Molecular modelling and endoplasmic reticulum retention of mutated TCR/CD3complexes

T cell receptor (TCR)/CD3 complex assembly takes place in the endoplasmic r eticulum (ER). Normal TCR/CD3 complexes egress from the ER to the cis-Golgi , where the interaction with zeta (2) homodimers occurs. This interaction l eads to further uncontrolled transport of TCR/CD3/zeta molecules to the cel l surface. The purpose of the present experiments was to determine firstly the basis for the impact of the phe(195)/(216) = > val mutations on TCR/CD3 expression in Jurkat cells, and secondly why mutated J79-cell TCR alpha be ta /CD3 hexamers are prevented from interacting with zeta (2) homodimers. W e found that phe = > val mutations cause serious perturbations in a so far undefined hydrophobic area formed by the two phe(195/216) on beta -strand F and aromatic/large hydrophobic amino acids on neighboring beta -strands B and A in C alpha and C beta domains, respectively. In addition, TCR/CD3 hex amers and zeta (2) homodimers colocalize in normal Jurkat T cells, in rever tant J79r58 cells, and in J79 cells transfected with wild-type TCR alpha cD NA but not in J79 mutant cells (confocal microscopy). Furthermore, mutated TCR/CD3 complexes seem to be actively retained in the ER in J79 cells but n ot in revertant J79r58 cells by a nondominant mechanism. We propose that a hitherto undefined ER-retention molecule controls both the protein structur e and egress of TCR/CD3 complexes from the ER of alpha beta and gamma delta T cells.