Wileman T, Kane L P, Young J, Carson G R, Terhorst C
Beth Israel Hospital, Department of Medicine, Harvard Medical School, Boston, Massachusetts 02215.
J Cell Biol. 1993 Jul;122(1):67-78. doi: 10.1083/jcb.122.1.67.
The T cell antigen receptor (TCR) is an oligomeric protein complex made from at least six different integral membrane proteins (alpha beta gamma delta epsilon and zeta). The TCR is assembled in the ER of T cells, and correct assembly is required for transport to the cell surface. Single subunits and partial receptor complexes are retained in the ER where TCR alpha, beta, and CD3 delta chains are degraded selectively. The information required for the ER degradation of the TCR beta chain is confined to the membrane anchor of the protein (Wileman et al., 1990c; Bonifacino et al., 1990b). In this study we show that the rapid degradation of the TCR beta chain is inhibited when it assembles with single CD3 gamma, delta, or epsilon subunits in the ER, and have started to define the role played by transmembrane anchors, and receptor ectodomains, in the masking proteolytic targeting information. Acidic residues within the membrane spanning domains of CD3 subunits were essential for binding to the TCR beta chain. TCR beta chains and CD3 subunits therefore interact via transmembrane domains. However, when sites of binding were restricted to the membrane anchor of the TCR beta chain, stabilization by CD3 subunits was markedly reduced. Interactions between membrane spanning domains were not, therefore, sufficient for the protection of the beta chain from ER proteolysis. The presence of the C beta domain, containing the first 150 amino acids of the TCR ectodomain, greatly increased the stability of complexes formed in the ER. For assembly with CD3 epsilon, stability was further enhanced by the V beta amino acids. The results showed that the efficient neutralization of transmembrane proteolytic targeting information required associations between membrane spanning domains and the presence of receptor ectodomains. Interactions between receptor ectodomains may slow the dissociation of CD3 subunits from the beta chain and prolong the masking of transmembrane targeting information. In addition, the close proximity of TCR and CD3 ectodomains within the ER may provide steric protection from the action of proteases within the ER lumen.
T细胞抗原受体（TCR）是一种寡聚蛋白复合体，由至少六种不同的整合膜蛋白（α、β、γ、δ、ε和ζ）组成。TCR在T细胞的内质网中组装，正确组装是转运至细胞表面所必需的。单个亚基和部分受体复合体保留在内质网中，其中TCRα、β和CD3δ链会被选择性降解。TCRβ链在内质网中降解所需的信息局限于该蛋白的膜锚定区（Wileman等人，1990c；Bonifacino等人，1990b）。在本研究中，我们发现当TCRβ链在内质网中与单个CD3γ、δ或ε亚基组装时，其快速降解受到抑制，并且我们已开始确定跨膜锚定区和受体胞外域在掩盖蛋白水解靶向信息中所起的作用。CD3亚基跨膜结构域内的酸性残基对于与TCRβ链结合至关重要。因此，TCRβ链和CD3亚基通过跨膜结构域相互作用。然而，当结合位点局限于TCRβ链的膜锚定区时，CD3亚基介导的稳定性显著降低。因此，跨膜结构域之间的相互作用不足以保护β链免受内质网蛋白水解作用。包含TCR胞外域前150个氨基酸的Cβ结构域的存在，极大地增加了在内质网中形成的复合体的稳定性。对于与CD3ε的组装，Vβ氨基酸进一步增强了稳定性。结果表明，有效中和跨膜蛋白水解靶向信息需要跨膜结构域之间的缔合以及受体胞外域的存在。受体胞外域之间的相互作用可能会减缓CD3亚基与β链的解离，并延长跨膜靶向信息的掩盖时间。此外，内质网中TCR和CD3胞外域的紧密相邻可能提供空间保护，使其免受内质网腔中蛋白酶的作用。
