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, 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 (Omega) 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.