This hypothesis, however, has yet to be formally proven

This hypothesis, however, has yet to be formally proven. Studies to understand the molecular basis of lectin activity in cellular systems have been hampered by several factors. lectin) stimulates signaling via receptor cross-linking rather than by binding to a specific epitope on the TCR. Moreover, PTx is able to activate signaling by binding either N-linked or O-linked glycan modified receptors as the TCR displays N-Linked glycans while CD8 displays O-linked glycans. Finally, studies with a diverse panel of lectins indicate that the signaling activity of the lectins does not always correlate with the biochemical reports of ligand preferences. Comparison of lectin signaling through TCR or CD8, allows us to better define the structural and functional properties of lectin-glycan interactions using a biological based signaling readout. lectinECLGal1C4GlcNAcN-glycans, O-glycans++?+ND+Jacalin (agglutininVVAGalNAc1-Ser/Thr (Tn antigen)O-glycans++?+++erythroagglutininPHA-EGlcNAc1C2Man1C3(GlcNAc1C4)(Gal1C4GlcNAc1C2Man1C6)ManN-glycans++?++?toxinPTxNeuAc, Complex-type N-glycansN-glycans, O-glycans++?++?Wheat germ agglutinin (agglutininRCATerminal GalN-glycans, O-glycans++?NDNDNDleucoagglutininPHA-LGal1C4GlcNAc1C2ManN-glycans+??++?lectinDSLGlcNAcb1C4, Galb1C4GlcNAcN-glycans, O-glycans+??++?agglutininLCABranched (Man)n with Fuc1C6GlcNAcN-glycans+??+??agglutininPSABranched (Man)n with Fuc1C6GlcNAcN-glycans+??+??Concanavalin A (lectin IIGSL IITerminal GlcNAcN-glycans, O-glycans+?????lectinLELGlcNAcN-glycans, O-glycans+?????AgglutininDBAGalNAc1C3GalNAc, GalNAc1C3(Fuc1C2)Gal (A antigen)N-glycans, O-glycans??????Peanut Agglutinin (agglutininSJATerminal GalNAc/GalN-glycans, O-glycans??????agglutininUEA-IFuc1C2Gal (H antigen)N-glycans, O-glycans?????? Open in a separate window 1The stated ligand preferences are based on information available at public databases [Consortium for Functional Glycomics (http://www.functionalglycomics.org) and Lectin frontier Database (http://riodb.ibase.aist.go.jp/rcmg/glycodb/LectinSearch)] and published reports of lectin/glycan binding (1C9, 23, 62C64). ND, Not Determined In addition to the plant lectins, it is now clear that pathogenic microorganisms express proteins with lectin-like activities. One example is the encoded pertussis toxin (PTx). PTx is an AB5 toxin comprised of a hexameric polypeptide complex with five binding (B) subunits arranged in a ring structure and a single Rabbit polyclonal to OPG active (A) subunit with enzymatic properties sitting on top of the pore of the ring structure. The A subunit of PTx, S1, is an ADP-ribosyltransferase that targets the -subunit of some GTP-binding proteins(10). The five B subunits of PTx (collectively referred to as B-pentamer or PTxB) are required for binding and cytosolic entry of S1 into mammalian cells. Unlike other AB5 toxins, which have five identical B subunits, PTxB is comprised of four different subunits S2, S3, S4, and S5 in the ratio 1:1:2:1. All the binding activities have thus far been mapped to the S2 and S3 subunits of PTxB(11C22). Analogous to WGA, each S2 and S3 subunit contains multiple glycan binding sites. Interestingly, although the S2 and Tubeimoside I S3 share 71% amino acid identity, each has distinct binding preferences. PTx has been shown to bind a broad array of glycans, including sialylated and non-sialylated N-glycans, sialylated O-glycans, and sialylated gangliosides(23). The glycan binding activity of PTxB mediates activities independent of its role in delivering the S1 catalytic subunit to cellular targets. Via the B-subunits, PTx can bind to Tubeimoside I a variety of cellular receptors and activate their associated signaling pathways(24). Receptor targets for PTxB include the TCR in T cells, Toll like receptor 4 (TLR4) in dendritic cells, and CD14 in myelomonocytic cells(25C33). In T-cells the binding of PTxB to the TCR leads to T-cell activation and mitogenesis(24). The various receptors that PTxB bind share little structural similarity Tubeimoside I but are heavily glycosylated(34C36). The broad binding specificity of PTx likely allows it to act as a super-lectin, able to trigger signaling by clustering a wide variety of glycosylated receptors(37C42). This hypothesis, however, Tubeimoside I has yet to be formally proven. Studies to understand the molecular basis of lectin activity in cellular systems have been hampered by several factors. One problem is the incomplete understanding of the Tubeimoside I repertoire of binding sites for each particular lectin. Glycans also typically interact with their cognate binding sites with very low affinity, and tight binding is often achieved by engaging multiple binding sites. Additionally, since cell-surface glycans are built by sequential enzymatic processing, they can be in various stages of completion, resulting in considerable heterogeneity. Furthermore, a single cell-surface protein may display both N-linked and.