The StarD4 subfamily of steroidogenic acute regulatory-related lipid transfer (START) domain proteins: new players in cholesterol metabolism.

Cholesterol levels in the body are maintained through the coordinated regulation of its uptake, synthesis, distribution, storage and efflux. However, the way cholesterol is sorted within cells remains poorly defined. The discovery of the newly described StarD4 subfamily, part of the steroidogenic acute regulatory lipid transfer (START) domain family of proteins, affords an opportunity for the study of intracellular cholesterol movement, metabolism and its disorders. The three members of this intracellular subfamily of proteins (StarD4, StarD5 and StarD6) have a similar lipid binding pocket specific for sterols (cholesterol in particular), but differing regulation and localization. The ability to bind and transport cholesterol through a non-vesicular mean suggests that they play a previously unappreciated role in cholesterol homeostasis.