Blood, Vol. 94 No. 1 (July 1), 1999:
pp. 310-318
Identification of a Defect in the Intracellular Trafficking of a
Kell Blood Group Variant
Karina Yazdanbakhsh,
Soohee Lee,
Qian Yu, and
Marion E. Reid
From the New York Blood Center, New York, NY.
Blood group polymorphisms have been used as tools to study the
architecture of the red blood cell (RBC) membrane. Some blood group
variants have reduced antigen expression at the cell surface. Understanding the underlying mechanism for this reduced expression can
potentially provide structural information and help to elucidate protein trafficking pathways of membrane proteins. 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
glycoprotein 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 reduced amount of total Kell glycoprotein at the
cell surface rather than to the inaccessibility of the antigens to Kell
antibodies. Using a heterologous expression system, we demonstrate that
the Kpa mutation causes retention of most of the Kell
glycoprotein in a pre-Golgi compartment due to differential processing,
thereby suggesting aberrant transport of the Kell protein to the cell surface. Furthermore, we demonstrated that single nucleotide
substitutions into the coding region of the common KEL allele,
as predicted by the molecular genotyping studies, was sufficient to
encode three clinically significant low incidence antigens. We found that two low incidence antigens can be expressed on a single Kell protein, thus showing that the historical failure to detect such a
variant is not due to structural constraints in the Kell protein. These
studies demonstrate the power of studying the molecular mechanisms of
blood group variants for elucidating the intracellular transport
pathways of membrane proteins and the requirements for cell surface expression.
