Phospholipid scramblase that promotes phosphatidylserine exposure on apoptotic cell surface (PubMed:25231987, PubMed:33725486). Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment (PubMed:25231987, PubMed:33725486). a 1,2-diacyl-sn-glycero-3-phospho-L-serine(in) = a 1,2-diacyl-sn-glycero-3-phospho-L-serine(out) Phospholipid scramblase activity is activated upon caspase cleavage to generate the XK-related protein 4, processed form (By similarity). Does not act prior the onset of apoptosis (By similarity). Homodimerizes upon caspase cleavage (By similarity). Phospholipid scramblase activity is activated following interaction with the processed C-terminus of XRCC4 (protein XRCC4, C-terminus) (PubMed:33725486). Homodimer; homodimerization takes place upon caspase cleavage (PubMed:33725486). Interacts with the processed C-terminus of XRCC4 (protein XRCC4, C-terminus); interaction promotes the phospholipid scramblase activity (PubMed:33725486). Undergoes proteolytic processing by caspase-3 (CASP3), caspase-6 (CASP6) and caspase-7 (CASP7) to generate the XK-related protein 4, processed form, leading to its activation. Belongs to the XK family.