title: Characterizing the Size and Composition of Saposin A Lipoprotein Picodiscs.
abstract: Saposin A (SapA) lipoprotein discs, also known as picodiscs (PDs), represent an attractive method to solubilize glycolipids for protein interaction studies in aqueous solution. Recent electrospray ionization mass spectrometry (ESI-MS) data suggest that the size and composition of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)-containing PDs at neutral pH differs from those of N,N-dimethyldodecylamine N-oxide determined by X-ray crystallography. Using high-resolution ESI-MS, multiangle laser light scattering (MALLS), and molecular dynamics (MD) simulations, the composition, heterogeneity, and structure of POPC-PDs in aqueous ammonium acetate solutions at pH 4.8 and 6.8 were investigated. The ESI-MS and MALLS data revealed that POPC-PDs consist predominantly of (SapA dimer + iPOPC) complexes, with i = 23-29, and have an average molecular weight (MW) of 38.2 ± 3.3 kDa at pH 4.8. In contrast, in freshly prepared solutions at pH 6.8, POPC-PDs are composed predominantly of (SapA tetramer + iPOPC) complexes, with i = 37-60, with an average MW of 68.0 ± 2.7 kDa. However, the (SapA tetramer + iPOPC) complexes are unstable at neutral pH and convert, over a period of hours, to (SapA trimer + iPOPC) complexes, with i = 29-36, with an average MW of 51.1 ± 2.9 kDa. The results of molecular modeling suggest spheroidal structures for the (SapA dimer + iPOPC), (SapA trimer + iPOPC), and (SapA tetramer + iPOPC) complexes in solution. Comparison of measured collision cross sections (Ω) with values calculated for gaseous (SapA dimer + 26POPC)8+, (SapA trimer + 33POPC)12+, and (SapA tetramer + 42POPC)16+ions produced from modeling suggests that the solution structures are largely preserved in the gas phase, although the lipids do not maintain regular bilayer orientations.
authors.affiliation: Alberta Glycomics Centre and Department of Chemistry, University of Alberta , Edmonton, Alberta T6G 2G2, Canada.
authors.forename: Jun
authors.initials: J
authors.lastname: Li
authors.affiliation: Alberta Glycomics Centre and Department of Chemistry, University of Alberta , Edmonton, Alberta T6G 2G2, Canada.
authors.forename: Michele R
authors.initials: MR
authors.lastname: Richards
authors.affiliation: Amgen, Discovery Analytical Sciences, Thousand Oaks, California 91320, United States.
authors.forename: Dhanashri
authors.initials: D
authors.lastname: Bagal
authors.affiliation: Amgen, Discovery Analytical Sciences, Thousand Oaks, California 91320, United States.
authors.forename: Iain D G
authors.initials: ID
authors.lastname: Campuzano
authors.affiliation: Alberta Glycomics Centre and Department of Chemistry, University of Alberta , Edmonton, Alberta T6G 2G2, Canada.
authors.forename: Elena N
authors.initials: EN
authors.lastname: Kitova
authors.affiliation: Department of Biochemistry, University of Toronto , Toronto, Ontario M5S 1A8, Canada.
authors.forename: Zi Jian
authors.initials: ZJ
authors.lastname: Xiong
authors.affiliation: Department of Biochemistry, University of Toronto , Toronto, Ontario M5S 1A8, Canada.|Princess Margaret Cancer Centre, University Health Network , Toronto, Ontario M5G 1L7, Canada.
authors.forename: Gilbert G
authors.initials: GG
authors.lastname: Privé
authors.affiliation: Alberta Glycomics Centre and Department of Chemistry, University of Alberta , Edmonton, Alberta T6G 2G2, Canada.
authors.forename: John S
authors.initials: JS
authors.lastname: Klassen
mesh_terms: D000085:Acetates / Q000737:chemistry; D000818:Animals; D002645:Chickens; D005740:Gases / Q000737:chemistry; D006736:Horses; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008074:Lipoproteins / Q000737:chemistry*; D056004:Molecular Dynamics Simulation; D008970:Molecular Weight; D010316:Particle Size; D010713:Phosphatidylcholines / Q000737:chemistry; D049231:Saposins / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization
chemical_list: D000085:Acetates; D005740:Gases; D008074:Lipoproteins; D010713:Phosphatidylcholines; D049231:Saposins; C018824:ammonium acetate; C028694:1-palmitoyl-2-oleoylphosphatidylcholine