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Characterizing the size and composition of saposin A lipoprotein picodiscs

  • Author(s) / Creator(s)
  • 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.

  • Date created
    2016
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  • Type of Item
    Article (Published)
  • DOI
    https://doi.org/10.7939/R34B2XH9J
  • License
    © 2016 Li, J., Richards, M. R., Bagal, D., Campuzano, I. D., Kitova, E. N., Xiong, Z. J., Privé, G. G., & Klassen, J. S. This version of this article is open access and can be downloaded and shared. The original author(s) and source must be cited.
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  • Citation for previous publication
    • Li, J., Richards, M. R., Bagal, D., Campuzano, I. D., Kitova, E. N., Xiong, Z. J., Privé, G. G., & Klassen, J. S. (2016). Characterizing the size and composition of saposin A lipoprotein picodiscs. Analytical Chemistry, 88(19), 9524-9531. http://doi.org/10.1021/acs.analchem.6b02097
  • Link to related item
    http://doi.org/10.1021/acs.analchem.6b02097