Roles of protrudin at interorganelle membrane contact sites.

Intracellular organelles were long viewed as isolated compartments floating in the cytosol. However, this view has been radically changed within the last decade by the discovery that most organelles communicate with the endoplasmic reticulum (ER) network via membrane contact sites (MCSs) that are essential for intracellular homeostasis. Protrudin is an ER resident protein that was originally shown to regulate neurite formation by promoting endosome trafficking. More recently, however, protrudin has been found to serve as a tethering factor at MCSs. The roles performed by protrudin at MCSs are mediated by its various domains, including inactivation of the small GTPase Rab11, bending of the ER membrane, and functional interactions with other molecules such as the motor protein KIF5 and the ER protein VAP. Mutations in the protrudin gene (ZFYVE27) are associated with hereditary spastic paraplegia, an axonopathy that results from defective ER structure. This review, examines the pleiotropic molecular functions of protrudin and its role in interorganellar communication.