title: Sequential proteolysis and high-field FTICR MS to determine disulfide connectivity and 4-maleimide TEMPO spin-label location in L126C GM2 activator protein.
abstract: The GM2 activator protein (GM2AP) is an 18 kDa nonenzymatic accessory protein involved in the degradation of neuronal gangliosides. Genetic mutations of GM2AP can disrupt ganglioside catabolism and lead to deadly lysosomal storage disorders. Crystallography of wild-type GM2AP reveals 4 disulfide bonds and multiple conformations of a flexible loop region that is thought to be involved in lipid binding. To extend the crystallography results, a cysteine construct (L126C) was expressed and modified with 4-maleimide TEMPO for electron paramagnetic resonance (EPR) studies. However, because a ninth cysteine has been added by site-directed mutagenesis and the protein was expressed in E. coli in the form of inclusion bodies, the protein could misfold during expression. To verify correct protein folding and labeling, a sequential multiple-protease digestion, nano-liquid chromatograph (LC) electrospray ionization 14.5 T Fourier transform ion cyclotron resonance mass spectrometry assay was developed. High-magnetic field and robust automatic gain control results in subppm mass accuracy for location of the spin-labeled cysteine and verification of proper connectivity of the four disulfide bonds. The sequential multiple protease digestion strategy and ultrahigh mass accuracy provided by FTICR MS allow for rapid and unequivocal assignment of relevant peptides and provide a simple pipeline for analyzing other GM2AP constructs.
authors.affiliation: Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005, USA.
authors.forename: Jeremiah D
authors.initials: JD
authors.lastname: Tipton
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Carter
authors.forename: Jordan D
authors.initials: JD
authors.lastname: Mathias
authors.forename: Mark R
authors.initials: MR
authors.lastname: Emmett
authors.forename: Gail E
authors.initials: GE
authors.lastname: Fanucci
authors.forename: Alan G
authors.initials: AG
authors.lastname: Marshall
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D018360:Crystallography, X-Ray; D003497:Cyclic N-Oxides / Q000737:chemistry*; D004220:Disulfides / Q000032:analysis* / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005583:Fourier Analysis; D049289:G(M2) Activator Protein / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008301:Maleimides / Q000737:chemistry*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010455:Peptides / Q000032:analysis / Q000737:chemistry; D017510:Protein Folding; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D013113:Spin Labels / Q000138:chemical synthesis; D014357:Trypsin / Q000378:metabolism
chemical_list: C545495:4-maleimide-2,2,6,6-tetramethyl-4-piperidine-N-oxide; D003497:Cyclic N-Oxides; D004220:Disulfides; D049289:G(M2) Activator Protein; D008301:Maleimides; D010455:Peptides; D011994:Recombinant Proteins; D013113:Spin Labels; D014357:Trypsin
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