Exceptional stability of a perilipin on lipid droplets depends on its polar residues, suggesting multimeric assembly

Numerous proteins target lipid droplets (LDs) through amphipathic helices (AHs). It is generally assumed that AHs insert bulky hydrophobic residues in packing defects at the LD surface. However, this model does not explain the targeting of perilipins, the most abundant and specific amphipathic proteins of LDs. The gigantic Plin4 contains a highly repetitive AH that lacks bulky hydrophobic residues, and its LD targeting depends strongly on its length. We show that Plin4 forms a remarkably immobile protein layer at the surface of cellular or artificial LDs, making them stable over days. This Plin4 AH feature is not shared with the AHs of other perilipins, which display much faster dynamics on lipid surfaces. Plin4 AH stability on LDs is exquisitely sensitive to the nature and distribution of its polar residues. These results suggest that Plin4 forms stable arrangements of adjacent AHs via polar interactions.