The Interaction Of Amphipathic Alpha-Helix Bundle Proteins With Neutral Lipid Droplets

Lipid droplets (LDs) are dynamic cell organelles with structure similar to that of lipoproteins: a core dominated by triglycerides and other neutral lipids surrounded by a phospholipid-protein monolayer. The phospholipid monolayer covering the neutral lipid core makes for a unique assembly for lipid-protein interaction. We study these interactions via liquid droplet tensiometry, using droplet shape to measure the surface tension of a triolein droplet in an aqueous medium. For the first time, we use lipid mixtures to form the phospholipid monolayer. We report two distinct types of experiments: 1) Dynamic measurements in which we oscillate the surface area of the lipid droplet, analogous to the area changes naturally occurring in LDs. These experiments allow us to deduce surface pressure vs. area isotherms for different lipid mixtures. They also allow us to test the stability of the phospholipid and protein layer on the surface of the triolein droplet. 2) Insertion measurements, which allow us to characterize the protein affinity and insertion for lipids with different negative curvatures and charges. We used two amphipathic α-helix bundle proteins namely apoLp-III (full length protein) and apoE-3 (N-terminus). We show that bundle unfolding is critical for protein interaction with the triolein-phospholipid interface. Important differences between apoLp-III and apoE-3 are discussed in terms of the ability of the helix bundle to unfold.