A Model of Human APOA2 on HDL

Apolipoprotein A-II, the second most abundant protein in HDL, plays a key role in the HDL maturation and reverse cholesterol transport. It has a stronger affinity for lipids than apolipoprotein A-I (APOA1), which forms HDL's structural scaffold. Like APOA1, APOA2 mainly consists of amphipathic alpha helices that help it interact with lipid surfaces. Using computer modeling and mutagenesis, we developed a structural model for lipid-bound human APOA2. The protein's single disulfide bond restricts it to a belt-like configuration in discoidal HDL particles. Our model shows several key features: i) most basic residues of APOA2 are in the hairpin loop; ii) salt bridges and π-π interactions stabilize the structure; iii) two high-lipid affinity helical regions are in each APOA2 monomer; and iv) the hairpin structure facilitates interactions with other proteins like APOA1 or APOA2. Chemical crosslinking and MS/MS analysis identified 16 crosslinks; Fifteen of the 16 crosslinks are consistent with the double hairpin belt model, strongly supporting the proposal that the disulfide-dimer double hairpin belt is the primary structure of APOA2 in humans.