Substrate-Enzyme Interactions in Intramembrane Proteolysis: γ-Secretase as the Prototype.

TitleSubstrate-Enzyme Interactions in Intramembrane Proteolysis: γ-Secretase as the Prototype.
Publication TypeJournal Article
Year of Publication2020
AuthorsLiu, X, Zhao, J, Zhang, Y, Ubarretxena-Belandia, I, Forth, S, Lieberman, RL, Wang, C
JournalFront Mol Neurosci
Volume13
Pagination65
Date Published2020
ISSN1662-5099
Abstract

Intramembrane-cleaving proteases (I-CLiPs) catalyze the hydrolysis of peptide bonds within the transmembrane regions of membrane protein substrates, releasing bioactive fragments that play roles in many physiological and pathological processes. Based on their catalytic mechanism and nucleophile, I-CLiPs are classified into metallo, serine, aspartyl, and glutamyl proteases. Presenilin is the most prominent among I-CLiPs, as the catalytic subunit of γ-secretase (GS) complex responsible for cleaving the amyloid precursor protein (APP) and Notch, as well as many other membrane substrates. Recent cryo-electron microscopy (cryo-EM) structures of GS provide new details on how presenilin recognizes and cleaves APP and Notch. First, presenilin transmembrane helix (TM) 2 and 6 are dynamic. Second, upon binding to GS, the substrate TM helix is unwound from the C-terminus, resulting in an intermolecular β-sheet between the substrate and presenilin. The transition of the substrate C-terminus from α-helix to β-sheet is proposed to expose the scissile peptide bond in an extended conformation, leaving it susceptible to protease cleavage. Despite the astounding new insights in recent years, many crucial questions remain unanswered regarding the inner workings of γ-secretase, however. Key unanswered questions include how the enzyme recognizes and recruits substrates, how substrates are translocated from an initial docking site to the active site, how active site aspartates recruit and coordinate catalytic water, and the nature of the mechanisms of processive trimming of the substrate and product release. Answering these questions will have important implications for drug discovery aimed at selectively reducing the amyloid load in Alzheimer's disease (AD) with minimal side effects.

DOI10.3389/fnmol.2020.00065
Alternate JournalFront Mol Neurosci
PubMed ID32508589
PubMed Central IDPMC7248309