Center for Evolutionary & Theoretical Immunology, Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA.
Immunogenetics. 2012 Aug;64(8):647-52. doi: 10.1007/s00251-012-0626-0. Epub 2012 Jun 9.
A VpreB surrogate light (SL) chain was identified for the first time in a marsupial, the opossum Monodelphis domestica. Comparing the opossum VpreB to homologues from eutherian (placental mammals) and avian species supported the marsupial gene being VpreB3. VpreB3 is a protein that is not known to traffic to the cell surface as part of the pre-B cell receptor. Rather, VpreB3 associates with nascent immunoglobulin chains in the endoplasmic reticulum. Homologues of other known SL chains VpreB1, VpreB2, and λ5, which are found in eutherian mammals, were not found in the opossum genome, nor have they been identified in the genomes of nonmammals. VpreB3 likely evolved from earlier gene duplication, independent of that which generated VpreB1 and VpreB2 in eutherians. The apparent absence of VpreB1, VpreB2, and λ5 in marsupials suggests that an extracellular pre-B cell receptor containing SL chains, as it has been defined in humans and mice, may be unique to eutherian mammals. In contrast, the conservation of VpreB3 in marsupials and its presence in nonmammals is consistent with previous hypotheses that it is playing a more primordial role in B cell development.
首次在有袋动物——袋貂属的袋貂中鉴定出 VpreB 替代轻 (SL) 链。将袋貂的 VpreB 与来自真兽亚纲（胎盘哺乳动物）和鸟类的同源物进行比较，支持该有袋动物基因是 VpreB3。VpreB3 是一种已知不会作为前 B 细胞受体的一部分转运到细胞表面的蛋白质。相反，VpreB3 与内质网中新生的免疫球蛋白链结合。在真兽亚纲哺乳动物中发现的其他已知 SL 链 VpreB1、VpreB2 和 λ5 的同源物在袋貂基因组中未找到，也未在非哺乳动物基因组中找到。VpreB3 可能是从早期基因复制进化而来的，与真兽亚纲中产生 VpreB1 和 VpreB2 的复制过程无关。VpreB1、VpreB2 和 λ5 在有袋动物中的明显缺失表明，含有 SL 链的细胞外前 B 细胞受体，如在人类和小鼠中定义的那样，可能是真兽亚纲哺乳动物所特有的。相比之下，VpreB3 在有袋动物中的保守性及其在非哺乳动物中的存在与之前的假设一致，即它在前 B 细胞发育中发挥着更为原始的作用。
