Intracellular lipid traffic is mediated both by membrane vesicles and by a number of non-vesicular pathways facilitated by cytoplasmic lipid binding proteins. For these proteins to act effectively they must be targeted accurately to specific membranes. Here we identify a novel short conserved determinant called the FFAT motif that is shared by several seemingly unrelated lipid binding proteins and is also found in Opi1p, a transcriptional regulator of phospholipid synthesis in yeast. FFAT motifs act as membrane- targeting determinants by their direct interaction with homologues of VAMP-associated protein (VAP), a conserved endoplasmic reticulum (ER) protein. In budding yeast, all four proteins with FFAT motifs interact with Scs2p, a homologue of VAP, to target the ER to some extent. The precise intracellular distribution of each of these proteins depends on the integration of the FFAT-Scs2p interaction with other targeting determinants, and the interaction is functionally significant. We conclude that binding to a VAP homologue is a common mechanism by which proteins with FFAT motifs, most of which are involved in lipid metabolism, target ER membranes.
