Protein-glycolipid interactions studied in vitro using ESI-MS and nanodiscs: insights into the mechanisms and energetics of binding.

Electrospray ionization-mass spectrometry (ESI-MS) analysis combined with the use of nanodiscs (NDs) to solubilize glycolipids (GLs) has recently emerged as a promising analytical method for detecting protein-GL interactions in vitro and, when applied to libraries of GLs, ranking their affinities. However, there is uncertainty regarding the mechanism(s) of complex formation in solution and the extent to which the relative abundances of protein-glycolipid complexes observed by ESI-MS reflect the relative concentrations in solution. Here, we describe the results of a systematic ESI-MS study aimed at elucidating the processes that influence binding of water-soluble proteins to GLs incorporated into NDs and to exploit these insights to quantify the binding energetics. The interactions between the cholera toxin B subunit homopentamer (CTB5) and its native ganglioside receptor, ß-D-Gal-(1 → 3)-ß-D-GalNAc-(1 → 4)-[α-D-Neu5Ac-(2 → 3)]-ß-D-Gal-(1 → 4)-ß-D-Glc-ceramide (GM1), and between a recombinant fragment of family 51 carbohydrate-binding module (CBM), originating from S. pneumoniae, with a synthetic B type 2 neoglycolipid, α-D-Gal-(1 → 3)-[α-L-Fuc-(1 → 2)]-ß-D-Gal-(1 → 4)-ß-D-GlcNAc-1,2-di-O-dodecyl-sn-glycero (B2NGL) served as model protein-GL complexes for this study. The results of the ESI-MS measurements reveal that proteins bind reversibly to ND-bound GLs and that proteins possessing multiple ligand binding sites are able to interact with GLs originating from different NDs. Experimental evidence suggests that the diffusion of GLs between NDs is rapid and influences the nature of the protein-GL complexes that are detected. Using a newly developed ESI-MS assay, the proxy ligand method, the association constants for the CBM-B2NGL and CTB5-GM1 interactions were quantified and found to be slightly smaller than those for the corresponding oligosaccharides in solution.

Picodiscs for facile protein-glycolipid interaction analysis.

Protein interactions with glycolipids are implicated in diverse cellular processes. However, the study of protein-glycolipid complexes remains a significant experimental challenge. Here, we describe a powerful new assay that combines electrospray ionization mass spectrometry (ESI-MS) and picodiscs, which are composed of human sphingolipid activator protein saposin A and a small number of phospholipids, to display glycolipids in a lipid environment for protein-glycolipid interaction studies in aqueous solution. Time-resolved measurements of enzyme catalyzed hydrolysis of glycolipid substrates and the detection of low, moderate, and high affinity protein-glycolipid interactions serve to demonstrate the reliability and versatility of the assay.

Mycobacteriophage cell binding proteins for the capture of mycobacteria.

Slow growing Mycobacteriumavium subsp. paratuberculosis (MAP) causes a deadly condition in cattle known as Johne's disease where asymptomatic carriers are the major source of disease transmission. MAP was also shown to be associated with chronic Crohn's disease in humans. Mycobacterium smegmatis is a model mycobacterium that can cause opportunistic infections in a number of human tissues and, rarely, a respiratory disease. Currently, there are no rapid, culture-independent, reliable and inexpensive tests for the diagnostics of MAP or M. smegmatis infections. Bacteriophages are viruses producing a number of proteins that effectively and specifically recognize the cell envelopes of their bacterial hosts. We demonstrate that the mycobacterial phage L5 minor tail protein Gp6 and lysin Gp10 are useful tools for the rapid capture of mycobacteria. Immobilized Gp10 was able to bind both MAP and M. smegmatis cells whereas Gp6 was M. smegmatis specific. Neither of the 2 proteins was able to capture E. coli, salmonella, campylobacter or Mycobacterium marinum cells. Gp6 was detected previously as a component of the phage particle and shows no homology to proteins with known function. Therefore, electrospray ionization mass spectrometry was used to determine whether recombinant Gp6 could bind to a number of chemically synthesized fragments of mycobacterial surface glycans. These findings demonstrate that mycobacteriophage proteins could be used as a pathogen capturing platform that can potentially improve the effectiveness of existing diagnostic methods.