The Cytoplasmic Tail of the T Cell Receptor CD3 ε Subunit Contains a Phospholipid-Binding Motif that Regulates T Cell Functions

Abstract

The CD3 ε subunit of the TCR complex contains two defined signaling domains, a proline-rich sequence and an ITAM. We identified a third signaling sequence in CD3 ε, termed the basic-rich stretch (BRS). Herein, we show that the positively charged residues of the BRS enable this region of CD3 ε to complex a subset of acidic phospholipids, including PI(3)P, PI(4)P, PI(5)P, PI(3,4,5)P3, and PI(4,5)P2. Transgenic mice containing mutations of the BRS exhibited varying developmental defects, ranging from reduced thymic cellularity to a complete block in T cell development. Peripheral T cells from BRS-modified mice also exhibited several defects, including decreased TCR surface expression, reduced TCR-mediated signaling responses to agonist peptide-loaded APCs, and delayed CD3 ε localization to the immunological synapse. Overall, these findings demonstrate a functional role for the CD3 ε lipid-binding domain in T cell biology.

Structural Biology of the T-cell Receptor: Insights into Receptor Assembly, Ligand Recognition, and Initiation of Signaling

“…Binding resulted in a substantial increase in a-helical content detected by circular dichroism measurements, and these authors proposed that the three ITAMs fold into a a-helical structure upon lipid binding. Binding to synthetic lipid vesicles was later also shown for the CD31 cytoplasmic domain as well as the cytoplasmic domain of Fcg, which serves as a signaling module for activating Fc receptors (Sigalov et al 2006;Xu et al 2008;Deford-Watts et al 2009). CD31 has a higher net positive charge than z and bound acidic lipid vesicles with higher affinity.…”

A conserved αβ transmembrane interface forms the core of a compact T-cell receptor–CD3 structure within the membrane

“…A mounting pool of evidence implicates ligand-induced alterations in receptor structure (1) and/or TCR-CD3 configuration (2)(3)(4)(5)(6) as the triggering event. Changes in the structured extracellular (EC) domains are proposed to translate, through an unknown mechanism, into alterations in the CD3 and ζζ cytoplasmic tails, converting them to a conformation that is receptive to phosphorylation by Lck and binding of other proximal signaling components (7)(8)(9)(10)(11)(12). This type of signaling model implies a pathway between EC and cytoplasmic domains that may involve alterations in the subunit transmembrane (TM) domains with respect to each other and/ or the lipid bilayer (13)(14)(15)(16).…”

Lipid-dependent conformational dynamics underlie the functional versatility of T-cell receptor

“…Therefore, the opening level of BRS can directly affect ITAM phosphorylation. This is supported by in vivo experiments showing that mutating BRS can significantly impair TCR phosphorylation and proximal signaling in peripheral T cells [25,28]. Other functions of BRS include the regulation of TCR surface expression level and the migration of TCR to immunological synapse [25,28,46].…”

“…There is emerging evidence showing that phospholipids in the plasma membrane play important roles in regulating CD3 conformations [46]. The cytoplasmic domains of CD3ε and ζ chains are enriched in basic residues and can ionically interact with the acidic phospholipids in the plasma membrane [17,[25][26][27][28][29]47]. The Nuclear Magnetic Resonance (NMR) structure of the lipid-bound CD3 cytoplasmic domain (CD3ε CD ) shows that CD3ε CD protein backbone is localized at the interface of the lipid hydrophobic acyl-chain region and hydrophilic headgroup region [29].…”

“…Besides the ITAM, other motifs in CD3 cytoplasmic domains also play important roles in TCR signaling, including the lipid-interacting Basic residue Rich Sequence (BRS) in CD3ε and CD3ζ chains and the Nck-interacting Proline-Rich Sequence (PRS) in CD3ε chain [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. The physiological importance of BRS and PRS has been demonstrated by in vivo experiments [25,28,33]. Mutation of CD3ε BRS leads to impaired thymocyte differentiation and positive selection as well as limited peripheral T cell function, due to abnormal TCR surface level and signaling.…”