Oxysterol binding protein (OSBP) and many of its homologs transfer sterol in vitro, but in vivo, they are not major sterol transporters. In this issue of Structure, Tong and colleagues find that the yeast OSBP homolog, Osh3, binds PI(4)P but not sterol, supporting the view that PI(4)P regulation, not sterol transport, is the key activity for OSBP homologs.
