Major driving forces in receptor triggering. Within the SCHOOL model, receptor triggering and signaling is an outcome of the ligand-induced interplay between three key protein-protein interactions: (1) ligand-receptor interactions, (2) interreceptor [single-chain receptors, (A)] and intrareceptor [multichain immune recognition receptors, (B)] transmembrane interactions, and (3) interreceptor cytoplasmic homointeractions. Circular arrows indicate ligand-induced receptor reorientation. Abbreviation: SCHOOL, signaling chain homooligomerization.

Receptor oligomerization (clustering) induced upon ligand binding outside the cell is translated across the membrane into protein oligomerization inside the cell with cytoplasmic homointeractions representing the major driving force of receptor triggering. Small solid black and magenta arrows indicate specific interunit hetero- and homointeractions between transmembrane and cytoplasmic domains, respectively. Circular arrows indicate ligand-induced receptor reorientation. Phosphate groups are shown as gray circles. Abbreviation: SCHOOL, signaling chain homooligomerization.

Within the model, formation of competent signaling oligomers in cytoplasmic milieu is necessary and sufficient to generate the activation signal, thus triggering downstream signaling pathways. Receptor clustering and reorientation (and/or receptor reorientation in pre-existing MIRR clusters) induced by antibodies to recognition (not shown) or signaling subunits (e.g., anti-TCRα, anti-TCRβ, anti-CD3ε, anti-Igβ, etc.,) lead to formation of a dimeric/oligomeric intermediate (stage 1). In this intermediate, receptors are in sufficient proximity and adopt the correct (permissive) relative orientation and geometry to promote trans-homointeractions between cytoplasmic domains of signaling subunits resulting in formation of competent signaling oligomers. Further stages and outcomes are similar to those described in the legend to Figure 6.

Ligand-induced SR clustering and reorientation (in pre-existing SR clusters, ligand binding induces receptor reorientation) results in SR oligomerization mediated by transmembrane interactions. In these oligomers, receptors are in sufficient proximity and adopt a correct (permissive) relative orientation and geometry to promote homointeractions between cytoplasmic domains. Within the model, formation of competent signaling oligomers in cytoplasmic milieu is necessary and sufficient to generate the activation signal (for receptor tyrosine kinases, this means trans-autophosphorylation of Tyr residues in cytoplasmic signaling sequences), thus triggering downstream signaling pathways. Protein-protein transmembrane and cytoplasmic interactions are shown by solid black and magenta arrows, respectively. Abbreviation: SCHOOL, signaling chain homooligomerization.

The extracellular portion of the receptors is on top and the cytoplasmic portion is on bottom. The lengths of the receptors as shown are only approximately to scale. The inset shows SR domain organization. Abbreviations: EpoR, erythropoietin receptor; G-CSF-R, granulocyte colony-stimulating factor receptor; TGFβ, transforming growth factor-beta; TNF, tumor necrosis factor; JAK, Janus kinase; EGFR, epidermal growth factor receptor; InsR, insulin receptor; IGF1R, insulin-like growth factor I receptor; IRR, insulin receptor-related receptor; PDGFR, platelet-derived growth factor receptor; CSF1R, colony-stimulating-factor 1 receptor; FGFR, fibroblast growth factor receptor; MuSK, muscle-specific receptor tyrosine kinase; Eph, ephrin; DDR, discoidin domain receptor; Flt1, KDR and Flt4, vascular endothelial growth factor (VEGF) receptors.

Schematic presentation of the MIRRs expressed on many different immune cells including T and B cells, natural killer cells, mast cells, macrophages, basophils, neutrophils, eosinophils, dendritic cells and platelets. The inset shows MIRR assembly. The extracellular recognition domains and intracellular ITAM-containing signaling domains are located on separate subunits bound together by noncovalent transmembrane interactions (solid arrow). ITAMs/YxxM are shown by green. Curved lines depict intrinsic disorder of the cytoplasmic domains of MIRR signaling subunits. Abbreviations: BCR, B cell receptor; CLR, C-type lectin receptor; DAP-10 and DAP-12, DNAX adapter proteins of 10 and 12 kD, respectively; DCAR, dendritic cell immunoactivating receptor; GPVI, glycoprotein VI; ILT, Ig-like transcript; KIR, killer cell Ig-like receptor; LIR, leukocyte Ig-like receptor; MAIR-II, myeloid-associated Ig-like receptor; MDL-1, myeloid DAP12-associating lectin 1; NITR, novel immune-type receptor; NK, natural killer cells; SIRP, signal regulatory protein; TCR, T cell receptor; TREM receptors, triggering receptors expressed on myeloid cells.

Within the model, formation of competent signaling oligomers in cytoplasmic milieu is necessary and sufficient to generate the activation signal, thus triggering downstream signaling pathways. Ligand-induced MIRR clustering and reorientation (in pre-existing MIRR clusters, ligand binding induces receptor reorientation) lead to formation of a dimeric/oligomeric intermediate (stage 1). In this intermediate, receptors are in sufficient proximity and adopt the correct (permissive) relative orientation and geometry to promote trans-homointeractions between cytoplasmic domains of signaling subunits resulting in formation of competent signaling oligomers. Upon formation of signaling oligomers, PTKs phosphorylate the tyrosine residues in the cytoplasmic signaling motifs, the immunoreceptor tyrosine-based activation or YxxM motifs (ITAMs/YxxM, shown by green), that leads to generation of the activation signal, dissociation of signaling oligomers and internalization of the engaged binding subunits (stage 1). Next, the signaling oligomers sequentially homointeract with the relevant signaling subunits of nonengaged receptors resulting in formation of higher-order signaling oligomers, thus propagating and amplifying the signals (stages 3 and 4). This also leads to the release and subsequent internalization of the nonengaged ligand-binding MIRR subunits (stage 4). Circular arrows indicate ligand-induced receptor reorientation. Black and magenta arrows indicate specific intersubunit hetero- and homointeractions between transmembrane and cytoplasmic domains, respectively. All interchain interactions in a dimeric intermediate are shown by dotted arrows reflecting their transition state. Curved lines depict intrinsic disorder of the cytoplasmic domains of MIRR signaling subunits. Phosphate groups are shown as dark circles. Abbreviation: PTK, protein tyrosine kinase; SCHOOL, signaling chain homooligomerization.

SCHOOL model of GPVI signaling. Interaction with multivalent ligand (not shown) clusters the receptors and pushes them to reorientate (I), to bring signaling subunits into a correct (permissive) relative orientation and in sufficient proximity in the formed receptor oligomer (for illustrative purposes, receptor dimer is shown), and thus to promote the trans-homointeractions between FcRγ homodimers (II). Formation of competent signaling oligomers results in phosphorylation of the ITAM tyrosines, thus triggering the activation signal. Then, the signaling oligomers formed subsequently dissociate from GPVI, resulting in downmodulation of the remaining engaged receptor chains (IV). Stages I and II can be reversible or irreversible depending on interreceptor proximity and relative orientation of the receptors in ligand-specific dimers/oligomers as well as on time duration of the receptor-ligand contact and lifetime of the receptor in these dimers/oligomers that generally correlate with the nature of the stimulus and its specificity and affinity/avidity. Further, the signaling oligomers formed can sequentially interact with the signaling subunits of nonengaged receptors resulting in formation of higher-order signaling oligomers, thus amplifying and propagating the activation signal (not shown). Also, this leads to the release and subsequent downmodulation of the nonengaged GPVI chains (not shown). Immunoreceptor tyrosine-based activation motifs (ITAMs) are shown as green rectangles. Receptor components are represented as whole polypeptides and as a simplified axial view. Black and magenta arrows indicate specific intersubunit hetero- and homointeractions between transmembrane and cytoplasmic domains, respectively. All interchain interactions in intermediate complexes are shown by dotted arrows reflecting their transition state. Circular arrow indicates ligand-induced receptor reorientation. Phosphate groups are shown as filled gray circles. In an axial view, one solid small black line depicts one phosphorylated ITAM domain. Abbreviations: PTK, protein tyrosine kinase; SCHOOL, signaling chain homooligomerization; GPVI, glycoprotein VI receptor.

Interaction with multivalent ligand (not shown) clusters the receptors and pushes them to reorientate (I), to bring signaling subunits into a correct (permissive) relative orientation and in sufficient proximity in the formed receptor oligomer (for illustrative purposes, receptor dimers are shown), and thus to promote the trans-homointeractions between Igα/Igβ heterodimers (II). Formation of competent signaling oligomers results in phosphorylation of the ITAM tyrosines, thus triggering the activation signal (III). Then, the signaling oligomers formed subsequently dissociate from mIg, resulting in internalization of the remaining engaged receptor chains (IV). Stages I and II can be reversible or irreversible depending on interreceptor proximity and relative orientation of the receptors in ligand-specific dimers/oligomers as well as on time duration of the receptor-ligand contact and lifetime of the receptor in these dimers/oligomers that generally correlate with the nature of the stimulus and its specificity and affinity/avidity. The BCR signaling module contains two different signaling chains, Igα and Igβ. This provides a possibility of the signal and cell response diversity depending on the particular set of the Igα and/or Igβ ITAM tyrosines that become phosphorylated. Further, the signaling oligomers formed can sequentially interact with the signaling subunits of nonengaged receptors resulting in formation of higher-order signaling oligomers, thus amplifying and propagating the activation signal (not shown). Also, this leads to the release and subsequent internalization/downmodulation of the nonengaged mIg chains (not shown). Immunoreceptor tyrosine-based activation motifs (ITAMs) are shown as green rectangles. Receptor components are represented as whole polypeptides and as a simplified axial view. Black and magenta arrows indicate specific intersubunit hetero- and homointeractions between transmembrane and cytoplasmic domains, respectively. All interchain interactions in intermediate complexes are shown by dotted arrows reflecting their transition state. Circular arrow indicates ligand-induced receptor reorientation. Phosphate groups are shown as filled gray circles. In an axial view, one solid small black line depicts one phosphorylated ITAM domain. Abbreviations: PTK, protein tyrosine kinase; SCHOOL, signaling chain homooligomerization; BCR, B cell receptor.

SCHOOL model of FcεRI signaling. Interaction with multivalent ligand (not shown) clusters the receptors and pushes them to reorientate (I), to bring signaling subunits into a correct (permissive) relative orientation and in sufficient proximity in the formed receptor oligomer (for illustrative purposes, receptor dimer is shown), and thus to promote the trans-homointeractions between γ homodimers (II). Then, two alternative pathways can take a place depending on the nature of activating stimuli. First is going through a stage IV resulting in formation of γ₂ dimer (dimer of dimers) and phosphorylation of the γ ITAM tyrosines, thus triggering the activation signal A. Then, the signaling γ oligomers formed subsequently dissociate from the α/β complex, resulting in internalization of the remaining engaged complexes (VII). Alternatively, the intermediate complex formed at the stage II can undergo further rearrangements, starting trans-homointeractions between β chains (III) and resulting in formation of an oligomeric intermediate. Stages I, II and III can be reversible or irreversible depending on interreceptor proximity and relative orientation of the receptors in FcεRI dimers/oligomers as well as on time duration of the receptor-ligand contact and lifetime of the receptor in FcεRI dimers/oligomers that generally correlate with the nature of the stimulus and its specificity and affinity/avidity. Next, in the signaling oligomers formed (III), the γ and β ITAM tyrosines undergo phosphorylation by PTKs that leads to generation of the activation signal, dissociation of signaling oligomers and internalization of the engaged α chains (VIII, XI). This pathway provides two different activation signals from the γ and β signaling oligomers (signals A and B), respectively, and results in full cell activation. In addition, the signaling oligomers formed can sequentially interact with the signaling subunits of nonengaged FcεRIs resulting in formation of higher-order signaling oligomers, thus amplifying and propagating the activation signal (not shown). Also, this leads to the release and subsequent internalization of the nonengaged α and/or αβ chains (not shown). Immunoreceptor tyrosine-based activation motifs (ITAMs) are shown as green rectangles. FceRIα, β and γ components are represented as whole polypeptides and as a simplified axial view. Black and magenta arrows indicate specific intersubunit hetero- and homointeractions between transmembrane and cytoplasmic domains, respectively. All interchain interactions in intermediate complexes are shown by dotted arrows reflecting their transition state. Circular arrow indicates ligand-induced receptor reorientation. Phosphate groups are shown as filled gray circles. In an axial view, one solid small black line depicts one phosphorylated ITAM domain. Abbreviations: PTK, protein tyrosine kinase; SCHOOL, signaling chain homooligomerization; FcεRI, high affinity IgE receptor.

SCHOOL model of T cell receptor (TCR) signaling. Interaction with multivalent ligand (not shown) clusters the receptors and pushes them to reorientate (I), to bring signaling subunits into a correct (permissive) relative orientation and in sufficient proximity in the formed receptor oligomer (for illustrative purposes, receptor dimer is shown), and thus to promote the trans-homointeractions between ζ molecules (II). Then, two alternative pathways can take a place depending on the nature of activating stimuli. First is going through a stage IV resulting in formation of ζ₂ dimer (dimer of dimers) and phosphorylation of the ζ ITAM tyrosines, thus triggering the activation signal A. Then, the signaling ζ oligomers formed subsequently dissociate from the TCR-CD3 complex, resulting in internalization of the remaining engaged TCR-CD3 complexes (VII). This pathway leads to partial (or incomplete) T cell activation. Alternatively, the intermediate complex formed at the stage II can undergo further rearrangements, starting trans-homointeractions between CD3 proteins (III) and resulting in formation of an oligomeric intermediate. The stages I, II and III can be reversible or irreversible depending on interreceptor proximity and relative orientation of the receptors in TCR dimers/oligomers as well as on time duration of the TCR-ligand contact and lifetime of the receptor in TCR dimers/oligomers that generally correlate with the nature of the stimulus and its specificity and affinity/avidity. Next, in the signaling oligomers formed (III), the ITAM tyrosines undergo phosphorylation by PTKs that leads to generation of the activation signal, dissociation of signaling oligomers and internalization of the remaining engaged TCRαβ chains (VIII, XI). This pathway provides at least two different activation signals from the ζ and CD3 signaling oligomers (signals A and B), respectively, and results in full T cell activation. The distinct signaling through ζ and CD3 oligomers (or through various combinations of signaling chains in CD3 oligomeric structures) might be also responsible for distinct functions such as T cell proliferation, effector functions, T cell survival, pathogen clearance, TCR anergy, etc. In addition, the signaling oligomers formed can sequentially interact with the signaling subunits of nonengaged TCRs resulting in formation of higher-order signaling oligomers, thus amplifying and propagating the activation signal (not shown). Also, this leads to the release and subsequent internalization of the remaining nonengaged TCR complexes and/or TCRαβ chains (not shown). Immunoreceptor tyrosine-based activation motifs (ITAMs) are shown as green rectangles. TCR-CD3-ζ components are represented as whole polypeptides and as a simplified axial view. Circular arrows indicate ligand-induced receptor reorientation. Black and magenta arrows indicate specific intersubunit hetero- and homointeractions between transmembrane and cytoplasmic domains, respectively. All interchain interactions in intermediate complexes are shown by dotted arrows reflecting their transition state. Phosphate groups are shown as filled gray circles. In an axial view, one solid small black line depicts one phosphorylated ITAM domain. Abbreviations: PTK, protein tyrosine kinase; SCHOOL, signaling chain homooligomerization.