Biochemical properties of the ligand-binding 20-kDa subunit of the beta-glucan receptors on human mononuclear phagocytes.
beta-Glucan receptors are present on mammalian leukocytes and initiate phagocytosis of particulate yeast beta-glucans, such as zymosan particles. Human monocytes and U937 cells express two membrane proteins of 180 and 160 kDa, each of which binds particulate yeast glucan through a 20-kDa polypeptide constituent. In this report, the structural composition of the two beta-glucan receptors and the biochemical properties of their polypeptide constituents were examined. The 180-kDa receptor was composed of three disulfide-linked polypeptides of 95, 60, and 20 kDa, whereas the 160-kDa receptor was a multimer of two polypeptides of 27 and 20 kDa. Unlike other receptor constituents, the 20-kDa polypeptide was nonglycosylated and focused at two distinct isoelectric points. Immunoblots of the focused polypeptides showed the two 20-kDa variants and the 95-kDa subunit to be constitutively tyrosine-phosphorylated, a feature not previously reported for receptors on human mononuclear phagocytes. Dephosphorylation of the receptor proteins resulted in the loss of antigenic phosphotyrosine without affecting the antigenicity of either 20-kDa variant for the anti-idiotypic antibody to beta-glucan receptors. Separate analysis of the 160-kDa receptor showed it contained both variants of the 20-kDa polypeptide. Thus, the 20-kDa subunit constituent of the two beta-glucan receptors is a functionally and chemically unique polypeptide with apparent microheterogeneity in its primary structure.
Szabó
T
T
Department of Medicine, Harvard Medical School, Boston, Massachusetts.
Kadish
J L
JL
Czop
J K
JK
D000596:Amino Acids / Q000032:analysis; D001665:Binding Sites; D002460:Cell Line; D005936:Glucans / Q000378:metabolism*; D006801:Humans; D007526:Isoelectric Point; D008562:Membrane Glycoproteins / Q000737:chemistry; D008970:Molecular Weight; D009000:Monocytes / Q000378:metabolism*; D019000:Phosphotyrosine; D011971:Receptors, Immunologic / Q000737:chemistry* / Q000378:metabolism; D014443:Tyrosine / Q000031:analogs & derivatives / Q000378:metabolism
D000596:Amino Acids; D005936:Glucans; D008562:Membrane Glycoproteins; D011971:Receptors, Immunologic; C045268:beta-glucan receptor; D019000:Phosphotyrosine; D014443:Tyrosine