Griffith I J, Ghogawala Z, Nabavi N, Golan D E, Myer A, McKean D J, Glimcher L H
Department of Cancer Biology, Harvard School of Public Health, Boston, MA.
Proc Natl Acad Sci U S A. 1988 Jul;85(13):4847-51. doi: 10.1073/pnas.85.13.4847.
The association of foreign antigen with Ia molecules on the surface of antigen-presenting cells is necessary for the interaction with the clonally distributed antigen receptor on T cells and is therefore critical in the initiation and regulation of immune responses. Ia polypeptides (alpha and beta) are composed of two extracellular domains, a transmembrane domain and a cytoplasmic domain. Although exon-shuffling experiments have demonstrated that antigen associates with the NH2-terminal alpha 1 and beta 1 domains, the roles that the other domains play in Ia function are still poorly understood. The B-hybridoma cell line 2B1 was selected in a series of positive and negative immunoselection steps for a mutation in the Ek alpha polypeptide. It was found to fortuitously contain a mutation in the Ak alpha polypeptide as well. Sequence analysis of the Ak alpha gene showed that a single base transition (C----T) resulted in a stop codon at amino acid residue 222. This caused the loss of 12 amino acids from the cytoplasmic domain of the mature polypeptide. This mutation results in a decreased level of Ak alpha polypeptide expression on the cell surface (50% of wild-type levels), an increased half-life of Ak alpha polypeptide in the cell, and a specific limited defect in antigen presentation.
外来抗原与抗原呈递细胞表面的Ia分子相结合，这对于与T细胞上克隆分布的抗原受体相互作用是必需的，因此在免疫反应的启动和调节中至关重要。Ia多肽（α和β）由两个细胞外结构域、一个跨膜结构域和一个胞质结构域组成。尽管外显子改组实验已证明抗原与NH2末端的α1和β1结构域相结合，但其他结构域在Ia功能中所起的作用仍知之甚少。在一系列阳性和阴性免疫选择步骤中筛选出了B杂交瘤细胞系2B1，以寻找Ekα多肽中的突变。结果发现它还偶然在Akα多肽中存在一个突变。Akα基因的序列分析表明，单个碱基转换（C→T）导致在氨基酸残基222处出现一个终止密码子。这使得成熟多肽的胞质结构域丢失了12个氨基酸。这种突变导致细胞表面Akα多肽的表达水平降低（为野生型水平的50%），细胞内Akα多肽的半衰期延长，并且在抗原呈递方面存在特定的有限缺陷。
