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The mode of cytokine receptor activation is diverse. Here, the authors find that the marine-sponge derived lectin ThC, a bivalent sugar binding protein, activates human cytokine receptor MPL. This mode of action resembles the pathogenic activation of MPL by mutant molecular chaperon calreticulin in hematologic malignancies. N-glycan-mediated activation of the thrombopoietin receptor (MPL) under pathological conditions has been implicated in myeloproliferative neoplasms induced by mutant calreticulin, which forms an endogenous receptor-agonist complex that traffics to the cell surface and constitutively activates the receptor. However, the molecular basis for this mechanism is elusive because oncogenic activation occurs only in the cell-intrinsic complex and is thus cannot be replicated with external agonists. Here, we describe the structure and function of a marine sponge-derived MPL agonist, thrombocorticin (ThC), a homodimerized lectin with calcium-dependent fucose-binding properties. In-depth characterization of lectin-induced activation showed that, similar to oncogenic activation, sugar chain-mediated activation persists due to limited receptor internalization. The strong synergy between ThC and thrombopoietin suggests that ThC catalyzes the formation of receptor dimers on the cell surface. Overall, the existence of sugar-mediated MPL activation, in which the mode of activation is different from the original ligand, suggests that receptor activation is unpredictably diverse in living organisms.

The mode of cytokine receptor activation is diverse. Here, the authors find that the marine-sponge derived lectin ThC, a bivalent sugar binding protein, activates human cytokine receptor MPL. This mode of action resembles the pathogenic activation of MPL by mutant molecular chaperon calreticulin in hematologic malignancies. N-glycan-mediated activation of the thrombopoietin receptor (MPL) under pathological conditions has been implicated in myeloproliferative neoplasms induced by mutant calreticulin, which forms an endogenous receptor-agonist complex that traffics to the cell surface and constitutively activates the receptor. However, the molecular basis for this mechanism is elusive because oncogenic activation occurs only in the cell-intrinsic complex and is thus cannot be replicated with external agonists. Here, we describe the structure and function of a marine sponge-derived MPL agonist, thrombocorticin (ThC), a homodimerized lectin with calcium-dependent fucose-binding properties. In-depth characterization of lectin-induced activation showed that, similar to oncogenic activation, sugar chain-mediated activation persists due to limited receptor internalization. The strong synergy between ThC and thrombopoietin suggests that ThC catalyzes the formation of receptor dimers on the cell surface. Overall, the existence of sugar-mediated MPL activation, in which the mode of activation is different from the original ligand, suggests that receptor activation is unpredictably diverse in living organisms.

The primary structure of a lectin,designated OAA, isolated from thefreshwater cyanobacterium, Oscillatoriaagardhii NIES-204, was determined by thecombination of Edman degradation andESI-mass spectrometry. OAA is apolypeptide (MW 13,925) consisting of twotandem repeats. Interestingly, each repeatsequence of OAA showed a high degree ofsimilarity to those of a myxobacterium,Myxococcus xanthus hemagglutinin(MBHA), and a marine red alga Eucheumaserra lectin (ESA-2). A systematic bindingassay with pyridylaminated oligosaccharidesrevealed that OAA exclusively binds to highmannose (HM) type N-glycans, but not toother N-glycans, including complex types,hybrid types and the pentasaccharide core,or oligosaccharides from glycolipids. OAAdid not interact with any of free mono-andoligomannoses that are constituents of thebranched oligomannosides. These resultssuggest that the core disaccharide, GlcNAc-GlcNAc, is also essential for binding to OAA.The binding activity of OAA to HMtype N-glycanswas dramatically decreased whenα1-2 Man was attached to α1-3 Manbranched from the α1-6 Man of thepentasaccharide core. This specificity ofOAA for HM type oligosaccharides isdistinct from other HM-binding lectins.Kinetic analysis with an HMheptasaccharide revealed that OAApossesses two carbohydrate-binding sitesper molecule, with an association constant of2.41×10^8M^-1. Furthermore, OAA potentlyinhibits HIV replication in MT-4 cells(EC50=44.5 nM). Thus, we have found anovel lectin family sharing similar structureand carbohydrate binding specificity amongbacteria, cyanobacteria, and marine algae.