Caspase cleavage releases a nuclear protein fragment that stimulates phospholipid scrambling at the plasma membrane

Mol Cell. 2021 Apr 1;81(7):1397-1410.e9. doi: 10.1016/j.molcel.2021.02.025. Epub 2021 Mar 15.


Phospholipid scrambling in dying cells promotes phosphatidylserine exposure, a critical process for efferocytosis. We previously identified the Xkr family protein Xkr4 as a phospholipid-scrambling protein, but its activation mechanisms remain unknown. Here we show that Xkr4 is activated in two steps: dimer formation by caspase-mediated cleavage and structural change caused by activating factors. To identify the factors, we developed a new screening system, "revival screening," using a CRISPR sgRNA library. Applying this system, we identified the nuclear protein XRCC4 as the single candidate for the Xkr4 activator. Upon apoptotic stimuli, XRCC4, contained in the DNA repair complex, is cleaved by caspases, and its C-terminal fragment with an intrinsically disordered region is released into the cytoplasm. Protein interaction screening showed that the fragment interacts directly with the Xkr4 dimer to activate it. This study demonstrates that caspase-mediated cleavage releases a nuclear protein fragment for direct regulation of lipid dynamics on the plasma membrane.

Keywords: CRISPR-Cas9 sgRNA library screening; Caspase; DNA repair; Efferocytosis; Nuclear fragment; Phosphatidylserine; Phospholipid scrambling; XRCC4; Xkr4; cDNA library screening.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Apoptosis Regulatory Proteins / genetics
  • Apoptosis Regulatory Proteins / metabolism*
  • Caspases / genetics
  • Caspases / metabolism*
  • Cell Line, Tumor
  • Cell Membrane / genetics
  • Cell Membrane / metabolism*
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism*
  • HEK293 Cells
  • Humans
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism*
  • Mice
  • Phospholipids / genetics
  • Phospholipids / metabolism*
  • Protein Multimerization
  • Proteolysis*


  • Apoptosis Regulatory Proteins
  • DNA-Binding Proteins
  • Membrane Proteins
  • Phospholipids
  • XKR4 protein, human
  • XRCC4 protein, human
  • XRCC4 protein, mouse
  • Xkr4 protein, mouse
  • Caspases