TgrB1 (Tiger protein B1; formerly LagB1) is a highly polymorphic, single-pass type I transmembrane glycoprotein of the social amoeba Dictyostelium discoideum. Its long, glycosylated extracellular region contains three IPT/TIG (immunoglobulin-like) domains, followed by a single transmembrane helix and a short cytoplasmic tail. TgrB1 is displayed on the cell surface and functions as the receptor of a matched receptor-ligand pair with the adjacent-gene product TgrC1, which is its ligand. TgrB1 binds TgrC1 in trans across the gap between neighbouring cells in an allele-specific manner; this heterophilic binding drives homodimerization of TgrB1 and assembly of large TgrB1-TgrC1 adhesion complexes, mediates cell-cell adhesion, and transduces a signal via the cytoplasmic tail (which becomes phosphorylated upon ligand engagement). Because tgrB1 and tgrC1 are among the most polymorphic loci in the genome and only matching allele pairs bind productively, the TgrB1-TgrC1 system provides a self/kin-recognition (allorecognition) mechanism that governs the transition from unicellular growth to cooperative multicellular development, and underlies preferential cooperation with kin, protection against cheaters, and the greenbeard behaviour of the organism.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: TgrB1 is a single-pass type I transmembrane protein displayed at the plasma membrane, where it engages TgrC1 on adjacent cells. This electronic subcellular-location assignment is confirmed by direct experimental evidence. Reason: Plasma membrane localization is directly established experimentally and is a core aspect of TgrB1 function as a cell-surface receptor. Supporting Evidence: PMID:29038229 The TgrB1 and TgrC1 proteins are located on the plasma membrane and they bind each other through interactions between specific protein domains in trans |
| GO:0099138 altruistic, chimeric sorocarp development | IMP PMID:38734736 The greenbeard gene tgrB1 regulates altruism and cheating in... | KEEP AS NON CORE | Summary: In chimeras, activation of the TgrB1 receptor drives carrier cells into the altruistic prestalk/stalk fate while boosting wild-type spore production, whereas tgrB1 inactivation causes allotype-specific cheating. TgrB1 thus governs altruistic cell-fate allocation in chimeric development. This is a downstream social/developmental outcome rather than the core molecular function. Reason: Well supported by IMP evidence, but this is a pleiotropic social-development phenotype downstream of TgrB1 receptor activity rather than the core molecular function. Supporting Evidence: PMID:38734736 activation of the TgrB1 receptor confers altruism |
| GO:0140986 G protein-coupled chemorepellent receptor signaling pathway | HMP PMID:35622529 Identification of novel proteins in the Dictyostelium discoi... | UNDECIDED | Summary: This annotation derives from a REMI genetic screen for components of the AprA-induced chemorepulsion pathway. The connection to tgrB1 is uncertain the paper labels its screen hit "TgrB1" as a protein encoded by DDB_G0293176, whereas this gene (tgrB1/lagB1) is DDB_G0280689, and TgrB1 is an Ig-domain single-pass adhesion receptor, not a G-protein-coupled receptor. The relationship cannot be verified from the available text. Reason: Cannot verify the annotation; the cited paper attributes the phenotype to a protein encoded by a different DDB_G identifier (DDB_G0293176) than this gene (DDB_G0280689), and a GPCR chemorepellent role is inconsistent with the well-established biology of TgrB1 as a non-GPCR Ig-domain allorecognition receptor. |
| GO:0005515 protein binding | IPI PMID:23477311 TgrC1 mediates cell-cell adhesion by interacting with TgrB1 ... | KEEP AS NON CORE | Summary: Direct interaction evidence (IPI) documenting that TgrB1 binds TgrC1 (UniProtKB:P42523) via mutual IPT/TIG domains. This physical interaction is genuine, but the bare "protein binding" term is uninformative; the interaction underlies TgrB1's transmembrane signaling receptor activity captured elsewhere. Reason: The TgrB1-TgrC1 interaction is real and central, but GO:0005515 protein binding is too generic to convey function. The informative molecular function is transmembrane signaling receptor activity (GO:0004888), annotated separately. Supporting Evidence: PMID:23477311 identify TgrB1 as the heterophilic binding partner |
| GO:0007157 heterophilic cell-cell adhesion | IDA PMID:23477311 TgrC1 mediates cell-cell adhesion by interacting with TgrB1 ... | ACCEPT | Summary: TgrB1 and TgrC1 are the heterophilic partners of a cell-adhesion system; TgrB1 binds TgrC1 in trans through their respective TIG domains, mediating cell-cell adhesion during aggregation and post-aggregative development. Reason: Directly supported; heterophilic adhesion via trans TgrB1-TgrC1 binding is a core activity of the protein. Supporting Evidence: PMID:23477311 mapped the cell-binding regions in these two proteins to PMID:29038229 The TgrB1 and TgrC1 proteins are located on the plasma membrane and they bind each other through interactions between specific protein domains in trans |
| GO:0007166 cell surface receptor signaling pathway | IGI PMID:27307293 Gene discovery by chemical mutagenesis and whole-genome sequ... | ACCEPT | Summary: Genetic screens for suppressors of the tgrB1-tgrC1 mismatch identified downstream signal-transduction components, consistent with TgrB1 acting as a cell-surface receptor whose engagement by TgrC1 triggers an intracellular signaling pathway. Reason: Supported by genetic-interaction evidence and by the demonstration that TgrB1 acts as a signaling receptor; this captures its role as a surface receptor initiating a signaling pathway. Supporting Evidence: PMID:27307293 TgrB1 and TgrC1 are cell-surface adhesion proteins that form heterophilic interactions in trans PMID:29038229 the receptor activity of TgrB1 has been constitutively activated with respect to cooperative development and differentiation |
| GO:0004888 transmembrane signaling receptor activity | IDA PMID:29038229 The polymorphic proteins TgrB1 and TgrC1 function as a ligan... | ACCEPT | Summary: TgrB1 acts as the receptor of the TgrB1-TgrC1 pair. Binding of compatible TgrC1 in trans induces phosphorylation of the TgrB1 cytoplasmic tail, and dominant tgrB1 alleles behave as constitutively activated receptors, demonstrating transmembrane signaling receptor activity. Reason: Directly demonstrated core molecular function; TgrB1 is a transmembrane signaling receptor for its ligand TgrC1. Supporting Evidence: PMID:29038229 Here, we show that TgrB1 acts as a receptor with TgrC1 as its ligand in cooperative aggregation and differentiation. PMID:29038229 the cytoplasmic tail of TgrB1 becomes phosphorylated upon binding to a matching TgrC1 ligand |
| GO:0098635 protein complex involved in cell-cell adhesion | IDA PMID:24490801 Assembly of the TgrB1-TgrC1 cell adhesion complex during Dic... | ACCEPT | Summary: Trans-interaction of monomeric TgrB1 with TgrC1 homodimers triggers TgrB1 homodimerization and the coalescence of TgrB1-TgrC1 clusters into large cell-adhesion complexes, of which TgrB1 is an active component. Reason: Directly supported; TgrB1 is an active subunit of the TgrB1-TgrC1 cell-adhesion complex. Supporting Evidence: PMID:24490801 the coalescence of TgrB1-TgrC1 clusters results in the formation of large PMID:24490801 cis-homodimerization is dependent on trans-interaction with TgrC1 |
| GO:0009897 external side of plasma membrane | IDA PMID:24490801 Assembly of the TgrB1-TgrC1 cell adhesion complex during Dic... | ACCEPT | Summary: The long glycosylated TgrB1 ectodomain, bearing the TIG domains that bind TgrC1, faces the extracellular space and engages ligand on adjacent cells, consistent with activity on the external side of the plasma membrane. Reason: Consistent with the type I topology (extracellular N-terminal region) and with trans-binding of TgrC1 across the intercellular gap. Supporting Evidence: PMID:24490801 In Dictyostelium discoideum, TgrB1 and TgrC1 are partners of a heterophilic PMID:29038229 The TgrB1 and TgrC1 proteins are located on the plasma membrane and they bind each other through interactions between specific protein domains in trans |
| GO:0009988 cell-cell recognition | IMP PMID:19285397 Polymorphic members of the lag gene family mediate kin discr... | ACCEPT | Summary: Disruption of lagB1 (tgrB1) causes strain segregation in chimeras with wild-type cells; tgrB1 and tgrC1 are polymorphic and function together in a kin/self-recognition mechanism. This is a core recognition function. Reason: Directly supported by the null-mutant phenotype; cell-cell recognition is a core biological role of TgrB1. Supporting Evidence: PMID:19285397 lagB1 and lagC1 participate in a common self- or kin-recognition mechanism |
| GO:0097656 cell-cell self recognition | IGI PMID:21700835 Self-recognition in social amoebae is mediated by allelic pa... | ACCEPT | Summary: A matching pair of tgrB1 and tgrC1 alleles is necessary and sufficient for attractive self-recognition, mediated by allele-specific heterotypic binding between the extracellular domains on adjacent cells. This is a core function of TgrB1. Reason: Strongly supported by allele-swap genetic evidence; self-recognition is a defining core role of the TgrB1-TgrC1 pair. Supporting Evidence: PMID:21700835 mediate self-recognition by heterotypic interactions between the extracellular protein domains on adjacent cells PMID:21700835 necessary for self-recognition and for development |
| GO:0099120 socially cooperative development | IMP PMID:23910661 Kin recognition protects cooperators against cheaters. | KEEP AS NON CORE | Summary: Kin recognition mediated by tgrB1 and tgrC1 protects cooperators against cheaters, enabling stable socially cooperative development. This is a downstream social outcome of TgrB1-mediated recognition. Reason: Supported by IMP evidence, but represents a pleiotropic social/population outcome downstream of TgrB1's core recognition/receptor function rather than the core function itself. Supporting Evidence: PMID:23910661 kin-recognition genes, tgrB1 and tgrC1 |
| GO:0099138 altruistic, chimeric sorocarp development | IDA PMID:28120827 A polychromatic 'greenbeard' locus determines patterns of co... | KEEP AS NON CORE | Summary: The polymorphic Tgr locus behaves as a polychromatic greenbeard in which partner-specific TgrB1-TgrC1 binding strength predicts partner-specific patterns of cooperation during chimeric development. This is a downstream social-development phenotype. Reason: Well supported, but this greenbeard/altruistic chimeric-development role is a social outcome downstream of the core allorecognition receptor function. Supporting Evidence: PMID:28120827 partner-specific protein-protein binding strength and recognition specificity |
| GO:0031152 aggregation involved in sorocarp development | IEP PMID:25887420 Leaps and lulls in the developmental transcriptome of Dictyo... | KEEP AS NON CORE | Summary: tgrB1 mRNA is developmentally regulated with expression during the aggregation-to-mound transition, when amoebae must clear an allorecognition checkpoint established by tgrB1 and tgrC1. The annotation reflects expression timing and the checkpoint role during aggregative development. Reason: Expression-based (IEP) evidence places TgrB1 at the aggregation stage, but this developmental-process involvement is secondary to its core receptor/recognition function. Supporting Evidence: PMID:25887420 amoebae must clear a checkpoint established by the allorecognition genes tgrB1 and tgrC1 |
| GO:0048870 cell motility | IGI PMID:18164290 Screening of genes involved in cell migration in Dictyosteli... | UNDECIDED | Summary: This genetic-interaction annotation derives from a REMI suppressor screen for cell-migration genes. The cached abstract concerns the phospholipase D gene pldB and does not describe tgrB1, so the specific basis for a tgrB1 cell-motility role cannot be verified from the available text. Reason: The relevant supporting evidence cannot be verified from the available (abstract-only) publication, which is about pldB and cell migration and does not mention tgrB1. |
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