Background: Peripheral membrane proteins are targeted to the cytoplasmic face of specific intracellular membranes. The organelle-specific ligands recognised by peripheral proteins include both other proteins and lipids. Oxysterol-binding protein (OSBP) translocates from the cytoplasm to the Golgi apparatus on binding oxygenated derivatives of cholesterol. The mechanism by which OSBP recognises the Golgi is unknown. It does, however, contain a pleckstrin homology (PH) domain, which in other proteins has been found to mediate regulated membrane binding, although in all previously studied examples the binding is to the plasma membrane. Results: The PH domains of OSBP and of a yeast homologue, Osh1p, were sufficient to target fusion proteins specifically to mammalian Golgi membranes. In addition, high level expression disrupted Golgi architecture and prevented forward traffic of cargo protein. In vitro, the OSBP PH domain bound to Golgi membranes in a manner apparently dependent on phosphatidylinositol-4,5-bisphosphate (Pl(4,5)P-2) or a related phosphatidylinositide. The OSBP PH domain bound to Pl(4,5)P-2 in liposomes with a submicromolar dissociation constant. Conclusions: The PH domains of OSBP and its yeast homologue recognise a determinant which is specific to Golgi membranes and important for Golgi function. The determinant appears to be a combination of a phosphatidylinositol polyphosphate and a second, Golgi-specific feature. (C) Current Biology Ltd.
