The T-Cell-Specific Adapter Protein Family: Tsad, ALX, and SH2D4A

Philip E. Lapinski The T-cell-specific adapter protein Jennifer A. Oliver family: TSAd, ALX, and Jennifer N. Bodie Francesc Marti SH2D4A⁄SH2D4B Philip D. King Authors’ address Summary: Adapter proteins play key roles in intracellular signal trans- Philip E. Lapinski1, Jennifer A. Oliver1, Jennifer N. Bodie1, Francesc Marti1, duction through complex formation with catalytically active signaling Philip D. King1 molecules. In T lymphocytes, the role of several different types of adapter 1Department of Microbiology and Immunology, University proteins in T-cell antigen receptor signal transduction is well established. of Michigan Medical School, Ann Arbor, MI, USA. An exception to this is the family of T-cell-specific adapter (TSAd) proteins comprising of TSAd, adapter protein of unknown function (ALX), Correspondence to: SH2D4A, and SH2D4B. Only recently has the function of these adapters Philip D. King in T-cell signal transduction been explored. Here, we discuss advances in Department of Microbiology and Immunology our understanding of the role of this family of adapter proteins in T cells. University of Michigan Medical School Their function as regulators of signal transduction in other cell types is 6606 Med Sci II also discussed. 1150 West Medical Center Drive Ann Arbor, MI 48109-5620, USA Keywords: adapter proteins, protein tyrosine kinases, signal transduction, T lymphocytes, Tel.: +1 734 615 9073 autoimmunity, knockout mice Fax: +1 734 764 3562 e-mail: [email protected] Introduction Acknowledgements Work relating to TSAd family adapter proteins in the King Adapter proteins play central roles in intracellular signal trans- Laboratory was supported by National Institutes of Health duction pathways initiated by different cell surface receptors grant RO1 AI050699, by the American Heart Association in response to ligand recognition (1–3). Adapter proteins lack grants 0615514Z and 0850170Z, and by funds from the Dorothy and Herman Miller award for Innovative Research in catalytic activity but instead usually contain one or more mod- Immunology. ular-binding domains or recognition sequences for modular- binding domains of other signaling proteins. As such, adapter proteins participate in the formation of signaling complexes with catalytically active molecules, resulting in direct or indi- rect modulation of catalytic activity and ⁄ or localization of catalytically active molecules to substrates. Testimony to the importance of adapter proteins in receptor signal transduction is the finding that gene-targeted mice that lack expression of specific adapter proteins frequently show signs of disease and disturbances of tissue homeostasis often associated with early mortality. In T lymphocytes, the function of several different adapter Immunological Reviews 2009 proteins such as growth receptor-bound-2 (Grb-2), Grb-2- Vol. 232: 240–254 Printed in Singapore. All rights reserved related adapter protein-2 (GRAP-2), Src homology-2 (SH2)- domain-containing leukocyte protein of 76 kDa (SLP-76), Ó 2009 John Wiley & Sons A/S Fyn-binding protein (FYB), and linker of activated T cells Immunological Reviews 0105-2896 (LAT) has been extensively characterized (4–8). In contrast, 240 Ó 2009 John Wiley & Sons A/S • Immunological Reviews 232/2009 Lapinski et al Æ TSAd family adapter proteins adapter proteins belonging to the T-cell-specific adapter activation with phorbol esters, phytohemagglutinin, or CD3 (TSAd) protein family have been less explored. This family plus CD28 mAbs, consistent with the nature of the original comprises of TSAd itself (also known as SH2D2A), adapter subtractive cDNA library screen (10). The kinetics of induced protein of unknown function (ALX or HSH2D), SH2D4A, and expression were fairly rapid in that transcripts were detected SH2D4B (9). In this article, we review current knowledge within hours and reached a plateau by 1 day after stimulation. relating to the function of these adapters in T cells. In addi- Western blot studies confirmed that the 52-kDa TSAd protein tion, a role for these adapters in other cell types is discussed. was not expressed in resting T cells but was induced upon T-cell activation with kinetics consistent with the increased Identification and expression of TSAd family adapter RNA expression (12). Such studies also revealed synergism proteins between CD3 and CD28 with regards to the induction of TSAd TSAd protein. Expression of TSAd in human T cells appears to be regulated in part by a cyclic adenosine monophosphate (AMP) TSAd was first identified in humans as part of a subtractive response element contained within the promoter of the TSAd cDNA library screen in which cDNA from peripheral blood gene (13). However, activation of the cyclic AMP signal trans- CD8+ T cells activated with CD3 plus CD28 monoclonal anti- duction pathway alone is not sufficient for induction of TSAd bodies (mAbs) was subtracted using cDNA prepared from the protein expression. Thus, a cyclic AMP analog, although able Jurkat human T-cell leukemia cell line (10). From this screen, to induce TSAd mRNA, is unable to induce TSAd protein an open reading frame (ORF) was identified that encoded a expression (14). Presumably, therefore, CD3 ⁄ CD28 engage- 389-amino acid protein that resembled a typical intracellular ment additionally results in the activation of pathways that adapter. Sequence inspection revealed a protein without any allow efficient translation of TSAd mRNA and ⁄ or promote recognizable catalytic activity but instead an SH2 domain stability of TSAd protein. located in the center of the linear sequence with predicted Subsequent to the identification of TSAd in humans, TSAd affinity for protein phosphotyrosine (Fig. 1). In addition, in was identified in mice by two independent groups, both using the region of the protein carboxyl to the SH2 domain, there yeast-hybrid screening technology. Choi et al. (15) used a bait are five tyrosine residues, four of which are contained within protein comprising of the SH2 domain plus kinase domain of consensus motifs for phosphorylation by protein tyrosine kin- the Src family PTK resting lymphocyte kinase (RLK). Rajagopal ases (PTKs), plus several proline-rich regions. Potentially, et al. (16) used the Tec-family PTK RLK as a bait. TSAd was therefore, these sequences could serve as docking sites for found to interact with both types of bait protein and also SH2 or protein tyrosine-binding domain (PTB)-containing or another Tec family PTK, interleukin-2-inducible T cell kinase SH3 domain-containing proteins, respectively (11). (ITK), hence the synonyms for TSAd, LCK-associated adapter Initial Northern blot analysis of different human tissues (LAD) and RLK ⁄ ITK-binding protein (RIBP), respectively. revealed that TSAd expression was restricted to thymus, Murine TSAd is a 366-amino acid protein that like human spleen, and peripheral blood T lymphocytes (10). Further- TSAd contains a single SH2 domain (Fig. 1). Four out of the more, TSAd was not expressed in resting T cells but rather five tyrosine residues found in the carboxyl region of human was induced in both CD4+ and CD8+ T cells following their TSAd are conserved in the carboxyl region of mouse TSAd as is one of the proline-rich stretches. Interaction of TSAd with LCK and RLK in yeast was shown to require a catalytically active kinase domain in both cases. In addition, for the interaction with LCK, binding was shown to be abolished by a point mutation of the LCK SH2 domain that prevented protein phosphotyrosine recognition by this domain. These findings suggested that the mechanism of interaction of TSAd with PTKs involved PTK-mediated phosphorylation of TSAd carboxyl region tyrosine residues followed by PTK SH2 Fig. 1. TSAd (T-cell-specific adapter) family adapter proteins. domain recognition of the same tyrosine residues. Positions of SH2 domains within linear sequences of each adapter are Similar to human TSAd, murine TSAd was initially reported indicated. Proline-rich stretches that are conserved between mouse and human are shown in purple. Conserved tyrosine residues in consensus to be relatively restricted in expression to T cells and natural phosphorylation motifs (NetPhos program) are also shown. killer (NK) cells. Northern blot analysis revealed strong Ó 2009 John Wiley & Sons A/S • Immunological Reviews 232/2009 241 Lapinski et al Æ TSAd family adapter proteins expression in thymus, spleen, and peripheral blood mononu- is significant homology between SH2D4A and SH2D4B in clear cells (15, 16). Transcripts were detected in T-helper 2 parts of the amino-terminal region (although there is no (Th2) and NK cell lines and in CD3 mAb-stimulated Th1 cells. homology of this region to regions outside of the SH2 In the latter case, transcripts were induced as soon as 2 h after domains of TSAd and ALX). The amino-terminal region of stimulation (16). Induced expression of TSAd in murine SH2D4A contains a conserved tyrosine residue in a phosphor- T cells is consistent, therefore, with findings in human T cells. ylation consensus motif and a proline-rich stretch near the However, particularly in mouse T cells, TSAd protein does SH2 domain. The amino-terminal region of SH2D4B contains appear to be expressed at functionally significant levels prior three conserved tyrosine residues in consensus phosphoryla- to activation (15, 17). Furthermore, TSAd also appears to be tion motifs and a proline-rich stretch located in the same posi- expressed at functionally significant levels in some other pri- tion as that found in SH2D4A. mary cell types in mice including lung epithelial cells, blood Analysis of expressed sequence tag and public microarray vessel endothelial cells, and mature B cells (15, 18–20). databases indicate that SH2D4A is expressed at low levels in a variety of human and murine tissues. With the use of a spe- ALX cific polyclonal antiserum, Lapinski et al. (25) demonstrated that in quiescent human T cells, the 52-kDa SH2D4A protein ALX was first identified as part of a search of databases for pro- is expressed poorly but, like TSAd, is induced in response to teins with SH2 domains with significant sequence similarity CD3 plus CD28 mAb activation.

Load more