Characterizing the Size and Composition of Saposin A Lipoprotein Picodiscs.

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, multi-angle 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 to 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 to 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 to 36, with an average MW of 51.1 ± 2.9 kDa. The results of molecular modelling 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 modelling suggests that the solution structures are largely preserved in the gas phase, although the lipids do not maintain regular bilayer orientations. .

Citation

Jun Li, Michele R Richards, Dhanashri Bagal, Iain David Grant Campuzano, Elena N Kitova, Zi Jian Xiong, Gilbert G Prive, John S Klassen. Characterizing the Size and Composition of Saposin A Lipoprotein Picodiscs. Analytical chemistry. 2016 Aug 17

PMID: 27532319

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