Placental alkaline phosphatase has a binding site for the human immunoglobulin-G Fc portion.
Affinity chromatography of human plasma on placental-alkaline-phosphatase-Sepharose columns (placental alkaline phosphatase, PLAP) yielded consistently a pure protein which was identified as IgG on the basis of electrophoretical and immunological comparisons with authentic human IgG. SDS/PAGE of the protein revealed, under reducing conditions, two polypeptides of 55 kDa and 25 kDa. The N-terminal amino acid sequence (12 residues) of the 55-kDa subunit presented high similarity (83-100%) with known sequences of immunoglobulin gamma chains. The IgG binds by its Fc portion to a fully exposed domain in the plasma-membrane-anchored PLAP. Scatchard analysis of the interaction gave a dissociation constant of 3.68 microM, a value close to those found for haematopoietic cells and syncytiotrophoblast Fc receptors. The latter was affinity purified from human placenta as the major IgG-binding component and presented cross-immunoreactivity with anti-PLAP antibodies, indicating that PLAP and the putative placental Fc receptor could be identical molecules.
Makiya
R
R
Department of Medical Biochemistry and Biophysics, University of Umeå, Sweden.
Stigbrand
T
T
D000469:Alkaline Phosphatase / Q000378:metabolism*; D000595:Amino Acid Sequence; D001665:Binding Sites; D015153:Blotting, Western; D002846:Chromatography, Affinity; D003429:Cross Reactions; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D006801:Humans; D005779:Immunodiffusion; D007141:Immunoglobulin Fc Fragments / Q000235:genetics / Q000378:metabolism*; D007074:Immunoglobulin G / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D010920:Placenta / Q000201:enzymology*; D011247:Pregnancy; D011961:Receptors, Fc / Q000302:isolation & purification
D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D011961:Receptors, Fc; D000469:Alkaline Phosphatase