Conserved transmembrane tyrosine residues of the TCR β chain are required for TCR expression and function in primary T cells and hybridomas

The T cell receptor (TCR) beta chain transmembrane domain contains two evolutionarily conserved tyrosines (Y). In this study, the functional basis for the evolutionary conservation is addressed by mutation of the residues, expression of the mutants in hybridoma and primary T cells, and examination of TCR signaling function. We find that the phenotype of the mutants, both surface expression and ability to signal for IL-2 production, is highly variable in different mouse T hybridoma lines. Although we have not been able to determine the basis for these differences in the hybridomas, expression of the mutants in primary T cells provides a definitive assessment of mutant phenotype. We show that mutation of the N-terminal Y to either leucine (L) or alanine (A) results in low surface expression in primary T cells, while mutation of both N- and C-terminal Y to A or L abrogates surface expression. However, the more conservative mutation of both transmembrane Y to phenylalanine maintained receptor surface expression and assembly while severely disrupting signaling in primary T cells. Our data demonstrate that TCR beta chain transmembrane Y are essential for TCR signal transduction as well as complex assembly. These findings suggest that protein-protein interactions involving membrane-spanning domains are likely relevant for TCR signal transduction mechanisms.