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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.