2.A.112 The KX Blood-group Antigen (KXA) Family

McLeod syndrome is an X-linked human disorder characterized by abnormalities in the neuromuscular and hematopoetic systems. The KX blood group antigen mRNA expression pattern correlates with the McLeod phenotype. The KX gene codes for a novel protein with characteristics of membrane transporters. The protein has been proposed to be a Na+ -dependent neutral amine and/or oligopeptide transporter. It is predicted to be 444 amino acyl residues in length and exhibits 10 putative TMSs (Ho et al. 1994). The protein has distant homologues in animals such as Ciona intestinalis (P91583; TC# 2.A.112.1.18). The McLeod phenotype is derived from various forms of XK gene defects that result in the absence of the XK protein, and is defined hematologically by the absence of Kx antigen, weakening of Kell system antigens, and red cell acanthocytosis (Peng et al. 2007). Several of these proteins are Phosphatidyl serine flippases (from the cytoplasmic side of the membrane to the external side).

Two covalently linked proteins, Kell and XK, constitute the Kell blood group system. Kell, a 93-Kd type II glycoprotein, is highly polymorphic and carries all but 1 of the known Kell antigens, and XK, which traverses the membrane 10 times, carries a single antigen, the ubiquitous Kx. The Kell/XK complex is not limited to erythroid tissues and may have multiple physiological roles. Absence of XK is associated with abnormal red cell morphology and late-onset forms of nerve and muscle abnormalities, whereas the other protein component, Kell, is an enzyme whose principal known function is the production of a potent bioactive peptide, ET-3 (Lee et al. 2000).

The X-linked McLeod syndrome is defined by absent Kx red blood cell antigen and weak expression of Kell antigens. Most carriers of this McLeod blood group phenotype have acanthocytosis and elevated serum creatine kinase levels and are prone to develop a severe neurological disorder resembling Huntington's disease. Onset of neurological symptoms ranges between 25 and 60 years, and the penetrance of the disorder appears to be high. Additional symptoms of the McLeod neuroacanthocytosis syndrome that warrant therapeutic and diagnostic considerations include generalized seizures, neuromuscular symptoms leading to weakness and atrophy, and cardiopathy mainly manifesting with atrial fibrillation, malignant arrhythmias and dilated cardiomyopathy (Jung et al. 2007)

A classic feature of apoptotic cells is the cell-surface exposure of phosphatidylserine (PtdSer) as an 'eat me' signal for engulfment. Suzuki et al. 2013 showed that the Xk-family protein Xkr8 mediates PtdSer exposure in response to apoptotic stimuli. Mouse Xkr8(-/-) cells or human cancer cells in which Xkr8 expression was repressed by hypermethylation failed to expose PtdSer during apoptosis and were inefficiently engulfed by phagocytes. Xkr8 was activated directly by caspases and required a caspase-3 cleavage site for its function. CED-8, the only Caenorhabditis elegans Xk-family homolog, also promoted apoptotic PtdSer exposure and cell-corpse engulfment. Thus, Xk-family proteins have evolutionarily conserved roles in promoting the phagocytosis of dying cells by altering the phospholipid distribution in the plasma membrane (Suzuki et al. 2013). 

During apoptosis, phosphatidylserine (PS), normally restricted to the inner leaflet of the plasma membrane, is exposed on the surface of apoptotic cells and serves as an 'eat-me' signal to trigger phagocytosis (see previous paragraph). Chen et al. 2013 reported that CED-8, a Caenorhabditis elegans protein implicated in controlling the kinetics of apoptosis and a homologue of the XK family proteins, is a substrate of the CED-3 caspase. Cleavage of CED-8 by CED-3 activates its proapoptotic function and generates a carboxyl-terminal cleavage product, acCED-8, that promotes PS externalization in apoptotic cells and can induce ectopic PS exposure in living cells. Consistent with its role in promoting PS externalization in apoptotic cells, ced-8 is important for cell corpse engulfment in C. elegans. Thus, there is a link between caspase activation and PS externalization, which triggers phagocytosis of apoptotic cells.

The generalized reactions proposed to be catalyzed by KXA family members are:

1)  Amino acid or peptide (out)  →  Amino acid or peptide  (in).

2)  Phospholipid (inner monolayer of the plasma membrane) →  Phospholipid (outer monolayer of the plasma membrane)



Calenda, G., J. Peng, C.M. Redman, Q. Sha, X. Wu, and S. Lee. (2006). Identification of two new members, XPLAC and XTES, of the XK family. Gene 370: 6-16.

Chen, Y.Z., J. Mapes, E.S. Lee, R.R. Skeen-Gaar, and D. Xue. (2013). Caspase-mediated activation of Caenorhabditis elegans CED-8 promotes apoptosis and phosphatidylserine externalization. Nat Commun 4: 2726.

Ho, M., J. Chelly, N. Carter, A. Danek, P. Crocker, and A.P. Monaco. (1994). Isolation of the gene for McLeod syndrome that encodes a novel membrane transport protein. Cell 77: 869-880.

Jung, H.H., A. Danek, and B.M. Frey. (2007). McLeod syndrome: a neurohaematological disorder. Vox Sang 93: 112-121.

Lee, S., D. Russo, and C. Redman. (2000). Functional and structural aspects of the Kell blood group system. Transfus Med Rev 14: 93-103.

Peng, J., C.M. Redman, X. Wu, X. Song, R.H. Walker, C.M. Westhoff, and S. Lee. (2007). Insights into extensive deletions around the XK locus associated with McLeod phenotype and characterization of two novel cases. Gene 392: 142-150.

Sivagnanam, U., S.K. Palanirajan, and S.N. Gummadi. (2017). The role of human phospholipid scramblases in apoptosis: An overview. Biochim. Biophys. Acta. 1864: 2261-2271.

Suzuki, J. and S. Nagata. (2014). Phospholipid scrambling on the plasma membrane. Methods Enzymol 544: 381-393.

Suzuki, J., D.P. Denning, E. Imanishi, H.R. Horvitz, and S. Nagata. (2013). Xk-related protein 8 and CED-8 promote phosphatidylserine exposure in apoptotic cells. Science 341: 403-406.

Suzuki, J., E. Imanishi, and S. Nagata. (2014). Exposure of phosphatidylserine by Xk-related protein family members during apoptosis. J. Biol. Chem. 289: 30257-30267.

Suzuki, J., E. Imanishi, and S. Nagata. (2016). Xkr8 phospholipid scrambling complex in apoptotic phosphatidylserine exposure. Proc. Natl. Acad. Sci. USA 113: 9509-9514.

Zhang, P., M. Maruoka, R. Suzuki, H. Katani, Y. Dou, D.M. Packwood, H. Kosako, M. Tanaka, and J. Suzuki. (2023). Extracellular calcium functions as a molecular glue for transmembrane helices to activate the scramblase Xkr4. Nat Commun 14: 5592.


TC#NameOrganismal TypeExample

The human KX blood group antigen (putative amino acid transporter), KX antigen (Suzuki et al. 2013). 


The KX blood group antigen of Homo sapiens


Uncharacterized XK-related protein of 450 aas


UP of Daphnia pulex (water flea)


Xkr4 of 650 aas and 10 TMSs in a 2 + 1 + 2 + 2 + 1 + 2 TMS arrangement, suggestive of a 5 TMS repeat unit.  Xrk4 catalyzes phosphatidyl serine flipping from the inner leaflet to the outer leaflet of the cell membrane to signal appoptosis.  It has a C-terminal caspase recognition signal that may play an important role.  Xkr4, 8 and 9 have this activity and have several essential residues in TMS2 and cytoplasmic loop 2 (Suzuki et al. 2014). Xkr4 is activated by caspase-mediated cleavage and binding of the XRCC4 fragment. Zhang et al. 2023 showed that extracellular calcium is a factor needed to activate Xkr4. A constitutively active mutant of Xkr4 was found to induce phospholipid scrambling in an extracellular, but not intracellular, calcium-dependent manner. Other Xkr family members also require extracellular calcium for activation. D123 and D127 of TMS1 and E310 of TMS3 coordinate calcium binding, and the E310K mutation-mediated salt bridge between TMS1 and TMS3 bypasses the requirement for calcium. Disulfide bond formation between these two TMSs also activates phospholipid scrambling without calcium (Zhang et al. 2023). 


Xkr4 of Homo sapiens


Xkr9 of 650 aas.  Catalyzes phosphatidyl serine flipping from the inner leaflet to the outer leaflet of the cell membrane to signal appoptosis.  It has a C-terminal caspase recognition signal that may play an important role.  Xkr4, 8 and 9 have this activity and have several essential residues in TMS2 and cytoplasmic loop 2 (Suzuki et al. 2014).


Xkr9 of Homo sapiens


Xkr8 (XK8, XRG8) is of 395 aas and 6 - 11 apparent TMSs based on hydropathy plots. It catalyzes phosphatidyl serine (PS) flipping from the inner leaflet to the outer leaflet of the cell membrane to signal appoptosis (Sivagnanam et al. 2017).  It has a C-terminal caspase recognition sequence that may play a role in apoptosis signalling. Xkr4 TC# 2.A.112.1.11), Xkr8 and Xkr9 (TC# 2.A.112.1.12) have this activity and have several essential residues in TMS2 and cytoplasmic loop 2 (Suzuki et al. 2014). It is a 6 TMS protein that is activated by caspases during apoptosis and promotes phospholipid scrambling, thus exposing PS as an "eat-me-signal" (Suzuki and Nagata 2014). Basigin (BSG; TC# 2.A.23.1.1) and neuroplastin (NPTN; TC# 2.A.23.1.8)) bind to Xkr8 and usher it to the plasma membrane (Suzuki et al. 2016).


Xkr8 of Homo sapiens


XKR5 of 686 aas and 5 or 6 TMSs

XKR5 of Homo sapiens


XK-related protein of 603 aas and 8 TMSs.

XK-protein of Capitella teleta (Polychaete worm)


XK-like protein of 667 aas and 9 TMSs

XK-like protein of Ciona intestinalis (Transparent sea squirt) (Ascidia intestinalis)


XK8 or XraB of 404 aas and 9 TMSs

XK8 of Ciona intestinalis (Transparent sea squirt) (Ascidia intestinalis)


Cell death abnormality protein 8; phospholipid flippase (in to out) (Suzuki et al. 2013; Chen et al. 2013).


ced-8 of Caenorhabditis elegans


The X Kell blood group precursor-related family member 8 homologue isoform CRAa of 401 aas (XK-related protein 8 or XkR8) (Suzuki et al. 2013).


XkR8 of Mus musculus


Uncharacterized protein of 374 aas and ~ 10 TMSs

Animals (Insects)

UP of Culex quinquefasciatus (Southern house mosquito) (Culex pungens)


XkR8 homologue of 352 aas and 8 - 10 TMSs.


XkR8 homologue of Drosophila melanogaster


The XK-related protein 2, XKP2 of 440 aas (Calenda et al. 2006).


XKP2 of Homo sapiens


The XK-related protein 3, XKR3; XRG3, XTES of 459 aas (Calenda et al. 2006).


XKR3 of Homo sapiens


XK protein 6 of 675 aas


XK protein of Hymenolepis microstoma (Rodent tapeworm) (Rodentolepis microstoma)


Uncharacterized protein of 470 aas


UP of Branchiostoma floridae (Florida lancelet) (Amphioxus)


TC#NameOrganismal TypeExample

XkR8 homologue of 884 aas and 8 - 10 TMSs.


XkR8 homologue of Otreococcus lucimarinus


Uncharacterized protein of 1119 aas and 10 TMSs.


UP of Bathycoccus prasinos


Uncharacterized protein of 781 aas and 9 TMSs


UP of Coccomyxa subellipsoidea


Unchracterized protein of 892 aas


UP of Ostriococcus tauri


TC#NameOrganismal TypeExample

TC#NameOrganismal TypeExample

Uncharacterized protein of 652 aas and 8 TMSs


UP of Drosophila grimshawi (Fruit fly) (Idiomyia grimshawi)


Uncharacterized protein of 694 aas and 9 TMSs.


UP of Solenopsis invicta (Red imported fire ant) (Solenopsis wagneri)


Uncharacterized protein of 615 aas and 9 TMSs


UP of Aedes aegypti (Yellowfever mosquito) (Culex aegypti)


TC#NameOrganismal TypeExample