PubMed · 409901
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J Yariv. 1977. beta-Galactosidase.. https://doi.org/10.1016/s0076-6879(77)46047-6
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Sambucus nigra agglutinin I (SNA-I) is a type 2 ribosome-inactivating protein. Site-directed mutagenesis was used to mimic the conversion of the highly active B-chain of fruit-specific SNA (SNA-If) into the completely inactive B-chain of the closely related and naturally occurring loss-of-activity mutant called S. nigra agglutinin lectin-related protein. In the first mutant SNA-If-M1 the high-affinity site 2 of SNA-If was disrupted by replacing the presumed critical residue Asp231 with Glu231. In the double mutant SNA-If-M2, site 1 of SNA-If-M1 was also disrupted by substituting the presumed critical residue Asn48 with Ser48. The parent type 2 ribosome-inactivating protein and both mutants were expressed in Nicotiana tabacum Samsun NN and the recombinant proteins were purified and analysed. Recombinant SNA-If agglutinated rabbit erythrocytes equally well as SNA-If, but both mutants were completely inactive in this test. Binding assays to immobilized galactose and fetuin revealed that the mutation Asp231-->Glu231 reduces the affinity of the B-chain for galactose and fetuin by more than 50%. Furthermore, the introduction of the second mutation Asn48-->Ser48 reduces the binding activity to less than 20% of the original activity.
An important characteristic of malaria parasite Plasmodium falciparum-infected red blood cells (IRBCs) is their ability to adhere to host endothelial cells and accumulate in various organs. Sequestration of IRBCs in the placenta, associated with excess perinatal and maternal mortality, is mediated in part by adhesion of parasites to the glycosaminoglycan chondroitin sulfate A (CSA) present on syncytiotrophoblasts lining the placental blood spaces. To define key structural features for parasite interactions, we isolated from CSA oligosaccharide fractions and established by electrospray mass spectrometry and high performance liquid chromatography disaccharide composition analysis their differing chain length, sulfate content, and sulfation pattern. Testing these defined oligosaccharide fragments for their ability to inhibit IRBC adhesion to immobilized CSA revealed the importance of non-sulfated disaccharide units in combination with 4-O-sulfated disaccharides for interaction with IRBCs. Selective removal of 6-O-sulfates from oligo- and polysaccharides to increase the proportion of non-sulfated disaccharides enhanced activity, indicating that 6-O-sulfation interferes with the interaction of CSA with IRBCs. Dodecasaccharides with four or five 4-O-sulfated and two or one non-sulfated disaccharide units, respectively, comprise the minimum chain length for effective interaction with IRBCs. Comparison of the activities of CSA and CSB oligo- and polysaccharides with a similar sulfation pattern and content achieved from partial desulfation demonstrated that glucuronic acid rather than iduronic acid residues are important for IRBC binding.
Lectins on antigen presenting cells are potentially involved in the antigen uptake and the cellular recognition and trafficking. Serial analysis of gene expression in monocyte-derived dendritic cells (DCs), monocytes, and macrophages revealed that 7 of the 19 C-type lectin mRNA were present in immature DCs. Two of these, the macrophage mannose receptor and the macrophage lectin specific for galactose/N-acetylgalactosamine (MGL), were found only in immature DCs, as confirmed by reverse transcriptase-PCR and flow cytometric analysis. By subcloning and sequencing the amplified mRNA, we obtained nucleotide sequences encoding seven different human MGL (hMGL) subtypes, which were apparently derived from alternatively spliced mRNA. In addition, the hMGL gene locus on human chromosome 17p13 contains one gene. A single nucleotide polymorphism was identified at a position in exon 3 that corresponds to the cytoplasmic region proximal to the transmembrane domain. Of all the splicing variants, the hMGL variant 6C was expressed at the highest levels on immature DCs from all donors tested. Immature DCs could incorporate alpha-GalNAc-modified soluble acrylamide polymers, and this was significantly inhibited by pretreatment of the cells with an anti-hMGL monoclonal antibody that blocks the lectin-carbohydrate interaction. We propose that hMGL is a marker of imDCs and that it functions as an endocytic receptor for glycosylated antigens.