Lassoued K, Guglielmi P, Benlagha K
Service d'immuno-hématologie, CNRS, Montpellier.
Bull Acad Natl Med. 1997 Oct;181(7):1465-75.
Ig alpha and Ig beta are two glycosylated transmembrane proteins of the Ig superfamily that are encoded by the B cell-specific genes mb-1 and B29, respectively. Ig alpha/Ig beta heterodimers may associate with the mu/surrogate light chains (psi LC) complex and with membrane Immunoglobulins on the surface of pre-B and B cells, respectively. They play a crucial role in the signal transduction that follows pre-B and B cell receptor cross-linking. Previous works have shown that mb-1 and B29 transcripts are expressed in normal mouse and human pro-B cells. However, little is known about the expression of Ig alpha and Ig beta molecules in pro-B cells. To address this issue we first analysed the expression of the Ig alpha and Ig beta molecules in the RS4; 11 and Nalm16 human pro-B cell lines using specific monoclonal antibodies. We found that both cell lines expressed Ig alpha and Ig beta but this expression was limited to the cytoplasm compartment. Three forms (44, 40 and 36 kDa) of the Ig alpha molecule and a single form (36 kDa) of the Ig beta molecule were detected in these lines. The heterogeneity of the Ig alpha molecule was partly related to the presence of a truncated Ig alpha protein which is likely the product of a short mb-1 transcript expressed in these cell lines. This short transcript is generated by alternative splicing of the mb1 mRNA with loss of exon 2. Ig alpha heterogeneity was also related to the expression of different glycosylated forms of the Ig alpha molecule. Only a minor fraction of the Ig alpha and Ig beta molecules associate with each other to form Ig alpha/Ig beta heterodimers and Ig beta homodimers. In these pro-B cell lines Ig alpha and Ig beta were found to associate with the lyn tyrosine kinase, suggesting that they may play some functional role at this B cell differentiation stage. Transfection of muHC gene in the Nalm16 cells results in the assembly of the pre-B receptor and in its expression on the cell surface. The level of surface expression of the pre-B receptor was found to correlate with the level of muHC and psi LC synthesis and with the level of association of the different components of the pre-B receptor with each other. Analysis of the 697 and Nalm6 pre-B cells and of the Ramos B cells indicated that heterogeneity of Ig alpha and Ig beta increases as a function of B cell differentiation.
Igα和Igβ是免疫球蛋白超家族的两种糖基化跨膜蛋白，分别由B细胞特异性基因mb-1和B29编码。Igα/Igβ异二聚体可能分别与前B细胞和B细胞表面的μ/替代轻链（ψLC）复合物以及膜免疫球蛋白结合。它们在B细胞受体交联后的信号转导中起关键作用。先前的研究表明，mb-1和B29转录本在正常小鼠和人原B细胞中表达。然而，关于原B细胞中Igα和Igβ分子的表达知之甚少。为了解决这个问题，我们首先使用特异性单克隆抗体分析了RS4;11和Nalm16人原B细胞系中Igα和Igβ分子的表达。我们发现这两种细胞系都表达Igα和Igβ，但这种表达仅限于细胞质区室。在这些细胞系中检测到了三种形式（44、40和36 kDa）的Igα分子和一种形式（36 kDa）的Igβ分子。Igα分子的异质性部分与截短的Igα蛋白的存在有关，该蛋白可能是这些细胞系中表达的短mb-1转录本的产物。这种短转录本是由mb1 mRNA的可变剪接产生的，外显子2缺失。Igα的异质性还与Igα分子不同糖基化形式的表达有关。只有一小部分Igα和Igβ分子相互结合形成Igα/Igβ异二聚体和Igβ同二聚体。在这些原B细胞系中，发现Igα和Igβ与lyn酪氨酸激酶结合，表明它们可能在这个B细胞分化阶段发挥一些功能作用。在Nalm16细胞中转染μHC基因导致前B细胞受体的组装及其在细胞表面的表达。发现前B细胞受体的表面表达水平与μHC和ψLC的合成水平以及前B细胞受体不同组分之间的结合水平相关。对697和Nalm6原B细胞以及Ramos B细胞的分析表明，Igα和Igβ的异质性随着B细胞分化而增加。
