Recent progress on paired immunoglobulin-like receptors

Almost 10 years ago the Takai and Kubagawa laboratories independently identified the paired immunoglobulin-like receptors (PIRs) in mice based on limited homology with the human Fcα receptor/CD89 (Hayami et al. 1997; Kubagawa et al. 1997). Two PIR isoforms were identified on the basis of their signaling properties as activating (PIR-A) and inhibitory (PIR-B) types. Pir is a multigene family located on the proximal end of mouse chromosome 7 (Kubagawa et al. 1997; Tun et al. 2003; Yamashita et al. 1998a), in a region syntenic with the human chromosome 19q13 where a cluster of structurally related gene families called the leukocyte receptor complex resides. Among these human genes are the closest PIR homologs, the immunoglobulin (Ig)-like transcripts (ILTs) [also called leukocyte Ig-like receptors (LIRs), monocyte/macrophage Ig-like receptors or CD85; see the new LILR nomenclature at www.gene.ucl.ac.uk/ nomenclature/genefamily/lilr.html] (Arm et al. 1997; Barten et al. 2001; Colonna et al. 1999; Cosman et al. 1999; Long 1999; Martin et al. 2002; Wagtmann et al. 1997). Paired immunoglobulin-like receptor-A and PIR-B are cell surface glycoproteins with very similar extracellular regions (>92% homology) containing six Ig-like domains, but with structurally and functionally distinct transmembrane and cytoplasmic regions (see Fig. 1). There are multiple PIR-A isoforms (>6), each encoded by a different Pira gene. Paired immunoglobulin-like receptor-As associate non-covalently with the Fc receptor common γ chain (FcRγc), a transmembrane signal transducer that contains immunoreceptor tyrosine-based activation motif (ITAM) D/ExxYxxL/Ix _6-8YxxL/I (single amino acid code, where x represents any amino acid) in the cytoplasmic tail, to form a cell activation complex (Kubagawa et al. 1999a; Maeda et al. 1998b; Ono et al. 1999; Taylor and McVicar 1999). Fig. 1 Schematic presentation of paired immunoglobulin-like receptor (PIR)-A and PIR-B. Both PIR-A and PIR-B cDNAs encode type I transmembrane proteins consisting of similar extracellular regions with six Ig-like domains, but having distinctive trans-membrane and cytoplasmic regions. The ectodomain has five or six potential sites for N-linked glycosylation (bars with closed circles). The predicted PIR-A has a short cytoplasmic tail and a positively charged arginine (R) residue in the transmembrane segment, which is noncovalently associated with a negatively charged aspartic acid (D) in the transmembrane domain of the disulfi de-linked homodimer of the Fc receptor common γ chain (FcRγc) carrying immunoreceptor tyrosine-based activation motifs (ITAMs). In contrast, the PIR-B protein has a typical uncharged transmembrane region and a long cytoplasmic tail with immunoreceptor tyrosine-based inhibitory motifs (ITIMs).