A conserved ER targeting motif in three families of lipid binding proteins and in Opi1p binds VAP

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Abstract
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.