Abstract
Glycophorin C (GPC) and glycophorin D (GPD) are homologous sialoglycoproteins in the human red blood cell membrane. Both are thought to be encoded by the GPC gene (GYPC). We report that the rare blood group antigen, Ana, is expressed on GPD but not on GPC. cDNA was synthesized from total RNA obtained from two unrelated, heterozygous Ana+ blood donors and analyzed by the polymerase chain reaction using primers that spanned sequences encoded by the GYPC gene. The expected 412-bp fragment was generated, and sequencing of the amplified product showed a G-->T substitution at nucleotide 67 of the coding sequence, resulting in the substitution of alanine by serine at amino acid residue 23 of GPC and, presumably, residue 2 of GPD. To explain the expression of Ana on GPD but not on GPC, we postulate that the conformation of the amino acid residues at the N-terminal region of GPD determines the antigenic expression as this conformation would be different from that of the same sequence of amino acids occurring within GPC. Other possible reasons for antigen expression on a shorter protein product but not on the full-length protein product of the same gene are discussed. We extrapolate this reasoning to account for the expression of the common GE2 blood group antigen on GPD but not on GPC.
