OSBP, the first member of the gene family to be characterized, was identified by Kandutsch and Shown (8) as a high affinity cytosolic receptor for a variety of oxysterol regulators of cholesterol synthesis, such as 25-hydroxycholesterol. Based on a correlation between the potency of oxysterols to suppress 3-hydroxy-3-methylglutaryl-CoA reductase activity and affinity for OSBP, it was proposed that OSBP played a role in mediating the effects of oxysterols on cholesterol homeostasis (9). Cloning and expression studies revealed that OSBP was a soluble protein that underwent translocation from a cytosolic/ vesicular compartment to the Golgi apparatus when cells were challenged with exogenous 25-hydroxycholesterol (10). Interaction with the Golgi apparatus has since been shown to involve the OSBP PH domain and is mediated by phosphatidylinositol 4,5-bisphosphate (PI-4,5-P 2 ) and possibly other unidentified protein or lipid factors (6, 11). The interaction of OSBP with the Golgi apparatus via the PH domain is important for function since overexpression of OSBP mutants with deletions of the PH domain did not cause alterations in cholesterol homeostasis that were evident with wild-type protein (6). In a model for oxysterol-mediated translocation, 25-hydroxycholesterol binding to the C-terminal region of OSBP could unmask the PH domain and thus facilitate binding to lipid or protein targets in the Golgi apparatus. Expression of the OSBP PH domain alone

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DNA topoisomerase (topo) II modulates DNA topology and is essential for cell division. There are two isoforms of topo II (α and β) that have limited functional redundancy, although their catalytic mechanisms appear the same. Using their COOH-terminal domains (CTDs) in yeast two-hybrid analysis, we have identified phospholipid scramblase 1 (PLSCR1) as a binding partner of both topo II α and β. Although predominantly a plasma membrane protein involved in phosphatidylserine externalization, PLSCR1 can also be imported into the nucleus where it may have a tumour suppressor function. The interactions of PLSCR1 and topo II were confirmed by pull-down assays with topo II α and β CTD fusion proteins and endogenous PLSCR1, and by co-immunoprecipitation of endogenous PLSCR1 and topo II α and β from HeLa cell nuclear extracts. PLSCR1 also increased the decatenation activity of human topo IIα. A conserved basic sequence in the CTD of topo IIα was identified as being essential for binding to PLSCR1 and binding of the two proteins could be inhibited by a synthetic peptide corresponding to topo IIα amino acids 1430-1441. These studies reveal for the first time a physical and functional interaction between topo II and PLSCR1.