X-Kell Blood Group Precursor Related Family, Member 3 (XKR3)

XKRX and XKR3 are homologs of the Kell blood group precursor XK, which is a putative membrane transporter and a component of the XK/Kell complex of the Kell blood group system. The deduced 459-amino acid protein has calculated molecular mass of 53.4 kD and contains 10 predicted transmembrane regions with a large second exoplasmic loop, consistent with XK family structure. XKR3 has 1 predicted N-glycosylation site and a cysteine residue, cys392, in the fifth exoplasmic loop that is conserved with a similar XK cys residue that forms a disulfide bond with the Kell protein. XKR3 shares 31% and 45% amino acid identity with XK and XKRX, respectively. Northern blot analysis of human tissues detected a 1.6-kb transcript in testis only. XKR3 had an experimental molecular mass of 40 kD, smaller than the predicted size, which may be due to XKR3 hydrophobicity.