γ-secretase, we find that substrate ectodomain is entirely dispensable for cleavage. Instead, γ-secretase–substrate binding is driven by an apparent tight-binding interaction derived from substrate transmembrane domain, a mechanism in stark contrast to rhomboid—another family of intramembrane-cleaving proteases. Disruption of the nicastrin fold allows for more efficient cleavage of substrates retaining longer ectodomains, indicating that nicastrin actively excludes larger substrates through steric hindrance, thus serving as a molecular gatekeeper for substrate binding and catalysis.
Footnotes
- ↵1To whom correspondence may be addressed. Email: mswolfe{at}partners.org or dselkoe{at}partners.org.
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Author contributions: D.M.B., D.R.M., D.J.S., and M.S.W. designed research; D.M.B., D.R.M., and Y.G. performed research; D.M.B., D.R.M., and Y.G. contributed new reagents/analytic tools; D.M.B., D.R.M., Y.G., D.J.S., and M.S.W. analyzed data; and D.M.B., D.J.S., and M.S.W. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
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See Commentary on page 1112.
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This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1512952113/-/DCSupplemental.
