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) compl...