Activation of TMEM16F by inner gate charged mutations and possible lipid/ion permeation mechanisms

Area of Science:

• Biophysics
• Molecular Biology
• Membrane Protein Function

Background:

• Transmembrane protein 16F (TMEM16F) is a Ca2+-activated protein with both phospholipid scramblase and ion channel activity.
• While closed and intermediate states of TMEM16F are known, its open, conductive state remains uncharacterized.
• It's debated whether a hydrophilic pathway is essential for TMEM16F lipid scrambling.

Purpose of the Study:

• To investigate the open state of mammalian TMEM16F (mTMEM16F) using atomistic simulations.
• To determine if specific mutations can induce pore opening and facilitate lipid scrambling.
• To explore the potential mechanisms of TMEM16F-mediated lipid transport.

Main Methods:

• Atomistic molecular dynamics simulations of mTMEM16F.
• Analysis of pore dilation, hydration, and lipid accessibility upon specific residue mutations (F518K, Y563K).
• Comparison of simulated states with known fungal scramblase structures and proposed scrambling mechanisms.

Main Results:

• Lysine substitutions at F518 and Y563 residues in mTMEM16F promote spontaneous pore opening in the Ca2+-bound state.
• The dilated pore exposes hydrophilic regions, increases hydration, and supports lipid scrambling.
• The open state resembles active fungal scramblases and represents a meta-stable state for wild-type mTMEM16F.
• Phospholipid and ion pathways overlap intracellularly but diverge extracellularly.

Conclusions:

• mTMEM16F can adopt an open, conductive state facilitating lipid scrambling via an in-groove mechanism.
• The findings suggest TMEM16F may utilize both in-groove and out-of-groove scrambling mechanisms.
• Distinct intracellular and extracellular pathways for ion and lipid transport exist within mTMEM16F.