Dataset Information

Investigating cell surface galectin-mediated cross-linking on glycoengineered cells.

ABSTRACT: The galectin family of glycan-binding proteins is thought to mediate many cellular processes by oligomerizing cell surface glycoproteins and glycolipids into higher-order aggregates. This hypothesis reflects the known oligomeric states of the galectins themselves and their binding properties with multivalent ligands in vitro, but direct evidence of their ability to cross-link ligands on a cell surface is lacking. A major challenge in fundamental studies of galectin-ligand interactions is that their natural ligands comprise a heterogeneous collection of glycoconjugates that share related glycan structures but disparate underlying scaffolds. Consequently, there is no obvious means to selectively monitor the behaviors of natural galectin ligands on live cell surfaces. Here we describe an approach for probing the galectin-induced multimerization of glycoconjugates on cultured cells. Using RAFT polymerization, we synthesized well-defined glycopolymers (GPs) functionalized with galectin-binding glycans along the backbone, a lipid group on one end and a fluorophore on the other. After insertion into live cell membranes, the GPs' fluorescence lifetime and diffusion time were measured in the presence and absence of galectin-1. We observed direct evidence for galectin-1-mediated extended cross-linking on the engineered cells, a phenomenon that was dependent on glycan structure. This platform offers a new approach to exploring the "galectin lattice" hypothesis and to defining galectin ligand specificity in a physiologically relevant context.

SUBMITTER: Belardi B

PROVIDER: S-EPMC3374418 | biostudies-literature | 2012 Jun

REPOSITORIES: biostudies-literature

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Investigating cell surface galectin-mediated cross-linking on glycoengineered cells.

Belardi Brian B O'Donoghue Geoff P GP Smith Adam W AW Groves Jay T JT Bertozzi Carolyn R CR

Journal of the American Chemical Society 20120530 23

The galectin family of glycan-binding proteins is thought to mediate many cellular processes by oligomerizing cell surface glycoproteins and glycolipids into higher-order aggregates. This hypothesis reflects the known oligomeric states of the galectins themselves and their binding properties with multivalent ligands in vitro, but direct evidence of their ability to cross-link ligands on a cell surface is lacking. A major challenge in fundamental studies of galectin-ligand interactions is that th ...[more]

PMID: 22540968

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Project description:A cross-linking method is developed to elucidate glycan-mediated interactions between membrane proteins through sialic acids. The method provides information on previously unknown extensive glycomic interactions on cell membranes. The vast majority of membrane proteins are glycosylated with complicated glycan structures attached to the polypeptide backbone. Glycan-protein interactions are fundamental elements in many cellular events. Although significant advances have been made to identify protein-protein interactions in living cells, only modest advances have been made on glycan-protein interactions. Mechanistic elucidation of glycan-protein interactions has thus far remained elusive. Therefore, we developed a cross-linking mass spectrometry (XL-MS) workflow to directly identify glycan-protein interactions on the cell membrane using liquid chromatography-mass spectrometry (LC-MS). This method involved incorporating azido groups on cell surface glycans through biosynthetic pathways, followed by treatment of cell cultures with a synthesized reagent, N-hydroxysuccinimide (NHS)-cyclooctyne, which allowed the cross-linking of the sialic acid azides on glycans with primary amines on polypeptide backbones. The coupled peptide-glycan-peptide pairs after cross-linking were identified using the latest techniques in glycoproteomic and glycomic analyses and bioinformatics software. With this approach, information on the site of glycosylation, the glycoform, the source protein, and the target protein of the cross-linked pair were obtained. Glycoprotein-protein interactions involving unique glycoforms on the PNT2 cell surface were identified using the optimized and validated method. We built the GPX network of the PNT2 cell line and further investigated the biological roles of different glycan structures within protein complexes. Furthermore, we were able to build glycoprotein-protein complex models for previously unexplored interactions. The method will advance our future understanding of the roles of glycans in protein complexes on the cell surface.

Project description:During the peptidoglycan (PG) maturation of mycobacteria, the glycan strands are interlinked by both 3-3 (between two meso-diaminopimelic acids [meso-DAPs]) and 4-3 cross-links (between d-Ala and meso-DAP), though there is a predominance (60 to 80%) of 3-3 cross-links. The dd-carboxypeptidases (dd-CPases) act on pentapeptides to generate tetrapeptides that are used by ld-transpeptidases as substrates to form 3-3 cross-links. Therefore, dd-CPases play a crucial role in mycobacterial PG cross-link formation. However, the physiology of dd-CPases in mycobacteria is relatively unexplored. In this study, we deleted two dd-CPase genes, msmeg_2433 and msmeg_2432, both individually and in combination, from Mycobacterium smegmatis mc2155. Though the single dd-CPase gene deletions had no significant impact on the mycobacterial physiology, many interesting functional alterations were observed in the double-deletion mutant, viz, a predominance in PG cross-link formation was shifted from 3-3 cross-links to 4-3, cell surface glycopeptidolipid (GPL) expression was reduced, and susceptibility to β-lactams and antitubercular agents was enhanced. Moreover, the survival rate of the double mutant within murine macrophages was higher than that of the parent. Interestingly, the complementation with any one of the dd-CPase genes could restore the wild-type phenotype. In a nutshell, we infer that the altered ratio of 4-3 to 3-3 PG cross-links might have influenced the expression of surface GPLs, colony morphology, biofilm formation, drug susceptibility, and subsistence of the cells within macrophages.IMPORTANCE The glycan strands in mycobacterial peptidoglycan (PG) are interlinked by both 3-3 and 4-3 cross-links. The dd-CPases generate tetrapeptides by acting on the pentapeptides, and ld-transpeptidases use tetrapeptides as substrates to form 3-3 cross-links. In this study, we showed that simultaneous deletions of two dd-CPases alter the nature of PG cross-linking from 3-3 cross-links to 4-3 cross-links. The deletions subsequently decrease the expression of glycopeptidolipids (significant surface lipid present in many nontuberculous mycobacteria, including Mycobacterium smegmatis) and affect other physiological parameters, like cell morphology, growth rate, biofilm formation, antibiotic susceptibility, and survival within murine macrophages. Thus, unraveling the physiology of dd-CPases might help us design antimycobacterial therapeutics in the future.

Dataset Information

Investigating cell surface galectin-mediated cross-linking on glycoengineered cells.

Publications

Investigating cell surface galectin-mediated cross-linking on glycoengineered cells.

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