Identification of a defect in the intracellular trafficking of a Kell blood group variant

Blood group polymorphisms have been used as tools to study the architecture of the red blood cell (RBC) membrane. Some blood group variants have reduc ed antigen expression at the cell surface. Understanding the underlying mec hanism for this reduced expression can potentially provide structural infor mation and help to elucidate protein trafficking pathways of membrane prote ins. The Kp(a+) phenotype is a variant in the Kell blood group system that is associated with a single amino acid substitution (R281W) in the Kell gly coprotein and serologically associated with a weakened expression of other Kell system antigens by an unknown mechanism. We found by immunoblotting of RBCs that the weakening of Kell antigens in this variant is due to a reduc ed amount of total Kell glycoprotein at the cell surface rather than to the inaccessibility of the antigens to Kelt antibodies. Using a heterologous e xpression system, we demonstrate that the Kp(a) mutation causes retention o f most of the Kell glycoprotein in a pre-Golgi compartment due to different ial processing, thereby suggesting aberrant transport of the Kell protein t o the cell surface. Furthermore, we demonstrated that single nucleotide sub stitutions into the coding region of the common KEL allele, as predicted by the molecular genotyping studies, was sufficient to encode three clinicall y significant low incidence antigens. We found that two low incidence antig ens can be expressed on a single Kell protein, thus showing that the histor ical failure to detect such a variant is not due to structural constraints in the Kell protein. These studies demonstrate the power of studying the mo lecular mechanisms of blood group variants for elucidating the intracellula r transport pathways of membrane proteins and the requirements for cell sur face expression. (C) 1999 by The American Society of Hematology.