Luisiri P, Lee Y J, Eisfelder B J, Clark M R
Department of Medicine, University of Chicago, Illinois 60637, USA.
J Biol Chem. 1996 Mar 1;271(9):5158-63. doi: 10.1074/jbc.271.9.5158.
The B cell antigen receptor complex contains heterodimers of Ig-alpha and Ig-beta. The cytoplasmic tails of each of these chains contain two conserved tyrosines, phosphorylation of which initiates the signal transduction cascades activated by the receptor complex. Although the cytoplasmic domains of Ig-alpha and Ig-beta have been expressed individually and demonstrated to be competent signal transduction units, we postulated that within the context of a heterodimer, Ig-alpha and Ig-beta could have new, complementary or even synergistic functions. Therefore we developed a system to compare the signal transducing capacities of dimers of Ig-alpha/Ig-alpha, Ig-beta/Ig-beta, or Ig-alpha/Ig-beta. This was done by fusing the extracellular and transmembrane domains of either human platelet-derived growth factor receptor (PDGFR) alpha or beta to the cytoplasmic tail of either Ig-alpha or Ig-beta. Three cell lines expressing PDGFRbeta/Ig-alpha, PDGFRbeta/Ig-beta, or PDGFRalpha/Ig-beta together with PDGFRbeta/Ig-alpha were established in the murine B cell line A20 IIA1.6. While aggregation of each dimer by itself could induce the tyrosine phosphorylation of cellular substrates, only aggregation of the heterodimer induced the phosphorylation of substrates similar in range and intensity to that induced by the endogenous B cell antigen receptor complex. Interestingly, Ig-beta remarkably enhanced the rapidity (Tmax decreased from 5 to 1 min) and intensity (greater than 10-fold enhancement) of Ig-alpha phosphorylation. Conversely, the phosphorylation of Ig-beta was reduced to undetectable levels when co-aggregated with Ig-alpha. The enhancement of Ig-alpha phosphorylation by Ig-beta correlated with a lowering of the stimulation threshold for tyrosine kinase activation.
B细胞抗原受体复合物包含Ig-α和Ig-β的异二聚体。这些链中每条链的胞质尾部都含有两个保守的酪氨酸，其磷酸化启动由受体复合物激活的信号转导级联反应。尽管Ig-α和Ig-β的胞质结构域已分别表达并被证明是有功能的信号转导单元，但我们推测在异二聚体的背景下，Ig-α和Ig-β可能具有新的、互补的甚至协同的功能。因此，我们开发了一个系统来比较Ig-α/Iα-α、Ig-β/Iβ-β或Ig-α/Iβ-β二聚体的信号转导能力。这是通过将人血小板衍生生长因子受体（PDGFR）α或β的细胞外和跨膜结构域与人血小板衍生生长因子受体（PDGFR）α或β的细胞外和跨膜结构域融合来实现的。Ig-α或Ig-β的胞质尾部。在小鼠B细胞系A20 IIA1.6中建立了三个表达PDGFRβ/Ig-α、PDGFRβ/Ig-β或PDGFRα/Ig-β以及PDGFRβ/Ig-α的细胞系。虽然每个二聚体自身的聚集都能诱导细胞底物的酪氨酸磷酸化，但只有异二聚体的聚集能诱导与内源性B细胞抗原受体复合物诱导的底物磷酸化范围和强度相似的底物磷酸化。有趣的是，Ig-β显著提高了Ig-α磷酸化的速度（Tmax从5分钟降至1分钟）和强度（增强超过10倍）。相反，当与Ig-α共聚集时，Ig-β的磷酸化降低到无法检测的水平。Ig-β对Ig-α磷酸化的增强与酪氨酸激酶激活的刺激阈值降低相关。
