Patel N, Brinkman-Van der Linden E C, Altmann S W, Gish K, Balasubramanian S, Timans J C, Peterson D, Bell M P, Bazan J F, Varki A, Kastelein R A
Molecular Biology Department, DNAX Research Institute, Palo Alto, California 94304, USA.
J Biol Chem. 1999 Aug 6;274(32):22729-38. doi: 10.1074/jbc.274.32.22729.
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.
我们报道了一种免疫球蛋白超家族的新型瘦素结合蛋白（OB - BP1）和一个交叉杂交克隆（OB - BP2）的表达克隆，后者与最近描述的一种名为Siglec - 5的唾液酸结合I型凝集素相同。与其他已知的Siglec家族成员（CD22、CD33、髓鞘相关糖蛋白和唾液酸粘附素）比较显示，OB - BP1、OB - BP2 / Siglec - 5和CD33 / Siglec - 3构成一个独特的相关亚组，具有较高的总体氨基酸同一性：OB - BP1与Siglec - 5（59%）、OB - BP1与CD33（63%）、OB - BP2 / Siglec - 5与CD33（56%）。胞质结构域的保守性没那么高，但显示出新型基序，这些基序是酪氨酸磷酸化的假定位点，包括一个免疫受体酪氨酸激酶抑制基序和一个在信号淋巴细胞激活分子（SLAM）及SLAM样蛋白中发现的基序。人体组织中，胎盘显示出高水平的OB - BP1 mRNA，脾脏、外周血白细胞和小肠有中度表达。在外周血白细胞、肺、脾脏和胎盘中检测到OB - BP2 / Siglec - 5 mRNA。一种对OB - BP1特异的单克隆抗体证实其在胎盘的细胞滋养层细胞和合胞体滋养层细胞中有高表达。在外周血白细胞上使用该抗体显示其几乎只在B细胞上表达。对OB - BP1、OB - BP2 / Siglec - 5和CD33 / Siglec - 3细胞外结构域的重组形式进行了瘦素特异性结合检测。虽然OB - BP1表现出紧密结合（解离常数K(d)为91 nM），但其他两种显示出较弱结合，K(d)值在1 - 2 microM范围内。对唾液酸化配体的研究表明，OB - BP1选择性结合Neu5Acalpha2 - 6GalNAcalpha（唾液酸 - Tn），因此将其正式命名为Siglec - 6。OB - BP1 / Siglec - 6作为Siglec家族成员的鉴定，以及其受限的表达模式，表明它可能通过与特定细胞群体上表达的唾液酸化糖蛋白配体相互作用来介导细胞 - 细胞识别事件。我们还提出OB - BP1在瘦素生理学中的作用，即作为调节瘦素血清水平的分子库。
