Insights & Perspectives Hypotheses Activation of transmembrane cell-surface receptors via a common mechanism? The ‘‘rotation model’’ Ichiro N. Maruyama It has long been thought that transmembrane cell-surface receptors, such as typically consist of an extracellular receptor tyrosine kinases and cytokine receptors, among others, are activated by domain (ECD) and an intracellular ligand binding through ligand-induced dimerization of the receptors. However, domain (ICD) separated by a single transmembrane domain (TMD), with there is growing evidence that prior to ligand binding, various transmembrane the exception of bacterial receptors receptors have a preformed, yet inactive, dimeric structure on the cell surface. such as the aspartate receptor (Tar) Various studies also demonstrate that during transmembrane signaling, ligand and the serine receptor (Tsr), which binding to the extracellular domain of receptor dimers induces a rotation of have another TMD at their amino transmembrane domains, followed by rearrangement and/or activation of termini. Ligand binding to their ECDs often regulates kinases that are either intracellular domains. The paper here describes transmembrane cell-surface integrated into the receptor ICD, or receptors that are known or proposed to exist in dimeric form prior toligand binding, physically associated with the ICD. and discusses how these preformed dimers are activated by ligand binding. Apart from receptors that initiate signaling pathways inside cells via Keywords: tyrosine phosphorylation, there are .cytokine; dimerization; ligand binding; preformed dimer; transmembrane receptors in bacteria, fungi, and plants signaling; tyrosine kinase that phosphorylate histidine residues upon ligand binding. Furthermore, natriuretic peptide receptors, which are receptor-type guanylyl cyclases, Introduction cell membranes to the cytoplasm, and produce cGMP upon peptide binding. include receptor tyrosine kinases There are also receptors that recruit Transmembrane, cell-surface receptors (RTKs) and cytokine receptors among adaptor/effector proteins through transmit extracellular signals across many others. Cell-surface receptors protein-protein interactions upon ligand binding. There are two major, mutually DOI 10.1002/bies.201500041 exclusive concepts to explain activation of transmembrane, cell-surface receptors. Ligand binding induces Okinawa Institute of Science and Technology GCY-14, receptor-type guanylyl cyclase-14; Graduate University, Onna, Okinawa, Japan GHR, growth hormone receptor; ICD, intracellular either (i) dimerization of receptors, or domain; IGF1R, insulin-like growth factor-1 recep- (ii) rearrangement of constitutively pre- Corresponding author: tor; IL-6R, interleukin 6 receptor; IL-12R, inter- formed dimeric receptors. The former Ichiro N. Maruyama leukin 12 receptor; IR, insulin receptor; IRR, insulin E-mail: [email protected] receptor-related receptor; JAK, Janus kinase; mechanism, known as ligand-induced LepR, leptin receptor; NPRA, natriuretic peptide receptor dimerization, was first pro- Abbreviations: receptor A; p75NTR, neurotrophin receptor; PRLR, posed for the epidermal growth factor BRET, bioluminescence resonance energy trans- prolactin receptor; RTK, receptor tyrosine kinase; receptor (EGFR; also called ErbB1 or fer; ECD, extracellular domain; EGFR, epidermal STAT, signal transducers and activators of tran- growth factor receptor; Eph, ephrin receptor; scription; Tar, bacterial aspartate receptor; TLR, HER1) almost three decades ago [1–3]. In EpoR, erythropoietin receptor; ErbB, originally toll-like receptor; TMD, transmembrane domain; this “dimerization model,” the receptor named because of the homology to the erythro- TNFR, tumor necrosis factor receptor; TpoR, is thought to exist in monomeric form blastoma viral gene product, v-erbB; FGFR, thrombopoietin receptor; Trk, tropomyosin- fibroblast growth factor receptor; FnIII, fibronectin related kinase receptor; Tsr, bacterial serine on the cell surface prior to ligand type III; FRET, Forster€ resonance energy transfer; receptor. binding, which induces receptor Bioessays 37: 959–967, ß 2015 The Author. BioEssays published by WILEY Periodicals, Inc. This is an www.bioessays-journal.com 959 open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. I. N. Maruyama Insights & Perspectives..... dimerization. According to this model, A variety of receptors kinase activity and in selective trans- ICDs of dimerized receptors are brought exist in dimeric form prior autophosphorylation of tyrosine resi- into close proximity to allow receptor to ligand binding dues. Some of these sites are involved in trans-autophosphorylation, subsequent maintaining active conformations of the tyrosine kinase activation, and initia- Receptor tyrosine kinases kinases, while others become docking tion of downstream signaling path- sites for various adaptor/effector scaf- ways [4, 5]. This mechanism has been The human RTK superfamily consists of fold proteins and enzymes. All RTKs, proposed for activation of many other 58 proteins grouped into 20 sub- except for the IR family, are expressed cell-surface receptors, including recep- families [8]. RTKs are integral mem- as single protomers. IR family members, tor tyrosine kinases and cytokine recep- brane proteins with a single TMD, and comprising IR, IGF1R, and IRR, are also tors. In contrast, the insulin receptor their N-terminal ECDs are generally expressed as single subunits, but they Hypotheses (IR), insulin-like growth factor-1 composed of various structural modules undergo processing into two a and two receptor (IGF1R), and insulin receptor- with multiple, intrachain, disulfide b polypeptide chains that are assembled related receptor (IRR) have covalent, bonds, and numerous N-linked into a heterotetramer, or an (ab)2 preformed, dimeric structures linked by glycosylation sites. Their ICDs have homodimer, stabilized by disulfide cysteine disulfide bridges [6, 7]. Like- tyrosine kinase domains flanked by bonds. wise, there is growing evidence that intracellular, juxtamembrane regions, prior to ligand binding, various trans- and C-terminal tails, which differ in size membrane receptors exist in preformed, and tyrosine content among family EGFR (ErbB) family yet inactive, dimeric form on the cell members. Ligand binding to the ECDs surface (Table 1). results in elevation of their tyrosine The ErbB receptor family consists of EGFR, ErbB2 (also known as Neu/ HER2), ErbB3 (HER3), and ErbB4 (HER4), and the receptors play crucial Table 1. Transmembrane cell-surface receptors that exist or are proposed to exist roles in cell growth, differentiation, in dimeric form prior to ligand binding survival, and migration. A number of studies demonstrate that prior to ligand References Rotation angle and reference binding, ErbB receptors exist in dimeric Receptor tyrosine kinases form on the cell surface (see [9], and EGFR [10–21] [140˚: 10] ErbB2 [13, 18, 80] references therein]. Chemical cross- ErbB3 [18, 21] linking showed that >80% of EGFR ErbB4 [18] molecules were dimeric in the absence EphA1 [40, 41] of bound ligand [10]. Forster€ resonance EphA2 [38, 39, 41] [60˚: 41] energy transfer (FRET) [11–14] and fluo- EphA3 [37] rescence correlation spectroscopic anal- FGFR3 [43] IGF1R [32] yses [13, 15, 16] further demonstrated that IR [32] preformed EGFR and ErbB2 dimers are IRR [33] present at physiological expression lev- MET [42] els on surfaces of living cells. Single- TrkA [28] molecule observations using total inter- TrkB [29] nal reflection fluorescence microscopy VEGFR2 [180˚: 99] with oblique illumination also supports Cytokine receptors EpoR [50, 51] [100˚: 91, 92] the existence of receptor dimers [17]. GHR [48, 49] [40˚: 93, 94; 45˚: 95] Fluorescent protein fragment comple- IL-6R [60] mentation indicates that all the members IL-12R [58] of the ErbB family exist in dimeric LepR [55–57] NTR form [18]. This is consistent with results p75 [30] of reversible firefly luciferase fragment PRLR [52] TNFR [62] complementation analysis, showing that TpoR [100˚: 53] 100% of EGFR and ErbB3 receptor Other cell-surface receptors molecules exist as dimers [19–21], since EnvZ [69] luciferase activity did not increase after GCY-14 [65] addition of EGF to the cell culture. LINGO-1 [66] Depending on methods used, dimer- NPRA [64] [40˚: 96] Tar [67] [50˚: 74] to-monomer ratios vary from 40 to TLR9 [63] 100%. Considering the inefficiency of Tsr [68] chemical cross-linking [22] and of fluo- rescent protein folding [23–25], these Receptors in which TMDs have been proposed to rotate during signaling are ratios are likely to be underestimated. indicated with bold letters, with or without rotation angles and reference(s). However, when EGFR mutants with 960 Bioessays 37: 959–967, ß 2015 The Author. BioEssays published by WILEY Periodicals, Inc. .....Insights & Perspectives I. N. Maruyama cysteine substitutions at different loca- extracellular a-subunits that contain (signal transducers and activators of tions in the TMD were expressed in a ligand-binding domains and two trans- transcription) proteins. A series of land- murine pre-B lymphocyte line, Ba/F3, membrane b-subunits that possess mark publications gave rise to the Hypotheses disulfide cross-linking of the receptors intracellular kinase domains [32, 33]. textbook view that ligand binding