Inter-organelle membrane contact sites: through a glass, darkly

Inter-organelle membrane contact sites are zones where heterologous membranes, usually the endoplasmic reticulum plus a partner organelle, come into close apposition. These sites are very poorly understood because so few of their components have been identified; however, it is clear that they are specialised for traffic of material and information between the two membranes. There have been recent advances in the study of lipid transfer proteins, such as ceramide transfer protein (CERT) and homologues of oxysterol binding protein (OSBP). Not only can these proteins carry lipids across the cytoplasm, but they have been found to target both the endoplasmic reticulum and a partnering organelle, and in some cases have been localised to membrane contact sites. Further work will be needed to test whether these lipid transfer proteins act when anchored at inter-organelle contact sites.