Review Tec Kinases: a Family with Multiple Roles in Immunity
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Immunity, Vol. 12, 373±382, April, 2000, Copyright 2000 by Cell Press Tec Kinases: A Family Review with Multiple Roles in Immunity Wen-Chin Yang,*³§ Yves Collette,*³ inositol phosphates, but they are thought to be relevant Jacques A. NuneÁ s,*³ and Daniel Olive*² for binding of PtdIns lipids to the same sites. In most *INSERM U119 cases, PH domains bind preferentially to PtdIns (4,5)P2 Universite de la Me diterrane e and inositol (1,4,5) P3 (Ins (1,4,5) P3). However, the Btk 13009 Marseille PH domain binds PtdIns (3,4,5)P3 and Ins (1, 3, 4, 5)P4 France the tightest. PtdIns (3, 4, 5)P3, one of the products of the action of PI3K, is thought to act as a second messen- ger to recruit regulatory proteins to the plasma mem- brane via their PH domains (see below). Many of the Antigen receptors on T, B, and mast cells are multimo- mutations in Btk that lead to XLA are point mutations lecular complexes that are activated by interactions with that cluster at one end of the PH domain and could be external signals. These signals are then transmitted to predicted to impair binding to Ins (3,4,5)P (for review regulate gene expression and posttranscriptional modi- 3 see Satterthwaite et al., 1998a) (Figure 1b). Similarly, fications. Nonreceptor tyrosine kinases (NRTK) are key CBA/N xid mice carry an R28C mutation in the Btk PH players that relay and integrate these signals. NRTK are domain. The recent structure of the PH domain from divided into distinct families defined by a prototypic Btk complexed with Ins (1,3,4,5)P4 provides an explana- member: Src, Tec, Syk, Csk, Fes, Abl, Jak, Fak, Ack, tion for several mutations associated with XLA: mis- Brk, and Srm (Bolen and Brugge, 1997). In 1993, several sense mutations either directly affect the interaction reports identified the first hereditary immunodeficiency with the phosphate of the lipid head group or change linked to the NRTK Btk, a member of the Tec family. electrostatic properties of the lipid binding site (Baraldi These immunodeficiencies are X-linked agammaglobu- et al., 1999). The crystal structure of BtkE41K complexed linemia (XLA) syndrome in humans and X-linked immuno- with Ins (1,3,4,5)P4 shows that two Ins (1,3,4,5)P4 mole- deficiency (xid) in mice. From these observations, a new cules are associated with this gain-of-function mutant era started for the understanding of the role of Tec family as opposed to the wild-type PH domain, which only kinases in the regulation of immune functions, first in binds one. Assuming that these structures are compati- B cells, then in mast cells, and now in T cells. They ble with the association of the Btk PH domain with PtdIns correspond to the first family of NRTK that integrate (3,4,5) P3, the increase in avidity showed for the PH signals generated not solely by the Src family kinases domain of BtkE41K may explain its constitutive mem- but also by phosphatidylinositol 3-OH kinase (PI3K), pro- brane association and activation. That XLA is also asso- tein kinase C (PKC), Janus kinases (Jak), and heterotri- ciated with a point mutation Q127H that maps to the meric G proteins. The present review discusses recent ␣1 region of the Btk PH domain (Figure 1b) cannot be data regarding the role of Tec family kinases in cells of explained by the crystal structure, suggesting that the the immune system and presents current models for PH domain of Btk carries an additional function (Baraldi their structure, regulation, and functions. The available et al., 1999). A general consensus has emerged that PH information regarding Tec family kinase expression and domains mediate intermolecular interactions and have overall structure are depicted in Table 1. evolved primarily in regulating protein±lipid interactions, yet in some instances PH domains may also mediate The Tec Family Kinases: Structural Features protein±protein interactions (see below). The general domain structure of Tec family kinases is The Tec Homology Domain formed of an N-terminal pleckstrin-homology (PH) do- The Tec homology (TH) domain contains a Btk motif, main; the so-called Tec homology domain (TH), which which is a globular core packed against the PH domain, includes a Btk motif and one or two proline-rich motifs and either one (Itk/Emt, Emb/Bmx, Rlk/Txk, and Tec29) (PR); Src homology 3 (SH3) and SH2 domains; and fi- or two (Btk and Tec) PR motifs linked to the SH3 domain nally, the catalytic kinase domain (SH1). Tec family ki- by the so-called Tec-loop (Figure 1c). The Btk motif nases generally lack the unique domain and myristoyla- contains characteristic conserved cysteine and histidine tion site (and frequently palmitoylation site) found in residues, forming a zinc binding fold. It is always adja- Src family kinases. Moreover, they lack the C-terminal cent to a PH domain and is found in other molecules regulatory tyrosine residue characteristic of Src (see such as Ras GAP. The PR motif adopts the dihedral Figure 1a). angles of a polyproline type II (PPII) helix and can bind The Pleckstrin Homology Domain SH3 domains (Mayer and Eck, 1995). Pleckstrin homology (PH) domains adopt a common fold The Src Homology 3 Domain with electrostatic polarization (Hirata et al., 1998; for The recent solution structures of Btk and of the Itk/Emt review see Rebecchi and Scarlata, 1998) (Figure 1b). Src homology (SH) 3 domains (SH3) show the typical The positively charged end of the module allows binding SH3 topology, consisting of two small  sheets that are packed against each other approximately at right angles to inositol phosphate in vitro with Kds as low as 40 nM. Most of the studies on the interaction of PH domains (Figure 1c). A 21 amino acid deletion in the Btk SH3 with their ligands have been performed using soluble domain leads to a severe B cell defect characteristic of XLA in affected patients (Zhu et al., 1994). The modeled ² To whom correspondence should be addressed (e-mail: olive@ mutant protein exhibits the loss of two C-terminal  marseille.inserm.fr). strands containing several residues critical for the for- ³ These authors contributed equally to this work. mation of the SH3 ligand binding pocket. Remarkably, § Present address: Department of Pathology, University of Massa- Itk SH3 can bind to the intramolecular PR motif (An- chusetts Medical School, Worcester, Massachusetts 01655. dreotti et al., 1997), which may regulate the enzyme Immunity 374 Table 1. Overview of Tec Family Kinases Tissue and Cell Chromosomal Activating Phenotype of Protein Protein Size (kDa) Lineage Expression localization Pathway Single Mutation Tec29/dsrc29A/ 66/55* tissue: embryo, oocyte, polytene unknown embryonic lethality Dsrc29C and imaginal disc chromosome 28C and defective oogenesis Bmx/Etk 80 tissue: bone marrow, Xp22.2 IL-6R unknown lung, testis, colon, (human) G␣12/13 and heart cell: macrophage, ? (mouse) neutrophil Btk 77 tissue: bone marrow, Xq22 BCR, Fc⑀RI, gp130 block in B cell /Bpk/Atk/Emb spleen, lymph node, (human) (IL-6R), IL-5R, development, and fetal liver IL-10R, CD19, CD28, defective BCR cell: B, myeloid, syntenic CD38, CD40 signaling and XLA, erythroid, mast, and Xq22 (mouse) defective degranulation megakaryocyte Itk/Tsk/Emt 72 tissue: thymus, spleen, 5q31-32 TCR, CD28, CD2 Reduction in CD4ϩ and lymph node (human) and FC⑀RI subset and defect cell: T, NK, and ? (mouse) in TCR signaling mast cells Rlk/Txk 52/58* tissue: thymus, spleen, 4p12 (human) TCR subtle, some reduction lymph node, tonsil, of IL-2 production and testis cell: T and myeloid chromosome 5 (mouse) Tec 72/70/66/58* tissue: bone marrow, 4p12 (human) TCR, CD28, BCR, unknown spleen, and thymus CD38, gp130, (IL-3R, cell: T, B, myeloid, and chromosome 5 IL-6R), EpoR, hepatocarcinoma (mouse) G-CSFR, TpoR c-kit, * Alternative splicing site of mRNA or alternative start site yields multiple isoforms. activity and is reminiscent of the recently described fold- B cell defect (for review see Satterthwaite et al., 1998a). ing of the Src kinases (Figure 1c) (Pawson, 1997). Based Immature B cells are arrested at the progression from on this intramolecular structure of Itk, the Tec splice the IgMlo to the IgMhi stage, which could reflect the first variant (Tec III) expressed in vivo and displaying a 66 immune tolerance checkpoint at which autoreactive B bp deletion in the C-terminal portion of the SH3 domain cells become susceptible to apoptosis (Maas et al., encoding region is predicted to make altered intramo- 1999). In addition to the BCR, multiple pathways induce lecular associations, which raises the question of its Btk phosphorylation and activation such as those induc- potential differential function (Merkel et al., 1999). ing the costimulatory molecules CD19 and CD38 and The SH2 Domain cytokine receptors. Btk could also be involved in the SH2 domains bind ligands containing phosphotyrosine homeostasis of the mature B cell population by the regu- residues included within a specific sequence. The three- lation of their emigration and maintenance in the periph- dimensional structure of an SH2 domain from a Tec ery, since a maintenance defect is observed in subsets family kinase is not yet available, but a model structure of B cells in xid mice (Cariappa et al., 1999). The basis based on v-Src suggests that Btk SH2 has a structure for this defect is unknown but could be at the level of very similar to the other SH2 domains (Vihinen et al., the regulation of chemokines or homing receptors and 1994). The binding properties of recombinant fusion pro- may shed light on new functions for Btk (Guinamard et teins containing the Itk SH2 domain suggest that while al., 1999).