Immune receptors that show high mutual sequence similarity and have antagonizing signaling properties are called paired receptors, and are believed to fine-tune immune responses. Siglecs are sialic acid-recognizing receptors of the immunoglobulin (Ig) superfamily expressed on immune cells. Human Siglec-5, encoded by SIGLEC5 gene, has four extracellular Ig-like domains and a cytosolic inhibitory motif. We discovered human Siglec-14 with three Ig-like domains, encoded by the SIGLEC14 gene, adjacent to SIGLEC5. Human Siglec-14 has almost complete sequence identity with human Siglec-5 at the first two Ig-like domains, shows a glycan binding preference similar to that of human Siglec-5, and associates with the activating adapter protein DAP12. Thus, Siglec-14 and Siglec-5 appear to be the first glycan binding paired receptors. Near-complete sequence identity of the amino-terminal part of human Siglec-14 and Siglec-5 indicates partial gene conversion between SIGLEC14 and SIGLEC5. Remarkably, SIGLEC14 and SIGLEC5 in other primates also show evidence of gene conversions within each lineage. Evidently, balancing the interactions between Siglec-14, Siglec-5 and their common ligand(s) had selective advantage during the course of evolution. The "essential arginine" critical for sialic acid recognition in both Siglec-14 and Siglec-5 is present in humans but mutated in almost all great ape alleles.

Siglecs are vertebrate cell-surface receptors that recognize sialylated glycans. Here we have identified and characterized a novel Siglec, named Siglec-15. Siglec-15 is a type-I transmembrane protein consisting of: (i) two immunoglobulin (Ig)-like domains, (ii) a transmembrane domain containing a lysine residue, and (iii) a short cytoplasmic tail. Siglec-15 is expressed on macrophages and/or dendritic cells of human spleen and lymph nodes. We show that the extracellular domain of Siglec-15 preferentially recognizes the Neu5Acalpha2-6GalNAcalpha- structure. Siglec-15 associates with the activating adaptor proteins DNAX activation protein (DAP)12 and DAP10 via its lysine residue in the transmembrane domain, implying that it functions as an activating signaling molecule. Siglec-15 is the second human Siglec identified to have an activating signaling potential; unlike Siglec-14, however, it does not have an inhibitory counterpart. Orthologs of Siglec-15 are present not only in mammals but also in other branches of vertebrates; in contrast, no other known Siglec expressed in the immune system has been conserved throughout vertebrate evolution. Thus, Siglec-15 probably plays a conserved, regulatory role in the immune system of vertebrates.

Copolymerization of cyclohexene oxide with carbon dioxide was investigated by using chromium complexes with salalen ligands which are reduced analogues of salen ones (salen-H2 = N,N ′-disalicylidene-1,2-ethylenediamine). Although the use of (salalen)CrCl complexes alone without additives was not effective for the copolymerization, a mixture of (salalen)CrCl complexes and onium salts catalyzed the copolymerization with relatively high catalytic activity. Particularly, the present catalyst system showed the highest catalytic activity under atmospheric pressure of CO2 among those ever reported. In general, the obtained copolymers consisted of almost perfectly alternating structure. Structural analysis of the copolymers by MALDI-TOF mass spectrometry revealed that chloride initiated the copolymerization. In addition, concomitant water was found to work as a chain transfer reagent and a bifunctional initiator.

E2F-1 is capable of promoting both cell cycle progression and apoptosis. The latter is important for suppressing untoward expansion of proliferating cells. In this study, we investigated its underlying mechanisms. E2F-1-induced apoptosis was accompanied by caspase-9 activation and inhibited by a specific inhibitor of caspase-9 in K562 sublines overexpressing E2F-1. E2F-1 enhanced the expression of Apaf-1 without the cytosolic accumulation of cytochrome c. Apaf-1-deficient melanoma cell lines were resistant to E2F-1, indicating that Apaf-1 is an essential element of E2F-1-mediated apoptosis. Finally, we isolated the promoter region of the Apaf-1 gene and found a putative binding site for E2F. A chromatin immunoprecipitation assay revealed that E2F-1 bound to Apaf-1 promoter upon E2F-1 overexpression, suggesting that Apaf-1 is under transcriptional regulation of E2F-1. These data demonstrate a novel mechanism of apoptosis in which an increase in Apaf-1 levels results in direct activation of caspase-9 without mitochondrial damage, leading to the initiation of a caspase cascade.

To date, 10 members of the UDP-N-acetyl-␣-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase (pp-GalNAc-T) family have been cloned and analyzed in human. In this study, we cloned and analyzed a novel human pp-GalNAc-T from an NT2 cell cDNA library, and we named it pp-GalNAc-T13. In amino acid sequences, pp-GalNAc-T13 was highly homologous, showing 84.3% identity, to pp-GalNAc-T1. Real time PCR analysis revealed pp-GalNAc-T13 to be highly and restrictively expressed in the brain and present at very low or undetectable levels in other tissues, in contrast to the ubiquitous expression of pp-GalNAc-T1. pp-GalNAc-T13 was abundantly expressed in all neuroblastoma cells examined and primary cultured neurons but not in glioblastoma cells and primary cultured astrocytes. pp-GalNAc-T13 exhibited much stronger activity to transfer GalNAc to mucin peptides, such as Muc5Ac and MUC7, than did pp-GalNAc-T1. In addition, pp-GalNAc-T13 differed in substrate specificity to pp-GalNAc-T1. pp-GalNAc-T13 was able to form a triplet Tn epitope, three consecutive GalNAc-Ser/Thr structures, on peptides encoded in syndecan-3, a proteoglycan expressed in neurons. pp-GalNAc-T13-deficient mice have been established in a previous work. Immunohistochemical study showed a remarkable decrease in Tn antigen expression in the cerebellum of the pp-GalNAc-T13 knockout mouse. pp-GalNAc-T13 would be a major enzyme responsible for the synthesis of O-glycan and specifically the Tn antigen epitope in neurons.O-Linked glycosylation of mucin is initiated by the transfer of N-acetylgalactosamine with an ␣-linkage to a serine or threonine residue in protein. The GalNAc residue attached to the peptide is usually extended to form more complex O-glycan structures by the action of multiple glycosyltransferases. The addition of GalNAc is controlled by multiple members of the pp-GalNAc-T 1 (EC 2.4.1.41) family. To date, 10 distinct members have been identified in human (pp-GalNAc-T1, -T2, -T3, -T4, -T6 to -T9, -T11, and -T12) (1-10) and 7 in rodent (ppGaN- Tase-T1, -T2, -T3, -T4, -T5, -T7, -T10 2 ) (11-17). All share a highly homologous primary sequence, 40 -60% homology at the amino acid level, to each other particularly in the predicted catalytic domain. Each member exhibits different substrate specificity toward peptide sequences. Thus, the positions of O-glycan in proteins are determined by the substrate specificity of each pp-GalNAc-T. The genes encoding these enzymes are distributed at different genomic localizations on chromosomes and have distinct genomic structures (4).pp-GalNAc-T1, which was the first pp-GalNAc-T to be cloned from bovine tissue (1), is the best characterized of the members. It shows activity toward Muc1, Muc2, Muc5Ac, and Muc7 peptides (4, 18). Peptide sequences derived from these mucins were used to determine the site of the O-glycosylation of the Muc2 and Muc5Ac mucin tandem repeat region by pp-GalNAc-T1 (19 -22).Many pp-GalNAc-Ts, including pp-GalNAc-T1, were found to * This work was performed as a part of the R&D Project o...