OB-BP1/Siglec-6. a leptin- and sialic acid-binding protein of the immunoglobulin superfamily.
We report the expression cloning of a novel leptin-binding protein of the immunoglobulin superfamily (OB-BP1) and a cross-hybridizing clone (OB-BP2) that is identical to a recently described sialic acid-binding I-type lectin called Siglec-5. Comparisons to other known Siglec family members (CD22, CD33, myelin-associated glycoprotein, and sialoadhesin) show that OB-BP1, OB-BP2/Siglec-5, and CD33/Siglec-3 constitute a unique related subgroup with a high level of overall amino acid identity: OB-BP1 versus Siglec-5 (59%), OB-BP1 versus CD33 (63%), and OB-BP2/Siglec-5 versus CD33 (56%). The cytoplasmic domains are not as highly conserved, but display novel motifs which are putative sites of tyrosine phosphorylation, including an immunoreceptor tyrosine kinase inhibitory motif and a motif found in SLAM and SLAM-like proteins. Human tissues showed high levels of OB-BP1 mRNA in placenta and moderate expression in spleen, peripheral blood leukocytes, and small intestine. OB-BP2/Siglec-5 mRNA was detected in peripheral blood leukocytes, lung, spleen, and placenta. A monoclonal antibody specific for OB-BP1 confirmed high expression in the cyto- and syncytiotrophoblasts of the placenta. Using this antibody on peripheral blood leukocytes showed an almost exclusive expression pattern on B cells. Recombinant forms of the extracellular domains of OB-BP1, OB-BP2/Siglec-5, and CD33/Siglec-3 were assayed for specific binding of leptin. While OB-BP1 exhibited tight binding (K(d) 91 nM), the other two showed weak binding with K(d) values in the 1-2 microM range. Studies with sialylated ligands indicated that OB-BP1 selectively bound Neu5Acalpha2-6GalNAcalpha (sialyl-Tn) allowing its formal designation as Siglec-6. The identification of OB-BP1/Siglec-6 as a Siglec family member, coupled with its restricted expression pattern, suggests that it may mediate cell-cell recognition events by interacting with sialylated glycoprotein ligands expressed on specific cell populations. We also propose a role for OB-BP1 in leptin physiology, as a molecular sink to regulate leptin serum levels.
Patel
N
N
Molecular Biology Department, DNAX Research Institute, Palo Alto, California 94304, USA.
Brinkman-Van der Linden
E C
EC
Altmann
S W
SW
Gish
K
K
Balasubramanian
S
S
Timans
J C
JC
Peterson
D
D
Bell
M P
MP
Bazan
J F
JF
Varki
A
A
Kastelein
R A
RA
D000595:Amino Acid Sequence; D015703:Antigens, CD / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D015214:Antigens, Differentiation, Myelomonocytic / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics; D019143:Evolution, Molecular; D005260:Female; D015870:Gene Expression; D006801:Humans; D007136:Immunoglobulins / Q000235:genetics*; D037102:Lectins; D020738:Leptin; D008024:Ligands; D008969:Molecular Sequence Data; D005810:Multigene Family; D019158:N-Acetylneuraminic Acid / Q000378:metabolism*; D010920:Placenta / Q000737:chemistry; D011247:Pregnancy; D011485:Protein Binding; D011506:Proteins / Q000378:metabolism*; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D063268:Sialic Acid Binding Ig-like Lectin 3; D014018:Tissue Distribution
D015703:Antigens, CD; D015214:Antigens, Differentiation, Myelomonocytic; C571163:CD33 protein, human; D018076:DNA, Complementary; D007136:Immunoglobulins; D037102:Lectins; D020738:Leptin; D008024:Ligands; D011506:Proteins; C120775:SIGLEC6 protein, human; D063268:Sialic Acid Binding Ig-like Lectin 3; D019158:N-Acetylneuraminic Acid