SHP2-interacting Transmembrane Adaptor Protein (SIT), A Novel Disulfide-linked Dimer Regulating Human T Cell Activation

Abstract

T lymphocytes express several low molecular weight transmembrane adaptor proteins that recruit src homology (SH)2 domain–containing intracellular molecules to the cell membrane via tyrosine-based signaling motifs. We describe here a novel molecule of this group termed SIT (SHP2 interacting transmembrane adaptor protein). SIT is a disulfide-linked homodimeric glycoprotein that is expressed in lymphocytes. After tyrosine phosphorylation by src and possibly syk protein tyrosine kinases SIT recruits the SH2 domain–containing tyrosine phosphatase SHP2 via an immunoreceptor tyrosine-based inhibition motif. Overexpression of SIT in Jurkat cells downmodulates T cell receptor– and phytohemagglutinin-mediated activation of the nuclear factor of activated T cells (NF-AT) by interfering with signaling processes that are probably located upstream of activation of phospholipase C. However, binding of SHP2 to SIT is not required for inhibition of NF-AT induction, suggesting that SIT not only regulates NF-AT activity but also controls NF-AT unrelated pathways of T cell activation involving SHP2.

FDF03, a Novel Inhibitory Receptor of the Immunoglobulin Superfamily, Is Expressed by Human Dendritic and Myeloid Cells

“…Moreover, as recently shown for the KIR molecules (67), the N-terminal and C-terminal ITIMs may have different efficiencies to associate with phosphatases in vivo, because the N-terminal ITIM was found to be sufficient to recruit SHP-2 but not SHP-1, while both ITIMs were required to recruit SHP-1. In keeping with this, the recently described T cell transmembrane adaptor protein SIT, which contains only one I/VxYxxV ITIM among its four cytoplasmic YxxL/V tyrosine motifs, associates in vivo with SHP-2, but not with SHP-1 and SHIP (68). Thus, this suggests that the presence of a threonine (T) instead of a hydrophobic residue at position Ϫ2 of the C-terminal tyrosine-based motif of FDF03 (TLYSVL), may decrease its potential association with SHP-1 but not with SHP-2, as seen in our study.…”

The Transmembrane Adapter Protein SIT Regulates Thymic Development and Peripheral T-Cell Functions

“…SIT has been reported to act as a modulator of TCR-mediated signaling (29,35). Indeed, when overexpressed in Jurkat T cells, wild-type (wt) SIT exerts strong negative regulatory effects upon TCR-mediated activation of the transcription factor NF-AT (29,35) This inhibitory effect seems to be exclusively mediated via a single TBSM within the SIT cytoplasmic domain (YASV) that binds a still-to-be-defined ligand. In contrast, overexpression of a SIT mutant lacking the YASV motif strongly amplifies TCR signals.…”

SIT and TRIM Determine T Cell Fate in the Thymus

“…As this regulatory system appears to be crucial for cell fate specifications in the thymus, it is not surprising that the molecules controlling the process are physically located at the plasma membrane as a part of, or in close proximity to, the receptor. In fact, we have shown that TRIM is an integral component of the TCR/CD3 complex (8,45), whereas SIT was identified as a protein co-immunoprecipitating with CD3 (9). Surprisingly, such an elaborated system for fine-tuning TCRmediated signals does not seem to play a role during peripheral T cell activation, as mutations in the number of TCR-ITAMs (41,42) or loss of SIT and TRIM (data not shown) only minimally affect activation of mature T cells.…”

“…SIT and TRIM are structurally strongly related proteins. Indeed, they both are non-raft-associated homodimeric transmembrane adaptors and both possess several tyrosine-based signaling motifs (TBSMs) within their cytoplasmic domains, two of which are highly conserved between the two molecules (YGNL and YASV in SIT, and YGNL and YASL in TRIM) (8,9). Interestingly, we have previously reported that the ability of SIT to modulate TCR-mediated signaling in the Jurkat T cell line is determined by these two TBSMs (10).…”