LAUSR

Protein-lipid interactions drive interorganelle sorting of proteins Experiments presented by Joseph Lorent of the Levental lab (University of Texas) define physical properties of integral membrane proteins that determine their inclusion in lipid domains of the plasma membrane (PM) called “rafts.” Through analyses of hundreds of transmembrane domains, it was concluded that three properties promote residence in a lipid raft: a small transmembrane domain (TMD) surface area, a long TMD, and palmitoylation of the protein. Mengxiao (Mandi) Ma of the Burd lab (Yale University) showed that endosometo-Golgi retrieval of a yeast SNARE protein, Snc1, depends critically on the sequence of its TMD and two endosomal sorting factors, Snx4 and Atg20, that bind to the cytoplasmic domain of Snc1. Mutations in the TMD ablate recognition by Snx4-Atg20 in vitro and retrieval of Snc1 from the endosome, leading to speculation that specific lipid–TMD interactions underlie proper presentation of the Snc1 retrieval signal. The lipid droplet (LD) is a storage organelle whose biogenesis and catabolism are governed by enzymes that are targeted to its surface via poorly characterized mechanisms. Coline Prévost (Walther-Farese lab, Harvard University) and collaborators used molecular dynamics simulations to identify candidate features of the LD surface that are recognized by an amphipathic helix that confers LD targeting. Their findings suggest that binding is promoted by lipid packing defects within the LD surface that facilitate insertion of bulky hydrophobic amino acid side chains into the LD surface, a result that was confirmed with elegant in vitro reconstitution studies.