Structure and function of the human apoptotic scramblase Xkr4

Area of Science:

• Cell Biology
• Molecular Mechanisms
• Membrane Dynamics

Background:

• Phosphatidylserine externalization on dying cells signals macrophage recognition and clearance, mediated by X-Kell related (Xkr) proteins.
• Defective Xkr-mediated scrambling leads to impaired clearance and subsequent inflammation.
• Previous models proposed Xkr4 activation involves caspase cleavage, dimerization, and ligand binding.

Purpose of the Study:

• To investigate the molecular mechanisms of human Xkr4 (hXkr4) lipid scrambling.
• To elucidate the structural basis of hXkr4 activity.
• To explore the role of membrane thinning in scramblase function.

Main Methods:

• Biochemical approaches using purified full-length monomeric hXkr4.
• Cryo-electron microscopy (CryoEM) for structural determination.
• Molecular dynamics (MD) simulations to analyze membrane interactions.

Main Results:

• Purified monomeric hXkr4 demonstrates lipid scrambling activity.
• CryoEM revealed a novel hXkr4 conformation with an electronegative surface formed by acidic residues.
• MD simulations showed this conformation induces membrane thinning, correlating with scrambling efficiency.

Conclusions:

• hXkr4 functions as a monomeric scramblase, challenging previous activation models.
• The electronegative surface and induced membrane thinning are key to hXkr4's scrambling mechanism.
• Membrane thinning may be a general property of active lipid scramblases.