XK-related protein 9 (XKR9)

XK-related protein 9 (XKR9) is a membrane protein belonging to the XK-related family, sharing structural features with XK and XKR8, which are known for their roles in apoptotic lipid scrambling[1][3]. XKR9 comprises eight transmembrane helices arranged in two homologous domains and is activated by caspase cleavage at its C-terminus, a mechanism that is fundamental to its putative function in facilitating the exposure of phosphatidylserine—an essential step in programmed cell death and subsequent cell clearance[1][3]. The precise substrate and mechanism are not fully characterized, and it is unclear whether XKR9 acts on its own or requires interaction with other proteins. While its function as a lipid scramblase is supported by structural evidence and analogy to XKR8, direct evidence of physiological activity is pending. Mutations in other members of the XK family are associated with neurodegenerative and hematologic disorders, highlighting the potential clinical relevance of these proteins[2][4].

Other names

XK-related protein 9XKR9XRG9hXKR9XK Kell blood group precursor-related family, member 9X-linked Kx blood group related 9XK, Kell blood group complex subunit-related family, member 9

For family members: Caspase-3-mediated cleavage leads to activation by removing an inhibitory C-terminal peptide, exposing phosphatidylserine by lipid scrambling during apoptosis. XKR9 activation is presumed similar to XKR8: proteolytic cleavage unlocks lipid translocation function