Selective Targeting of Lipid Droplets by Proteins

Abstract

Lipid droplets (LDs) are organelles that store neutral lipids in cells. LDs have a core of neutral lipids, including triacylglycerols and sterol esters, surrounded by a phospholipid monolayer. LDs can grow dramatically to store neutral lipids in its core, which requires a commensurate increase in the size of the LD membrane. CTP:phosphocholine cytidylyltransferase (CCT) plays a key role in regulating the size of LDs, as it the rate-limiting enzyme in phosphatidylcholine (PC) synthesis, which is a major component of the LD outer coat. CCT has been shown to have a binding preference for LD as opposed to bilayer membranes, although the cause of this preference is not known. Previous studies have shown that large hydrophobic packing defects are key for the binding of amphipathic helices (AH) to membranes. We have studied LDs and bilayer membranes by extensive long time scale molecular dynamics (MD) simulations. In our results, we observe differing properties between LD membranes and bilayer membranes, including an increase in the frequency of large defects in LD membranes. We also find, using μs-long MD simulations, that an increase in the size of hydrophobic lipid packing defects in LDs compared to bilayer membranes plays a large role in the specificity of CCT AH initial binding to membranes as compared to bilayer membranes. Moreover, we frequently observe detachment of CCT from bilayer membranes, suggesting the lack of large hydrophobic defects destabilizes this interaction sufficiently to make binding to bilayers thermodynamically less favorable.