Oncogene (1998) 17, 1353 ± 1364 1998 Stockton Press All rights reserved 0950 ± 9232/98 $12.00 http://www.stockton-press.co.uk/onc Antigen receptor signaling: integration of protein tyrosine kinase functions Idan Tamir1 and John C Cambier1 1Division of Basic Sciences, Department of Pediatrics, National Jewish Medical and Research Center, 1400 Jackson Street, Denver, Colorado 80206, USA Antigen receptors on T and B cells function to processing and presentation to T cells, and to transduce signals leading to a variety of biologic transduce signals that lead to multiple, often alter- responses minimally including antigen receptor editing, native responses. This receptor, termed the B cell apoptotic death, developmental progression, cell activa- antigen receptor (BCR), belongs to the family of tion, proliferation and survival. The response to antigen Multichain Immune Recognition Receptors (MIRR), depends upon antigen anity and valence, involvement which includes the T cell receptor (TCR) and receptors of coreceptors in signaling and dierentiative stage of for the Fc portions of IgG (FcgRI, FcgRIIA, FcgRIIC, the responding cell. The requirement that these FcgRIIIA) and IgE (FceRI). Common to this family of receptors integrate signals that drive an array of receptors is an oligomeric structure which uses dierent responses may explain their evolved structural complex- membrane-spanning subunits for the purpose of ity. Antigen receptors are composed of multiple subunits antigen/Ig recognition vs signal transduction. The compartmentalized to provide antigen recognition and BCR is comprised of a membrane-associated form of signal transduction function. In lieu of on-board immunoglobulin (mIg), noncovalently associated with enzymatic activity these receptors rely on associated a disul®de linked CD79a/CD79b heterodimer (Figure Protein Tyrosine Kinases (PTKs) for their signaling 1) (for review see Cambier, 1995a; Gold and function. By aggregating the receptors, and hence their DeFranco, 1994). Immunoglobulins of any isotype appended PTKs, antigens induce PTK transphosphor- can function as the mIg component of the BCR. ylation, activating them to phosphorylate the receptor While immature B cells express only mIgM-containing within conserved motifs termed Immunoreceptor Tyr- receptors, mature B cells co-express mIgM and mIgD, osine-based Activation Motifs (ITAMs) found in and memory B cells express the isotype, i.e., IgG, IgA transducer subunits. The tyrosyl phosphorylated ITAMs or IgE, that their daughter cells will secrete. then interact with Src Homology 2 (SH2) domains Membrane-associated immunoglobulins dier from within the PTKs leading to their further activation. As their secreted counterparts in containing a short receptor phosphorylation is ampli®ed, other eectors, spacer sequence at the normal heavy chain C- such as Shc, dock by virtue of SH2 binding, and serve, terminus, a single transmembrane spanning region in-turn, as substrates for these PTKs. This sequence of and a short [3 (mIgM and mIgD) to 28 (mIgM) events not only provides a signal ampli®cation mechan- residues] cytoplasmic tail. Although the mIg tail ism by combining multiple consecutive steps with contributes to signaling (Pleiman et al., 1994b; Weiser positive feedback, but also allows for signal diversi®ca- et al., 1997), CD79a and CD79b function as the tion by dierential recruitment of eectors that provide receptor's primary signal transducers (Cambier, 1995a; access to distinct parallel downstream signaling path- Gold and DeFranco, 1994; Pao et al., 1997). They are ways. The subject of antigen receptor signaling has been members of the immunoglobulin superfamily, contain- recently reviewed in depth (DeFranco, 1997; Kurosaki, ing a single extracellular immunoglobulin-like domain, 1997). Here we discuss the biochemical basis of antigen a single transmembrane spanning region and cytoplas- receptor signal transduction, using the B cell receptor mic tails of 48 (CD79b) and 61 (CD79a) amino acid (BCR) as a paradigm, with speci®c emphasis on the residues. The cytoplasmic domains of both CD79a and involved PTKs. We review several speci®c mechanisms CD79b contain the sequence motif YX2LX7YX2L, by which responses through these receptors are termed the Immunoreceptor Tyrosine-Based Activa- propagated and modi®ed by accessory molecules, and tion Motif or ITAM (Cambier, 1995b), that functions discuss how signal ampli®cation and diversi®cation are as the receptor's interface with cytoplasmic eectors. achieved. This motif is highly conserved among the cytoplasmic tails of MIRR and has been shown to be both Keywords: signal transduction; antigen receptors; sucient and necessary for signal transduction. Both protein tyrosine kinases the residue spacing (9 ± 11) between the two tyrosines in this motif and the presence of hydrophobic residues (leucine or isoleucine) at position +3 to both tyrosyl residues are required for proper ITAM function. As Antigen receptors-the MIRR family will be discussed below, these structural constraints are important for both ITAM phosphorylation and signal As pre-B cells transit into the immature stage they propagation by eector binding. begin to express speci®c receptors that function to recognize and internalize antigen for subsequent Antigen receptor activation of Src family PTKs The resting BCR associates with protein tyrosine Correspondence: JC Cambier kinases (PTKs) of the Src-family by virtue of an Signal transduction in lymphocytes I Tamir and JC Cambier 1354 interaction involving the receptor's ITAM and a these domains in Src-family PTK activation is domain found in the kinases' N-terminus (Clark et discussed below. al., 1992; Pleiman et al., 1994a; Yamanashi et al., 1991) The resting BCR-associated Src-family PTKs exist (Figure 1). The kinases bind preferentially to the in a dynamic equilibrium between inactive and CD79a ITAM via a site which involves the DCSM partially active states. The transition between these sequence found in CD79a, but not in CD79b (Clark et states has been shown to correlate with the phosphor- al., 1994). This low anity interaction of non- ylation of a single tyrosyl residue found at the kinase's phosphorylated ITAM with the kinase N-terminal C-terminal tail (Cooper and Howell, 1993; Pao and region is stabilized in vivo by concurrent kinase Cambier, 1998). The phosphorylation of this residue, association with the adjacent plasma membrane via which is conserved among Src-family members, is its covalent N-terminal myristylation/palmitylation mediated by the cytosolic PTK Csk (for C-terminal (Resh, 1993, 1994). In the absence of these mem- Src-family kinase) and results in an enzymatically- brane-docking modi®cations, the association of these inactive state (Hata et al., 1994). Dephosphorylation kinases with the plasma membrane and thus their of this residue, which is mediated by the transmem- ability to respond to receptor aggregation are inhibited branal protein tyrosine phosphatase (PTP) CD45, (Kabouridis et al., 1997; Timson-Gauen et al., 1996). leads to partial activation of the kinase (Mayer, The N-terminal sequence of Src-family PTKs, which is 1997; Thomas, 1989). The steady-state equilibrium of responsible for their association with the resting Src-family kinases in resting lymphocytes between the receptor, diers extensively among Src-family mem- C-terminally phosphorylated and non-phosphorylated bers and is thus termed `unique'. In addition to this forms favors the latter, probably due to the higher domain, Src-family members share four structurally- intrinsic activity of and dierent localization of CD45 homologous domains, namely the catalytic domain (membrane) compared to Csk (cytosolic). Hence, most (termed SH1 ± for src-homology 1), the SH2, SH3 (for of the resting receptor-associated Src-family PTK Src-Homology 2 and 3, respectively) and a C-terminal molecules are in the partially active state that allows tail containing a conserved tyrosyl residue (Brown and them to rapidly respond to receptor aggregation. Cooper, 1996). SH2 and SH3 domains are found in Several studies have suggested that kinase inactivity many proteins and are involved in mediating protein- results from an intramolecular interaction of the C- protein interactions. SH2 domains bind inter- or intra- terminal phosphotyrosyl residue with the kinase's SH2 molecularly to phosphotyrosyl residues with speci®city domain (Brown and Cooper, 1996). The recent three- which is governed by the phosphotyrosyl ¯anking dimensional structures of two Src-family kinases, residues. SH3 domains interact with proline-rich namely c-Src and Hck, solved by X-ray crystal- domains found within many adaptor proteins (Cohen lography (Sicheri et al., 1997; Xu et al., 1997), have and Baltimore, 1995; Pawson, 1995). The role(s) of actually demonstrated such interaction. Figure 1 Schematic representation of early signaling events activated upon BCR aggregation Signal transduction in lymphocytes I Tamir and JC Cambier 1355 These crystallographic studies have also shed light studies using B cells from Lyn knock-out mice (Chan on another facet of Src regulation involving its SH3 et al., 1997) have shown delayed, yet normal tyrosyl domain. It has been known for some time that the phosphorylation of CD79a, Syk and Shc upon BCR kinase's SH3 domain plays some role in regulating it's aggregation. Taken together, these data suggest that activity (Pleiman et al., 1994c; Superti-Fuga et al., Src-family members are the most proximal element 1993), however the mechanism was obscure