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

Area of Science:

• Biochemistry
• Structural Biology
• Neuroscience

Background:

• Intramembrane-cleaving proteases (I-CLiPs) release bioactive fragments crucial for physiological and pathological processes.
• Presenilin, the catalytic subunit of the gamma-secretase (GS) complex, cleaves key substrates like amyloid precursor protein (APP) and Notch.
• Dysregulation of GS activity is implicated in Alzheimer's disease (AD) pathogenesis.

Purpose of the Study:

• To elucidate the structural mechanisms of substrate recognition and cleavage by the gamma-secretase complex.
• To provide atomic-level insights into presenilin's role in intramembrane proteolysis.
• To identify potential targets for therapeutic intervention in Alzheimer's disease.

Main Methods:

• Recent cryo-electron microscopy (cryo-EM) studies of the gamma-secretase complex.
• Structural analysis of presenilin and its interaction with membrane protein substrates.
• Biochemical and biophysical approaches to study enzyme-substrate dynamics.

Main Results:

• Cryo-EM structures reveal dynamic presenilin transmembrane helices (TM 2 and 6).
• Substrate binding induces unwinding of the C-terminal transmembrane helix, forming an intermolecular beta-sheet with presenilin.
• This conformational change exposes the scissile peptide bond for hydrolysis.

Conclusions:

• The structural findings provide a mechanistic basis for gamma-secretase-mediated intramembrane proteolysis.
• Significant questions remain regarding substrate recruitment, translocation, catalytic water coordination, and product release.
• Understanding these mechanisms is critical for developing targeted therapies for Alzheimer's disease.