title: Substitution Glu480Lys in erythroid band 3 corresponds to the Fr(a) blood group antigen and supports existence of the second ectoplasmic loop of band 3.
abstract: BACKGROUND
  Polymorphisms in extracellular loops of RBC band 3 correspond to antigens of the Diego blood group system. Of the seven putative extracellular loops, no mutations have until recently been found in the second, fifth, and sixth loops. We detected a substitution Glu480Lys that would be located in its second ectoplasmic loop. We hypothesized the substitution may underlie a novel antigen of the Diego system.
  
  
  METHODS
  IAT was performed using two different multispecific sera containing anti-Fr(a) and a series of multispecific sera containing antibodies against other blood group antigens of the Diego blood group system but not agglutinating Fr(a+) RBCs. Biosynthesis of band 3 was studied by RT-PCR of reticulocyte RNA and electrophoresis of solubilized RBC membranes. Anion exchange function of band 3 was studied by measuring the influx of radiolabeled sulfate.
  
  
  RESULTS
  RBCs from the Glu480Lys carrier were agglutinated with sera containing anti-Fr(a) and not by sera with specificities for other antigens of the Diego system. We detected identical quantities of mRNA corresponding to the two band-3 alleles and normal content of band 3 in the RBC membranes, as well as normal sulfate influx into RBCs from the Fr(a) heterozygote.
  
  
  CONCLUSIONS
  We confirmed the previously reported molecular basis of the Fr(a) antigen, thus providing supportive evidence for the existence of the second extracellular loop of band 3. We also demonstrated that this substitution does not affect mRNA stability, surface expression, and anion exchange function of band 3.
authors.affiliation: Department of Pathology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA. pjarolim@partners.org
authors.forename: Petr
authors.initials: P
authors.lastname: Jarolim
authors.forename: Dana
authors.initials: D
authors.lastname: Kalábová
authors.forename: Marion E
authors.initials: ME
authors.lastname: Reid
mesh_terms: D000595:Amino Acid Sequence; D001457:Anion Exchange Protein 1, Erythrocyte / Q000737:chemistry* / Q000235:genetics; D001789:Blood Group Antigens / Q000737:chemistry*; D008969:Molecular Sequence Data; D012333:RNA, Messenger / Q000737:chemistry
chemical_list: D001457:Anion Exchange Protein 1, Erythrocyte; D001789:Blood Group Antigens; D012333:RNA, Messenger