The low-resolution structure of nHDL reconstituted with DMPC with and without cholesterol reveals a mechanism for particle expansion

Small-angle neutron scattering (SANS) with contrast variation was used to obtain the low-resolution structure of nascent HDL (nHDL) reconstituted with dimyristoyl phosphatidylcholine (DMPC) in the absence and presence of cholesterol, apoA1:DMPC (1:80, mol:mol) and apoA1:DMPC:cholesterol (1:86:9, mol:mol:mol)]. The overall shape of both particles is discoidal with the low-resolution structure of apoA1 visualized as an open, contorted, and out of plane conformation with three arms in nascent HDL/dimyristoyl phosphatidylcholine without cholesterol (nHDL_DMPC) and two arms in nascent HDL/dimyristoyl phosphatidylcholine with cholesterol (nHDL_DMPC+Chol). The low-resolution shape of the lipid phase in both nHDL_DMPC and nHDL_DMPC+Chol were oblate ellipsoids, and fit well within their respective protein shapes. Modeling studies indicate that apoA1 is folded onto itself in nHDL_DMPC, making a large hairpin, which was also confirmed independently by both cross-linking mass spectrometry and hydrogen-deuterium exchange (HDX) mass spectrometry analyses. In nHDL_DMPC+Chol, the lipid was expanded and no hairpin was visible. Importantly, despite the overall discoidal shape of the whole particle in both nHDL_DMPC and nHDL_DMPC+Chol, an open conformation (i.e., not a closed belt) of apoA1 is observed. Collectively, these data show that full length apoA1 retains an open architecture that is dictated by its lipid cargo. The lipid is likely predominantly organized as a bilayer with a micelle domain between the open apoA1 arms. The apoA1 configuration observed suggests a mechanism for accommodating changing lipid cargo by quantized expansion of hairpin structures.