Apolipoprotein A-I structural organization in high-density lipoproteins isolated from human plasma

Abstract

High density lipoproteins (HDL) mediate cholesterol transport and protection from cardiovascular disease. Although synthetic HDLs have been studied for 30 years, the structure of human plasma-derived HDL, and its major protein apolipoprotein (apo)A-I, is unknown. We separated normal human HDL into 5 density subfractions and then further isolated those containing predominantly apoA-I (LpA-I). Using cross-linking chemistry and mass spectrometry, we found that apoA-I adopts a structural framework in these particles that closely mirrors that in synthetic HDL. We adapted established structural models for synthetic HDL to generate the first detailed models of authentic human plasma HDL in which apoA-I adopts a symmetrical cage-like structure. The models suggest that HDL particle size is modulated via a twisting motion of the resident apoA-I molecules. This understanding offers insights into how apoA-I structure modulates HDL function and its interactions with other apolipoproteins.

ApoA1 and ApoA1-specific self-antibodies in cardiovascular disease

“…Reduction in the spherical particle size increases the proteinpacking density that in turn leads to a less folded structural organization. 51,53 HDL size also influences the conformation of all ApoA1 structural segments, with the central region (a.a. 121-165), whose conformation is especially sensitive to the particle size. 54 A dynamic ApoA1 conformation involves a possibility of ApoA1 reversible dissociation/association with the HDL particle.…”

Apolipoprotein A-I configuration and cell cholesterol efflux activity of discoidal lipoproteins depend on the reconstitution process

“…Interestingly, the cross-linking pattern found in reconstituted spherical HDL was highly similar to that found in discs, regardless of whether they contained two or three molecules of apoA-I in each particle [28]. Moreover, the same methodology applied to 5 HDL subfractions isolated from human plasma revealed the presence of several crosslinks supporting the double belt 5/5 or 5/2 arrangement [29]. Bath et al [30] used a very similar approach with CD reconstituted D-HDL to find three intermolecular cross-links, two of which were consistent with the 5/5 double-belt model.…”

“…Thus, it is probable that "in vivo" generated D-HDL by the action of ABCA1 would present the same major configuration (5/2 registry) as those D-HDL generated by the microsolubilization of DMPC vesicles at 24°C. This possibility must be confirmed, but it is worth noting that evidence obtained by cross-linking indicates that both, the 5/2 and 5/5 registries are present in natural HDL complexes [28,29].…”

“…Juxtaposition of helices 5 is also observed in a recently obtained crystal structure of the C-terminal truncated form D(185-243) apoAI [26]. The question was addressed by the group of WS Davidson in discoidal [27] and spherical HDL [28] reconstituted particles as well as in HDL isolated from human plasma [29] using lysine cross-linking followed by trypsinolysis and mass spectrometry analysis of the resulting peptides. In D-HDL reconstituted with POPC through the CD method [27], they found several intermolecular distance constraints supporting the double-belt model with the 5/5 registry, but data also indicate the presence of an additional double-belt orientation with a shifted helical registry with juxtaposition between helices 5 and 2 (5/2 registry) (see figure 1).…”

Targeted Proteomics Identifies Paraoxonase/Arylesterase 1 (PON1) and Apolipoprotein Cs as Potential Risk Factors for Hypoalphalipoproteinemia in Diabetic Subjects Treated with Fenofibrate and Rosiglitazone

“…Internal Standard Selection-Approximately 70% of HDL's protein mass is composed of APOA1, HDL's major structural protein (41). Previous studies validated the use of [ 15 N]APOA1 as a global internal standard to correct for digestion efficiency and run-to-run variability in LCMS analyses (31,34).…”