Unwinding of the Substrate Transmembrane Helix in Intramembrane Proteolysis

Area of Science:

• Biochemistry
• Molecular Biology
• Biophysics

Background:

• Intramembrane-cleaving proteases (I-CLiPs) regulate signaling by cleaving membrane proteins within their transmembrane domains (TMDs).
• The mechanism by which I-CLiPs hydrolyze the helical TMDs of their substrates remains poorly understood.
• Understanding this process is vital for comprehending cellular signaling pathways in prokaryotes and eukaryotes.

Purpose of the Study:

• To elucidate the structural basis for intramembrane proteolysis of single-pass helical membrane proteins.
• To investigate the role of transmembrane domain conformation in substrate recognition and cleavage by I-CLiPs.
• To provide biophysical evidence for a model of I-CLiP substrate processing.

Main Methods:

• Deep-ultraviolet resonance Raman spectroscopy with isotopic labeling to analyze TMD conformation.
• Microscale thermophoresis to measure substrate binding affinity to I-CLiPs.
• Site-directed mutagenesis (Proline to Alanine substitution) to assess the role of specific residues.
• Molecular dynamics simulations to model TMD behavior.

Main Results:

• The transmembrane domain of the I-CLiP substrate Gurken exhibits a 3₁₀-helical geometry, deviating from the canonical α-helix.
• Gurken's TMD binds with high affinity to I-CLiPs (GlpG rhomboid and MCMJR1 presenilin homolog).
• Binding induces conformational changes, including unwinding of the 3₁₀-helical region, facilitating cleavage. Mutation of a proline residue disrupted 3₁₀-helical content and abolished cleavage while maintaining binding.

Conclusions:

• A local 3₁₀-helical conformation within the TMD is essential for intramembrane proteolysis by I-CLiPs.
• Enzyme binding triggers conformational flexibility and unwinding of the TMD, enabling protease activity.
• This work provides a biophysical mechanism for how I-CLiPs process their membrane-bound substrates.