XTES Background Information Kell and XK are two covalently linked plasma membrane proteins that constitute the Kell blood group system; a group of antigens on the surface of red blood cells that are important determinants of blood type and targets for autoimmune or alloimmune diseases. XK is a 444 amino acid protein that spans the membrane 10 times and carries the ubiquitous antigen, Kx, which determines blood type. XK also plays a role in the sodium-dependent membrane transport of oligopeptides and neutral amino acids. XTES, also known as XKR3 (XK-related protein 3) or XRG3, is a 459 amino acid homolog of XK and is expressed predominately, if not exclusively, in testis. Localized to the cell membrane, XTES contains 10 transmembrane regions and a large second exoplasmic loop; both features consistent with the XK family structure.
