Disease relevance of PCYT1B

• This drop in toxicity is consistent with an observed 100-fold loss in binding capacity of the CT B subunit and a 20- to 50-fold reduction in adenylate cyclase activation by the CT A subunit [1].
• Vibrio cholerae non-O1 strains were screened for the presence of cholera enterotoxin (CT) genes by means of digoxigenin-labelled polynucleotide CTA and CTB probes [2].
• These results suggest that protection against autoimmune diabetes can be achieved by feeding minute amounts of a pancreas islet cell autoantigen linked to CTB and appears to involve the selective migration and retention of protective T cells into lymphoid tissues draining the site of organ injury [3].
• We have previously shown that feeding a single dose of minute amounts of antigens conjugated to cholera toxin B subunit (CTB) can effectively suppress delayed-type hypersensitivity reactions in systemically immune animals [4].
• We now report that feeding small amounts of myelin basic protein (MBP) conjugated to CTB either before or after disease induction protected rats from experimental autoimmune encephalomyelitis [4].

High impact information on PCYT1B

• Coupling of antigen-containing particles to the pentameric binding subunit of cholera toxin (CTB) has been proposed as a means for increasing antigen uptake because the CTB receptor, ganglioside GM1, is a glycolipid present in apical membranes of all intestinal epithelial cells [5].
• To test the accessibility of enterocyte and M cell membrane glycolipids to ligands in the size ranges of viruses, bacteria, and particulate mucosal vaccines, we analyzed binding of CTB probes of different sizes to rabbit Peyer's patch epithelium [5].
• Thus, the barrier function of the intestinal epithelial cell glycocalyx may be important in limiting microbial adherence to membrane glycolipids, and in CTB-mediated targeting of vaccines to M cells and the mucosal immune system [5].
• CTB-coated, fluorescent microparticles (final diameter, 1.13 microns) failed to adhere to enterocytes or M cells in vivo or to well-differentiated Caco-2 intestinal epithelial cells in vitro [5].
• Stimulation of TCR signalling in Jurkat cells resulted in localized increases in fluorescence of GPI-linked fluorescent proteins and cholera toxin B-subunit (CTB) [6].

Chemical compound and disease context of PCYT1B

• Amino acid substitutions that caused decreased binding of mutant CT-B to ganglioside GM1 and abolished toxicity included negatively charged or large hydrophobic residues for Gly-33 and negatively or positively charged residues for Trp-88 [7].
• The closely related B-subunits of cholera toxin (CTB) and Escherichia coli heat-labile enterotoxin (LTB) both bind strongly to GM1 ganglioside receptors but LTB can also bind to additional glycolipids and glycoproteins [8].
• Substitution of lysine or arginine for Gly-33 did not affect immunoreactivity or GM1-binding activity of CT-B but abolished or reduced toxicity of the mutant holotoxins, respectively [7].
• The vaccine consisted of formalin-killed E. coli bacteria expressing the most common colonization factor antigens (CFAs), i.e., CFA/I, -II, and -IV, and recombinantly produced cholera B subunit (CTB) [9].
• We now report the structure determination and 2.3-A refinement of the CTB mutant Gly 33-->Arg complexed with the GM1 oligosaccharide, as well as the 2.2-A refinement of a Gly 33-->Asp mutant of the closely related Escherichia coli heat-labile enterotoxin B-pentamer (LTB) [10].

Biological context of PCYT1B

• Hybrid CTB (hCTB), having only one or two functional binding sites, has been constructed from two chemically inactivated derivatives of CTB [11].
• Whereas CCTalpha activity is regulated by reversible phosphorylation, CCTbeta lacks most of the corresponding carboxyl-terminal domain and contained only 3 potential phosphorylation sites of the 16 identified in CCTalpha [12].
• CCTbeta and CCTalpha proteins share highly related, but not identical, catalytic domains followed by three amphipathic helical repeats [12].
• Transfection of COS-7 cells with a CCTbeta expression construct led to the overexpression of CCT activity, the accumulation of cellular CDP-choline, and enhanced radiolabeling of phosphatidylcholine [12].
• Expression of CCTbeta/CCTalpha chimeric proteins showed that the amino-terminal portion of CCTbeta was required for posttranslational modification [12].

Anatomical context of PCYT1B

• CCTbeta protein was posttranslationally modified in COS-7 cells, resulting in slower migration during polyacrylamide gel electrophoresis [12].
• CCTbeta transcripts were detected in human adult and fetal tissues, and very high transcript levels were found in placenta and testis [12].
• CT-B binds to ganglioside GM1, which functions as the plasma membrane receptor for CT, and the enzymatic activity of A1 causes the toxic effects of CT on target cells [13].
• CTB coupled to 14 nm colloidal gold (final diameter, 28.8 nm) failed to adhere to enterocytes but did adhere to M cells [5].
• Nonobese diabetic mice fed transformed potato tuber tissues containing microgram amounts of the CTB-insulin fusion protein showed a substantial reduction in pancreatic islet inflammation (insulitis), and a delay in the progression of clinical diabetes [14].

Associations of PCYT1B with chemical compounds

• Cross-linking of either CD3 or CT-B strongly induced tyrosine phosphorylation and recruitment of a ZAP-70(SH2)(2)-green fluorescent protein (GFP) fusion protein to the lipid patches [15].
• Transgenic potato tubers produced 0.1% of total soluble protein as the pentameric CTB-insulin fusion, which retained GM1-ganglioside binding affinity and native antigenicity of both CTB and insulin [14].
• Moreover, FGF2 competes with FITC-CTB for the binding to cell membrane GM1 in different CHO cell lines independently of their capacity to express heparan sulfate proteoglycans [16].
• CTB accumulation in NPC1(-) cells was abolished by expression of wild-type NPC1 but not by mutant proteins with a mutation either in the NPC domain or the sterol-sensing domain [17].
• While both CTB and LTB bind to the GM1 ganglioside, LTB also binds to N-acetyllactosamine-terminated glycoconjugates [18].

Physical interactions of PCYT1B

• CT consists of one A polypeptide and five B polypeptides associated by noncovalent bonds, and CT-B interacts with CT-A primarily via the A2 domain [13].

Other interactions of PCYT1B

• Both CT B subunits and to a lesser extent CT A subunits were delivered intact to the serosal surface of the basolateral membrane [19].
• These data indicate that GM1/ligand interaction does not necessarily lead to neuritogenesis and suggest that a specialisation of CTB, not shared by anti-GM1 antibodies or rETxB, is required to activate TrkA [20].
• The levels of CCTbeta were highly correlated (r = 0.606) with those of the proliferating cell nuclear antigen (PCNA), which was used as an indicator of cell growth [21].
• Expression levels of CCTbeta in tumor tissues was significantly higher than in nontumor tissues in all patients with hepatocellular carcinoma (n = 15) and 83% of patients with colonic carcinoma (n = 17) [21].
• Moreover, the amino acid sequences of the N. coriiceps CCT beta and theta chains contained residue substitutions in the equatorial, apical, and intermediate domains that would be expected to increase the flexibility of the subunits, thus facilitating function of the chaperonin in an energy poor environment [22].

Analytical, diagnostic and therapeutic context of PCYT1B

• We have recently shown that oral administration of microgram amounts of antigen coupled to cholera toxin B subunit (CTB), can effectively suppress systemic T cell reactivity in naive as well as in immune animals [3].
• Higher interleukin (IL)-4 amounts were observed in both splenocytes and pancreatic lymph node (PLN) cell cultures from CTB-insulin-fed mice as soon as 4 h after the feeding [23].
• Oligonucleotide-directed mutagenesis of ctxB was used to produce mutants of cholera toxin B subunit (CT-B) altered at residues Cys-9, Gly-33, Lys-34, Arg-35, Cys-86 and Trp-88 [7].
• In ultrastructural studies using the scanning electron microscope (SEM) and immunofluorescence (IF) analyses to discriminate microtubule distributions, neurites of GM1-supplemented cells on CTB were virtually identical with pFN-adherent neurites, whereas unsupplemented cells on CTB generated processes with fine-structural differences [24].
• Both oral and nasal vaccination resulted in 5- to 6-fold CTB-specific IgA and 20- to 30-fold specific IgG increases in vaginal secretions [25].

References