Structure‐function studies of the sterol transfer protein, STARD4 (606.2)

Abstract

Significant differences in lipid distribution are maintained between intracellular organelles. Cholesterol comprises ~30% of the lipid molecules in the plasma membrane and is also enriched in the endocytic recycling compartment (ERC). However, in the endoplasmic reticulum (ER) cholesterol accounts for 5% of the lipid molecules. Sterols are transported as components of lipid bilayers in vesicular transport and are transferred by non‐vesicular processes involving carrier proteins. The steroidogenic acute regulator‐related lipid‐transfer (START) domain containing proteins are involved in several pathways of non‐vesicular trafficking of sterols. Among the soluble START proteins, StARD4 has been shown to increase cholesteryl ester accumulation in lipid droplets, in an acyl‐CoA:cholesterol acyl‐transferase (ACAT) dependent manner, and is controlled at the transcriptional level by sterols. Our laboratory has previously shown that StARD4 is efficient in transporting sterols between membranes in vivo and in vitro. However, the precise molecular mechanisms that mediate StARD4 membrane targeting, interaction and sterol extraction are unknown. Here we show that STARD4 has a marked preference for negatively charged membranes, for membranes rich in unsaturated lipids and that specific phosphatidylinositol phosphates (PIPs) modulate the sterol transfer activity of STARD4. Additionally, mutations in either the basic patch or Ω1 loop greatly alter the sterol transfer activity of STARD4, likely by perturbing membrane interaction or sterol absorption/release. Overall, these findings provide insight into the biophysical and cellular mechanisms that facilitate StARD4 activity and distribution required to maintain cholesterol homeostasis.