Calcium dependent activation of the TMEM16F scramblase and ion channel

Area of Science:

• Structural Biology
• Membrane Protein Function
• Biophysics

Background:

• Transmembrane protein 16F (TMEM16F) is crucial for cellular processes like blood coagulation and membrane fusion.
• The precise molecular mechanisms of TMEM16F activation and function remain largely unknown.
• TMEM16F functions as both a calcium-activated ion channel and a lipid scramblase.

Purpose of the Study:

• To elucidate the molecular mechanisms of TMEM16F activation and function.
• To capture high-resolution structures of TMEM16F in active states.
• To understand how TMEM16F facilitates ion flux and lipid scrambling.

Main Methods:

• High-resolution cryo-electron microscopy (cryo-EM) of TMEM16F reconstituted in liposomes.
• Site-directed mutagenesis to probe functional roles of identified structural features.
• Functional assays to measure ion flux and lipid scrambling activity.
• Molecular dynamics simulations to analyze protein behavior and pathways.

Main Results:

• Two distinct TMEM16F conformations were observed: a closed state and an active state with an X-shaped groove.
• The X-shaped groove is essential for TMEM16F activity, mediating both ion and lipid transport.
• Ions traverse a protein-delimited pore, while lipids move around the X-shaped groove.
• Distinct pathways exist for ion and lipid translocation within TMEM16F.

Conclusions:

• The study provides a comprehensive understanding of TMEM16F calcium-dependent gating.
• Imaging membrane proteins in native-like environments is effective for capturing active states.
• The identified X-shaped groove represents a key functional element for TMEM16F's dual activity.