A Model of Human APOA2 on HDL

Area of Science:

• Biochemistry
• Structural Biology
• Lipid Metabolism

Background:

• Apolipoprotein A-II (APOA2) is the second most abundant protein in high-density lipoprotein (HDL), vital for HDL maturation and reverse cholesterol transport.
• APOA2 exhibits a strong affinity for lipids and, like apolipoprotein A-I (APOA1), is composed primarily of amphipathic alpha helices that facilitate interaction with lipid surfaces.

Purpose of the Study:

• To develop a structural model for lipid-bound human Apolipoprotein A-II (APOA2).
• To elucidate the configuration and stabilizing interactions of APOA2 within discoidal HDL particles.

Main Methods:

• Computer modeling and site-directed mutagenesis were employed to generate a structural model of lipid-bound APOA2.
• Chemical crosslinking coupled with mass spectrometry (MS/MS) analysis was used to identify crosslinks within the protein structure.

Main Results:

• The structural model revealed that APOA2 adopts a belt-like configuration stabilized by its single disulfide bond.
• Key features include basic residues in a hairpin loop, salt bridges, π-π interactions, and two helical regions with high lipid affinity per monomer.
• Fifteen out of sixteen identified crosslinks support the proposed double hairpin belt model for APOA2 structure.

Conclusions:

• The disulfide-dimer double hairpin belt is proposed as the primary structure of human APOA2 in HDL particles.
• This structure explains APOA2's role in lipid interactions and its potential to interact with other proteins like APOA1.