Kell is a 93kDa type II membrane glycoprotein that exists in over 25 different polymorphic forms and, because various forms are immunogenic, it is considered to be an important blood group protein. On red cells, Kell protein is linked by a single disulfide bond to another protein, XK that is lacking in the McLeod phenotype. Kell and XK are preferentially expressed in erythroid tissue but are also present, in lesser amount, in a large number of other tissues including testis, brain, and skeletal muscle. Kell protein is an endothelin converting enzyme that preferentially activates endothelin-3 while XK has the structural characteristics of a membrane transporter.
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