T cell activation occurs when a T cell receptor (TCR) is exposed to antigen bound to a major histocompatability complex (MHC) on the surface of an antigen-presenting cell. The TCR is noncovalently associated with the δ, ε, γ, and ζ subunits of CD3, which among them contain 10 cytoplasmic immunoreceptor tyrosine-based activation motifs (ITAMs). Binding of antigen to the TCR triggers the tyrosine phosphorylation of these ITAMs, which results in downstream signaling events, but how ITAM phosphorylation is initiated is unclear. This led Xu et al. to investigate the role of the transmembrane CD3ε subunit in T cell activation. They first showed that a fluorescently labeled peptide corresponding to the cytoplasmic domain of CD3ε (CD3εCD) bound to phospholipid vesicles and that this interaction depended on a segment of basic (positively charged) residues of CD3εCD but not on its ITAM motif. Fluorescence resonance energy transfer (FRET) between labeled CD3ε and the membrane demonstrated that wild-type CD3ε, but not a mutant CD3ε in which six basic residues were mutated to serines, bound to the plasma membrane of a transfected human T cell line. Analysis of the CD3εCD in complex with a phospholipid bicelle structure by nuclear magnetic resonance (NMR) spectroscopy revealed that lipid-binding induced a fold in the structure of the CD3εCD such that its ITAM motif was embedded in the hydrophobic region of the bilayer, which made it inaccessible to the tyrosine kinase Lck in vitro. As Kuhns and Davis discuss, binding of CD3ε to phospholipids in the plasma membrane shields its ITAM from tyrosine kinases, which implies that some other signal, perhaps mechanical, is required to enable ITAM-mediated signaling events to occur upon engagement of the TCR.
C. Xu, E. Gagnon, M. E. Call, J. R. Schnell, C. D. Schwieters, C. V. Carman, J. J. Chou, K. W. Wucherpfennig, Regulation of T cell receptor activation by dynamic membrane binding of the CD3ε cytoplasmic tyrosine-based motif. Cell 135, 702–713 (2008). [PubMed]
M. S. Kuhns, M. M. Davis, The safety on the TCR trigger. Cell 135, 594–596 (2008). [PubMed]